2.3: Chemical Formulas
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- Scott Van Bramer
- Widener University
Learning Objectives
- Symbolize the composition of molecules using molecular formulas and empirical formulas
- Represent the bonding arrangement of atoms within molecules using structural formulas
- Define ionic and molecular (covalent) compounds
- Predict the type of compound formed from elements based on their location within the periodic table
- Determine formulas for simple ionic compounds
- Derive names for common types of inorganic compounds using a systematic approach.
- Describe how to name binary covalent compounds including acids and oxyacids.
Elements, Molecules, and Compounds
Elements and Molecules
Although many elements consist of discrete, individual atoms, some exist as molecules made up of two or more atoms of the element chemically bonded together. For example, most samples of the elements hydrogen, oxygen, and nitrogen are composed of molecules that contain two atoms each (called diatomic molecules) and thus have the molecular formulas H 2 , O 2 , and N 2 , respectively. Other elements commonly found as diatomic molecules are fluorine (F 2 ), chlorine (Cl 2 ), bromine (Br 2 ), and iodine (I 2 ). The most common form of the element sulfur is composed of molecules that consist of eight atoms of sulfur; its molecular formula is S 8 (Figure \(\PageIndex{2}\)).
A molecular formula is a representation of a molecule that uses chemical symbols to indicate the types of atoms followed by subscripts to show the number of atoms of each type in the molecule. (A subscript is used only when more than one atom of a given type is present.) Molecular formulas are also used as abbreviations for the names of compounds. It is important to note that a subscript following a symbol and a number in front of a symbol do not represent the same thing; for example, H 2 and 2H represent distinctly different species. H 2 is a molecular formula; it represents a diatomic molecule of hydrogen, consisting of two atoms of the element that are chemically bonded together. The expression 2H, on the other hand, indicates two separate hydrogen atoms that are not combined as a unit. The expression 2H 2 represents two molecules of diatomic hydrogen (Figure \(\PageIndex{3}\)).
Compounds and Molecules
Compounds are formed when two or more elements chemically combine, resulting in the formation of bonds. If these two elements are non-metals the compound is a molecular compound. The structural formula for a compound gives the same information as its molecular formula (the types and numbers of atoms in the molecule) but also shows how the atoms are connected in the molecule. The structural formula for methane contains symbols for one C atom and four H atoms, indicating the number of atoms in the molecule (Figure \(\PageIndex{1}\)). The lines represent bonds that hold the atoms together. (A chemical bond is an attraction between atoms or ions that holds them together in a molecule or a crystal.) These molecular compounds (covalent compounds) result when atoms share electrons. Covalent bonding is an important and extensive concept in chemistry, and it will be treated in considerable detail in a later chapter of this text. We can often identify molecular compounds on the basis of their physical properties. Under normal conditions, molecular compounds often exist as gases, low-boiling liquids, and low-melting solids, although many important exceptions exist.We will discuss chemical bonds and see how to predict the arrangement of atoms in a molecule later. For now, simply know that the lines are an indication of how the atoms are connected in a molecule. A ball-and-stick model shows the geometric arrangement of the atoms with atomic sizes not to scale, and a space-filling model shows the relative sizes of the atoms.
We can describe a compound with a molecular formula, in which the subscripts indicate the
actual numbers of atoms
of each element in a molecule of the compound. We can also describe the composition of these compounds with an
empirical formula
, which indicates the types of atoms present and
the simplest whole-number ratio of the number of atoms (or ions) in the compound
. For example, it can be determined experimentally that benzene contains two elements, carbon (C) and hydrogen (H), and that for every carbon atom in benzene, there is one hydrogen atom. Thus, the empirical formula is CH. An experimental determination of the molecular mass reveals that a molecule of benzene contains six carbon atoms and six hydrogen atoms, so the molecular formula for benzene is C
6
H
6
(Figure \(\PageIndex{5}\)).
If we know a compound’s formula, we can easily determine the empirical formula. For example, the molecular formula for acetic acid, the component that gives vinegar its sharp taste, is C 2 H 4 O 2 . This formula indicates that a molecule of acetic acid (Figure \(\PageIndex{6}\)) contains two carbon atoms, four hydrogen atoms, and two oxygen atoms. The ratio of atoms is 2:4:2. Dividing by the lowest common denominator (2) gives the simplest, whole-number ratio of atoms, 1:2:1, so the empirical formula is CH 2 O. The molecular formula is always a whole-number multiple of an empirical formula.
Example \(\PageIndex{1}\): Empirical and Molecular Formulas
Molecules of glucose (blood sugar) contain 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. What are the molecular and empirical formulas of glucose?
Solution
The molecular formula is C 6 H 12 O 6 because one molecule actually contains 6 C, 12 H, and 6 O atoms. The simplest whole-number ratio of C to H to O atoms in glucose is 1:2:1, so the empirical formula is CH 2 O.
Exercise \(\PageIndex{1}\)
A molecule of metaldehyde (a pesticide used for snails and slugs) contains 8 carbon atoms, 16 hydrogen atoms, and 4 oxygen atoms. What are the molecular and empirical formulas of metaldehyde?
- Answer
-
Molecular formula, C 8 H 16 O 4 ; empirical formula, C 2 H 4 O
It is important to be aware that it may be possible for the same atoms to be arranged in different ways: Compounds with the same molecular formula may have different atom-to-atom bonding and therefore different structures. For example, could there be another compound with the same formula as acetic acid, C 2 H 4 O 2 ? And if so, what would be the structure of its molecules?
If you predict that another compound with the formula C 2 H 4 O 2 could exist, then you demonstrated good chemical insight and are correct. Two C atoms, four H atoms, and two O atoms can also be arranged to form methyl formate, which is used in manufacturing, as an insecticide, and for quick-drying finishes. Methyl formate molecules have one of the oxygen atoms between the two carbon atoms, differing from the arrangement in acetic acid molecules. Acetic acid and methyl formate are examples of isomers —compounds with the same chemical formula but different molecular structures (Figure \(\PageIndex{7}\)). Note that this small difference in the arrangement of the atoms has a major effect on their respective chemical properties. You would certainly not want to use a solution of methyl formate as a substitute for a solution of acetic acid (vinegar) when you make salad dressing.
Many types of isomers exist (Figure \(\PageIndex{8}\)). Acetic acid and methyl formate are structural isomers , compounds in which the molecules differ in how the atoms are connected to each other. There are also various types of spatial isomers , in which the relative orientations of the atoms in space can be different. For example, the compound carvone (found in caraway seeds, spearmint, and mandarin orange peels) consists of two isomers that are mirror images of each other.
Naming Molecular Compounds
When two nonmetallic elements form a molecular compound, several combination ratios are often possible. For example, carbon and oxygen can form the compounds CO and CO 2 . Since these are different substances with different properties, they cannot both be called carbon oxide. To distinguish these two compounds prefixes are added to specify the numbers of atoms of each element. The name of the more metallic element (the one farther to the left and/or bottom of the periodic table) is first, followed by the name of the more nonmetallic element (the one farther to the right and/or top) with its ending changed to the suffix – ide . The numbers of atoms of each element are designated by the Greek prefixes shown in Table \(\PageIndex{5}\).
| Number | Prefix | Number | Prefix | |
|---|---|---|---|---|
| 1 (sometimes omitted) | mono- | 6 | hexa- | |
| 2 | di- | 7 | hepta- | |
| 3 | tri- | 8 | octa- | |
| 4 | tetra- | 9 | nona- | |
| 5 | penta- | 10 | deca- |
When only one atom of the first element is present, the prefix mono - is usually deleted from that part. Thus, \(\ce{CO}\) is named carbon monoxide, and \(\ce{CO2}\) is called carbon dioxide. When two vowels are adjacent, the a in the Greek prefix is usually dropped. Some other examples are shown in Table \(\PageIndex{6}\).
| Compound | Name | Compound | Name | |
|---|---|---|---|---|
| SO 2 | sulfur dioxide | BCl 3 | boron trichloride | |
| SO 3 | sulfur trioxide | SF 6 | sulfur hexafluoride | |
| NO 2 | nitrogen dioxide | PF 5 | phosphorus pentafluoride | |
| N 2 O 4 | dinitrogen tetroxide | P 4 O 10 | tetraphosphorus decaoxide | |
| N 2 O 5 | dinitrogen pentoxide | IF 7 | iodine heptafluoride |
There are a few common names that you may encounter as you continue your study of chemistry. For example, although the proper name for NO is nitrogen monoxide, it is often called nitric oxide. Similarly, dinitrogen monioxide, N 2 O, may be called nitrous oxide. And H 2 O is usually called water, not dihydrogen monoxide. Be careful when you encounter these common names to make sure you know what formula to use - it is a good idea to double check by looking these up.
Example \(\PageIndex{2}\): Naming Covalent Compounds
Name the following covalent compounds:
- SF 6
- N 2 O 3
- Cl 2 O 7
- P 4 O 6
Solution
Because these compounds consist solely of nonmetals, we use prefixes to designate the number of atoms of each element:
- sulfur hexafluoride
- dinitrogen trioxide
- dichlorine heptoxide
- tetraphosphorus hexoxide
Exercise \(\PageIndex{2}\)
Write the formulas for the following compounds:
- phosphorus pentachloride
- dinitrogen monoxide
- iodine heptafluoride
- carbon tetrachloride
- Answer a
-
PCl 5
- Answer b
-
N 2 O
- Answer c
-
IF 7
- Answer d
-
CCl 4
Ionic Compounds
Introduction
Compounds are formed when two or more elements chemically combine, resulting in the formation of bonds. When two non-metals like hydrogen and oxygen react they form water molecules H 2 O - which is a molecular compound. However, when a metal and a non-metal like sodium and chlorine react they form table salt which is an ionic compound. The result is not a molecule. This ionic compound has elements in a fixed ratio 1 Na to 1 Cl but the structure is very different from a molecular compound. The empirical formula for table salt is NaCl and the structure contains a large number of interconnected atoms. The empirical formula just indicates the types of atoms present and the simplest whole-number ratio of the number of atoms (or ions) in the compound .
Another example is titanium (IV) oxide. Titanium (IV) oxide is used as pigment in white paint and in the thick, white, blocking type of sunscreen. It has an empirical formula of TiO 2 that identifies the elements titanium (Ti) and oxygen (O) as the constituents of titanium (IV) oxide. The empirical formula shows there are twice as many oxygen atoms as titanium atoms. The structure, however, is shown in Figure \(\PageIndex{4b}\).
Forming Ions
In ordinary chemical reactions, the nucleus of each atom (and thus the identity of the element) remains unchanged. Electrons, however, can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. The transfer and sharing of electrons among atoms govern the chemistry of the elements. During the formation of some compounds, atoms gain or lose electrons, and form electrically charged particles called ions (Figure \(\PageIndex{5}\)).
You can use the periodic table to predict whether an atom will form an anion or a cation, and you can often predict the charge of the resulting ion. Atoms of many main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. To illustrate, an atom of an alkali metal (group 1) loses one electron and forms a cation with a 1+ charge; an alkaline earth metal (group 2) loses two electrons and forms a cation with a 2+ charge, and so on. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized Ca 2 + . The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca 2 + is called a calcium ion.
When atoms of nonmetal elements form ions, they generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. Atoms of group 17 gain one electron and form anions with a 1− charge; atoms of group 16 gain two electrons and form ions with a 2− charge, and so on. For example, the neutral bromine atom, with 35 protons and 35 electrons, can gain one electron to provide it with 36 electrons. This results in an anion with 35 protons, 36 electrons, and a 1− charge. It has the same number of electrons as atoms of the next noble gas, krypton, and is symbolized Br − . (A discussion of the theory supporting the favored status of noble gas electron numbers reflected in these predictive rules for ion formation is provided in a later chapter of this text.)
Note the usefulness of the periodic table in predicting likely ion formation and charge (Figure \(\PageIndex{6}\)). Moving from the far left to the right on the periodic table, main-group elements tend to form cations with a charge equal to the group number. That is, group 1 elements form 1+ ions; group 2 elements form 2+ ions, and so on. Moving from the far right to the left on the periodic table, elements often form anions with a negative charge equal to the number of groups moved left from the noble gases. For example, group 17 elements (one group left of the noble gases) form 1− ions; group 16 elements (two groups left) form 2− ions, and so on. This trend can be used as a guide in many cases, but its predictive value decreases when moving toward the center of the periodic table. In fact, transition metals and some other metals often exhibit variable charges that are not predictable by their location in the table. For example, copper can form ions with a 1+ or 2+ charge, and iron can form ions with a 2+ or 3+ charge.
Example \(\PageIndex{2}\): Composition of Ions
An ion found in some compounds used as antiperspirants contains 13 protons and 10 electrons. What is its symbol?
Solution
Because the number of protons remains unchanged when an atom forms an ion, the atomic number of the element must be 13. Knowing this lets us use the periodic table to identify the element as Al (aluminum). The Al atom has lost three electrons and thus has three more positive charges (13) than it has electrons (10). This is the aluminum cation, Al 3 + .
Exercise \(\PageIndex{2}\)
Give the symbol and name for the ion with 34 protons and 36 electrons.
- Answer
-
Se 2 − , the selenide ion
Example \(\PageIndex{3}\): Formation of Ions
Magnesium and nitrogen react to form an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.
Solution
Magnesium’s position in the periodic table (group 2) tells us that it is a metal. Metals form positive ions (cations). A magnesium atom must lose two electrons to have the same number electrons as an atom of the previous noble gas, neon. Thus, a magnesium atom will form a cation with two fewer electrons than protons and a charge of 2+. The symbol for the ion is Mg 2 + , and it is called a magnesium ion.
Nitrogen’s position in the periodic table (group 15) reveals that it is a nonmetal. Nonmetals form negative ions (anions). A nitrogen atom must gain three electrons to have the same number of electrons as an atom of the following noble gas, neon. Thus, a nitrogen atom will form an anion with three more electrons than protons and a charge of 3−. The symbol for the ion is N 3− , and it is called a nitride ion.
Exercise \(\PageIndex{3}\)
Aluminum and carbon react to form an ionic compound. Predict which forms an anion, which forms a cation, and the charges of each ion. Write the symbol for each ion and name them.
- Answer
-
Al will form a cation with a charge of 3+: Al 3 + , an aluminum ion. Carbon will form an anion with a charge of 4−: C 4− , a carbide ion.
Polyatomic Ions
The ions that we have discussed so far are called monatomic ions , that is, they are ions formed from only one atom. We also find many polyatomic ions . These ions, which act as discrete units, are electrically charged molecules (a group of bonded atoms with an overall charge). Some of the more important polyatomic ions are listed in Table \(\PageIndex{1}\). Oxyanions are polyatomic ions that contain one or more oxygen atoms. At this point in your study of chemistry, you should memorize the names, formulas, and charges of the most common polyatomic ions. Because you will use them repeatedly, they will soon become familiar.
| Name | Formula |
|---|---|
| ammonium | \(\ce{NH4+}\) |
| hydronium | \(\ce{H_3O^+}\) |
| hydroxide | \(\ce{OH^-}\) |
| acetate | \(\ce{CH_3COO^-}\) |
| cyanide | \(\ce{CN^-}\) |
| carbonate | \(\ce{CO_3^{2-}}\) |
|
hydrogen carbonate (bicarbonate) |
\(\ce{HCO_3^-}\) |
| nitrate | \(\ce{NO_3^-}\) |
| nitrite | \(\ce{NO_2^-}\) |
| sulfate | \(\ce{SO_4^{2-}}\) |
| hydrogen sulfate (bisulfate) | \(\ce{HSO_4^-}\) |
| sulfite | \(\ce{SO_3^{2-}}\) |
| hydrogen sulfite (bisulfite) | \(\ce{HSO_3^-}\) |
| phosphate | \(\ce{PO_4^{3-}}\) |
| hydrogen phosphate (biphosphate) | \(\ce{HPO_4^{2-}}\) |
| dihydrogen phosphate (dibiphosphate) | \(\ce{H_2PO_4^-}\) |
Note that there is a system for naming some polyatomic ions; -ate and -ite are suffixes designating polyatomic ions containing more or fewer oxygen atoms. Unfortunately, the number of oxygen atoms corresponding to a given suffix or prefix is not consistent; for example, nitrate is \(\ce{NO3-}\) while sulfate is \(\ce{SO4^{2-}}\).
The nature of the attractive forces that hold atoms or ions together within a compound is the basis for classifying chemical bonding. When electrons are transferred and ions form, ionic bonds result. Ionic bonds are electrostatic forces of attraction, that is, the attractive forces experienced between objects of opposite electrical charge (in this case, cations and anions). When electrons are “shared” and molecules form, covalent bonds result. Covalent bonds are the attractive forces between the positively charged nuclei of the bonded atoms and one or more pairs of electrons that are located between the atoms. Compounds are classified as ionic or molecular (covalent) on the basis of the bonds present in them.
Forming Ionic Compounds
When an element composed of atoms that readily lose electrons (a metal) reacts with an element composed of atoms that readily gain electrons (a nonmetal), a transfer of electrons usually occurs, producing ions. The compound formed by this transfer is stabilized by the electrostatic attractions (ionic bonds) between the ions of opposite charge present in the compound. For example, when each sodium atom in a sample of sodium metal (group 1) gives up one electron to form a sodium cation, Na + , and each chlorine atom in a sample of chlorine gas (group 17) accepts one electron to form a chloride anion, Cl − , the resulting compound, NaCl, is composed of sodium ions and chloride ions in the ratio of one Na + ion for each Cl − ion. Similarly, each calcium atom (group 2) can give up two electrons and transfer one to each of two chlorine atoms to form CaCl 2 , which is composed of Ca 2 + and Cl − ions in the ratio of one Ca 2 + ion to two Cl − ions.
A compound that contains ions and is held together by ionic bonds is called an ionic compound . The periodic table can help us recognize many of the compounds that are ionic: When a metal is combined with one or more nonmetals, the compound is usually ionic. This guideline works well for predicting ionic compound formation for most of the compounds typically encountered in an introductory chemistry course. However, it is not always true (for example, aluminum chloride, AlCl 3 , is not ionic).
You can often recognize ionic compounds because of their properties. Ionic compounds are solids that typically melt at high temperatures and boil at even higher temperatures. For example, sodium chloride melts at 801 °C and boils at 1413 °C. (As a comparison, the molecular compound water melts at 0 °C and boils at 100 °C.) In solid form, an ionic compound is not electrically conductive because its ions are unable to flow (“electricity” is the flow of charged particles). When molten, however, it can conduct electricity because its ions are able to move freely through the liquid (Figure \(\PageIndex{7}\)).
In every ionic compound, the total number of positive charges of the cations equals the total number of negative charges of the anions. Thus, ionic compounds are electrically neutral overall, even though they contain positive and negative ions. We can use this observation to help us write the formula of an ionic compound. The formula of an ionic compound must have a ratio of ions such that the numbers of positive and negative charges are equal.
Example \(\PageIndex{3}\): Predicting the Formula of an Ionic Compound
The gemstone sapphire (Figure \(\PageIndex{4}\)) is mostly a compound of aluminum and oxygen that contains aluminum cations, Al 3 + , and oxygen anions, O 2− . What is the formula of this compound?
Solution Because the ionic compound must be electrically neutral, it must have the same number of positive and negative charges. Two aluminum ions, each with a charge of 3+, would give us six positive charges, and three oxide ions, each with a charge of 2−, would give us six negative charges. The formula would be Al 2 O 3 .
Exercise \(\PageIndex{3}\)
Predict the formula of the ionic compound formed between the sodium cation, Na + , and the sulfide anion, S 2− .
- Answer
-
Na 2 S
Many ionic compounds contain polyatomic ions (Table \(\PageIndex{1}\)) as the cation, the anion, or both. As with simple ionic compounds, these compounds must also be electrically neutral, so their formulas can be predicted by treating the polyatomic ions as discrete units. We use parentheses in a formula to indicate a group of atoms that behave as a unit. For example, the formula for calcium phosphate, one of the minerals in our bones, is Ca 3 (PO 4 ) 2 . This formula indicates that there are three calcium ions (Ca 2 + ) for every two phosphate \(\left(\ce{PO4^{3-}}\right)\) groups. The \(\ce{PO4^{3-}}\) groups are discrete units, each consisting of one phosphorus atom and four oxygen atoms, and having an overall charge of 3−. The compound is electrically neutral, and its formula shows a total count of three Ca, two P, and eight O atoms.
Example \(\PageIndex{4}\): Predicting the Formula of a Compound with a Polyatomic Anion
Baking powder contains calcium dihydrogen phosphate, an ionic compound composed of the ions Ca 2 + and \(\ce{H2PO4-}\). What is the formula of this compound?
Solution
The positive and negative charges must balance, and this ionic compound must be electrically neutral. Thus, we must have two negative charges to balance the 2+ charge of the calcium ion. This requires a ratio of one Ca 2 + ion to two \(\ce{H2PO4-}\) ions. We designate this by enclosing the formula for the dihydrogen phosphate ion in parentheses and adding a subscript 2. The formula is Ca(H 2 PO 4 ) 2 .
Exercise \(\PageIndex{4}\)
Predict the formula of the ionic compound formed between the lithium ion and the sulfate ion, \(\ce{SO4^2-}\) (Hint: Use the periodic table to predict the sign and the charge on the lithium ion.)
- Answer
-
Li 2 SO 4
Because an ionic compound is not made up of single, discrete molecules, it may not be properly symbolized using a molecular formula. Instead, ionic compounds must be symbolized by a formula indicating the relative numbers of its constituent ions. For compounds containing only monatomic ions (such as NaCl) and for many compounds containing polyatomic ions (such as CaSO 4 ), these formulas are just the empirical formulas introduced earlier in this chapter. However, the formulas for some ionic compounds containing polyatomic ions are not empirical formulas. For example, the ionic compound sodium oxalate is comprised of Na + and \(\ce{C2O4^2-}\) ions combined in a 2:1 ratio, and its formula is written as Na 2 C 2 O 4 . The subscripts in this formula are not the smallest-possible whole numbers, as each can be divided by 2 to yield the empirical formula, NaCO 2 . This is not the accepted formula for sodium oxalate, however, as it does not accurately represent the compound’s polyatomic anion, \(\ce{C2O4^2-}\).
Naming Ionic Compounds
This section describes how to name simple ionic compounds, such as NaCl, CaCO 3 , and N 2 O 4 . The simplest of these are binary compounds , those containing only two elements, but we will also consider how to name ionic compounds containing polyatomic ions. There are different systems for ionic compounds where the metal always forms ions with the same charge vs metals that form ions with different charges.
Ionic Compounds Containing Metals With a Single Charge
The name of a binary compound containing monatomic ions consists of the name of the metal followed by the name of the nonmetal. The nonmetal has its ending replaced by the suffix – ide ). Some examples are given in Table \(\PageIndex{2}\).
| NaCl, sodium chloride | Na 2 O, sodium oxide |
| KBr, potassium bromide | CdS, cadmium sulfide |
| CaI 2 , calcium iodide | Mg 3 N 2 , magnesium nitride |
| CsF, cesium fluoride | Ca 3 P 2 , calcium phosphide |
| LiCl, lithium chloride | Al 4 C 3 , aluminum carbide |
Compounds Containing Polyatomic Ions
Compounds containing polyatomic ions are named similarly to those containing only monatomic ions, except there is no need to change to an – ide ending, since a suffix is already present in the name of the anion. Examples are shown in Table \(\PageIndex{3}\).
| KC 2 H 3 O 2 , potassium acetate | (NH 4 )Cl, ammonium chloride |
| NaHCO 3 , sodium bicarbonate | CaSO 4 , calcium sulfate |
| Al 2 (CO 3 ) 3 , aluminum carbonate | Mg 3 (PO 4 ) 2 , magnesium phosphate |
Ionic Compounds in Your Cabinets
Every day you encounter and use a large number of ionic compounds. Some of these compounds, where they are found, and what they are used for are listed in Table \(\PageIndex{3}\). Look at the label or ingredients list on the various products that you use during the next few days, and see if you run into any of those in this table, or find other ionic compounds that you could now name or write as a formula.
| Ionic Compound | Name | Use |
|---|---|---|
| NaCl | sodium chloride | ordinary table salt |
| NaHCO 3 |
sodium hydrogen carbonate or sodium bicarbonate |
baking soda; used in cooking (and in antacids) |
| Na 2 CO 3 | sodium carbonate | washing soda; used in cleaning agents |
| CaCO 3 | calcium carbonate | ingredient in antacids |
| Mg(OH) 2 | magnesium hydroxide | ingredient in antacids |
| K 3 PO 4 | potassium phosphate | food additive (many purposes) |
| Na 2 HPO 4 |
sodium hydrogen phosphate or sodium biphosphate |
anti-caking agent; used in powdered products |
| Na 2 SO 3 | sodium sulfite | preservative |
Compounds Containing a Metal Ion with a Variable Charge
Most of the transition metals form two or more cations with different charges. Compounds of these metals with nonmetals are named with the same method as compounds in the first category, except the charge of the metal ion is specified by a Roman numeral in parentheses after the name of the metal. The charge of the metal ion is determined from the formula of the compound and the charge of the anion. For example, consider binary ionic compounds of iron and chlorine. Iron typically exhibits a charge of either 2+ or 3+, and the two corresponding compound formulas are FeCl 2 and FeCl 3 . The simplest name, “iron chloride,” will, in this case, be ambiguous, as it does not distinguish between these two compounds. In cases like this, the charge of the metal ion is included as a Roman numeral in parentheses immediately following the metal name. These two compounds are then unambiguously named iron(II) chloride and iron(III) chloride, respectively. Other examples are provided in Table \(\PageIndex{4}\).
| Transition Metal Ionic Compound | Name |
|---|---|
| FeCl 3 | iron(III) chloride |
| FeCl 2 | Iron (II) chloride |
| Cu 3 (PO 4 ) 2 | copper(II) phosphate |
Out-of-date nomenclature used the suffixes – ic and – ous to designate metals with higher and lower charges, respectively: Iron(III) chloride, FeCl 3 , was previously called ferric chloride, and iron(II) chloride, FeCl 2 , was known as ferrous chloride. Though this naming convention has been largely abandoned by the scientific community, it remains in use by some segments of industry. For example, you may see the words stannous fluoride on a tube of toothpaste. This represents the formula SnF 2 , which is more properly named tin(II) fluoride. The other fluoride of tin is SnF 4 , which was previously called stannic fluoride but is now named tin(IV) fluoride. If you see one of these names, look it up to make sure you have the correct charge for the metal.
Example \(\PageIndex{1}\): Naming Ionic Compounds
Name the following ionic compounds, which contain a metal that can have more than one ionic charge:
- Fe 2 S 3
- CuSe
- GaN
- CrCl 3
- Ti 2 (SO 4 ) 3
Solution
The anions in these compounds have a fixed negative charge (S 2− , Se 2 − , N 3− , Cl − , and \(\ce{SO4^2-}\)), and the compounds must be neutral. Because the total number of positive charges in each compound must equal the total number of negative charges, the positive ions must be Fe 3 + , Cu 2 + , Ga 3 + , Cr 3 + , and Ti 3 + . These charges are used in the names of the metal ions:
- iron(III) sulfide
- copper(II) selenide
- gallium(III) nitride
- chromium(III) chloride
- titanium(III) sulfate
Exercise \(\PageIndex{1}\)
Write the formulas of the following ionic compounds:
- chromium(III) phosphide
- mercury(II) sulfide
- manganese(II) phosphate
- copper(I) oxide
- chromium(VI) fluoride
- Answer a
-
CrP
- Answer b
-
HgS
- Answer c
-
Mn 3 (PO 4 ) 2
- Answer d
-
Cu 2 O
- Answer e
-
CrF 6
Erin Brokovich and Chromium Contamination
In the early 1990s, legal file clerk Erin Brockovich (Figure \(\PageIndex{2}\)) discovered a high rate of serious illnesses in the small town of Hinckley, California. Her investigation eventually linked the illnesses to groundwater contaminated by Cr(VI) used by Pacific Gas & Electric (PG&E) to fight corrosion in a nearby natural gas pipeline. As dramatized in the film Erin Brokovich (for which Julia Roberts won an Oscar), Erin and lawyer Edward Masry sued PG&E for contaminating the water near Hinckley in 1993. The settlement they won in 1996—$333 million—was the largest amount ever awarded for a direct-action lawsuit in the US at that time.
Chromium compounds are widely used in industry, such as for chrome plating, in dye-making, as preservatives, and to prevent corrosion in cooling tower water, as occurred near Hinckley. In the environment, chromium exists primarily in either the Cr(III) or Cr(VI) forms. Cr(III), an ingredient of many vitamin and nutritional supplements, forms compounds that are not very soluble in water, and it has low toxicity. Cr(VI), on the other hand, is much more toxic and forms compounds that are reasonably soluble in water. Exposure to small amounts of Cr(VI) can lead to damage of the respiratory, gastrointestinal, and immune systems, as well as the kidneys, liver, blood, and skin.
Despite cleanup efforts, Cr(VI) groundwater contamination remains a problem in Hinckley and other locations across the globe. A 2010 study by the Environmental Working Group found that of 35 US cities tested, 31 had higher levels of Cr(VI) in their tap water than the public health goal of 0.02 parts per billion set by the California Environmental Protection Agency.
Distinguishing Ionic and Molecular Compounds
Since the systems for naming molecular compounds and ionic compounds are very different, it is important to know how to distinguish these two types of compounds. M olecular compounds (covalent compounds) result when atoms share, rather than transfer (gain or lose), electrons. Covalent bonding is an important and extensive concept in chemistry, and it will be treated in considerable detail in a later chapter of this text. We can often identify molecular compounds on the basis of their physical properties. Under normal conditions, molecular compounds often exist as gases, low-boiling liquids, and low-melting solids, although many important exceptions exist. Covalent compounds are usually formed by a combination of nonmetals
In contrast, ionic compounds are usually formed when a metal and a nonmetal combine. As a result, the periodic table helps distinguish these two types of compounds. While we can use the positions of a compound’s elements in the periodic table to predict whether it is ionic or covalent at this point in our study of chemistry, you should be aware that this is a very simplistic approach that does not account for a number of interesting exceptions. Shades of gray exist between ionic and molecular compounds, and you’ll learn more about those later.
Example \(\PageIndex{5}\): Predicting the Type of Bonding in Compounds
Predict whether the following compounds are ionic or molecular:
- KI, the compound used as a source of iodine in table salt
- H 2 O 2 , the bleach and disinfectant hydrogen peroxide
- CHCl 3 , the anesthetic chloroform
- Li 2 CO 3 , a source of lithium in antidepressants
Solution
- Potassium (group 1) is a metal, and iodine (group 17) is a nonmetal; KI is predicted to be ionic.
- Hydrogen (group 1) is a nonmetal, and oxygen (group 16) is a nonmetal; H 2 O 2 is predicted to be molecular.
- Carbon (group 14) is a nonmetal, hydrogen (group 1) is a nonmetal, and chlorine (group 17) is a nonmetal; CHCl 3 is predicted to be molecular.
- Lithium (group 1) is a metal, and carbonate is a polyatomic ion; Li 2 CO 3 is predicted to be ionic.
Exercise \(\PageIndex{5}\)
Using the periodic table, predict whether the following compounds are ionic or molecular:
- SO 2
- CaF 2
- N 2 H 4
- Al 2 (SO 4 ) 3
- Answer a
-
molecular
- Answer b
-
ionic
- Answer c
-
molecular
- Answer d
-
ionic
Naming Acids
Acids are another important class of compounds. They share characteristics with both molecular and ionic compounds but they are named using a separate system of chemical nomenclature. If you are familiar with anions - both atomic and polyatomic - it is easy to identify when you need to name something as an acid because the cation is hydrogen. These compounds act as acids by releasing hydrogen ions, H + , when dissolved in water.To denote this distinct chemical property, a mixture of water with an acid is given a name derived from the compound’s name. The chemistry of acids is discussed in much greater detail in later chapters, but for now it is important that you know how to name these compounds.
Naming Binary Acids
If the compound is a binary acid (comprised of hydrogen and one other nonmetallic element):
- The word “hydrogen” is changed to the prefix hydro-
- The other nonmetallic element name is modified by adding the suffix - ic
- The word “acid” is added as a second word
For example, when the gas \(\ce{HCl}\) (hydrogen chloride) is dissolved in water, the solution is called hydrochloric acid . Several other examples of this nomenclature are shown in Table \(\PageIndex{5}\).
| Name of Gas | Name of Acid |
|---|---|
| HF( g ), hydrogen fluoride | HF( aq ), hydrofluoric acid |
| HCl( g ), hydrogen chloride | HCl( aq ), hydrochloric acid |
| H 2 S( g ), hydrogen sulfide | H 2 S( aq ), hydrosulfuric acid |
Naming Oxyacids
O xyacids are compounds that contain hydrogen and a polyatomic ion with oxygen. These compounds and are bonded in such a way as to impart acidic properties to the compound (you will learn the details of this in a later chapter). To name oxyacids:
- Omit “hydrogen”
- Start with the root name of the anion
- Replace – ate with – ic , or – ite with – ous
- Add “acid”
For example, consider H 2 CO 3 (which you might be tempted to call “hydrogen carbonate”). To name this correctly, “hydrogen” is omitted; the – ate of carbonate is replace with – ic ; and acid is added—so its name is carbonic acid. Other examples are given in Table \(\PageIndex{6}\). There are some exceptions to the general naming method (e.g., H 2 SO 4 is called sulfuric acid, not sulfic acid, and H 2 SO 3 is sulfurous, not sulfous, acid).
| Formula | Anion Name | Acid Name |
|---|---|---|
| HC 2 H 3 O 2 | acetate | acetic acid |
| HNO 3 | nitrate | nitric acid |
| HNO 2 | nitrite | nitrous acid |
| H 2 SO 4 | sulfate | sulfuric acid |
| H 2 SO 3 | sulfite | sulfurous acid |
| H 3 PO 4 | phosphate | phosphoric acid |
Summary
A molecular formula uses chemical symbols and subscripts to indicate the exact numbers of different atoms in a molecule or compound. An empirical formula gives the simplest, whole-number ratio of atoms in a compound. A structural formula indicates the bonding arrangement of the atoms in the molecule. Ball-and-stick and space-filling models show the geometric arrangement of atoms in a molecule. Isomers are compounds with the same molecular formula but different arrangements of atoms.
Metals (particularly those in groups 1 and 2) tend to lose the number of electrons that would leave them with the same number of electrons as in the preceding noble gas in the periodic table. By this means, a positively charged ion is formed. Similarly, nonmetals (especially those in groups 16 and 17, and, to a lesser extent, those in Group 15) can gain the number of electrons needed to provide atoms with the same number of electrons as in the next noble gas in the periodic table. Thus, nonmetals tend to form negative ions. Positively charged ions are called cations, and negatively charged ions are called anions. Ions can be either monatomic (containing only one atom) or polyatomic (containing more than one atom).
Compounds that contain ions are called ionic compounds. Ionic compounds generally form from metals and nonmetals. Compounds that do not contain ions, but instead consist of atoms bonded tightly together in molecules (uncharged groups of atoms that behave as a single unit), are called covalent compounds. Covalent compounds usually form from two or more nonmetals.
Chemists use nomenclature rules to clearly name compounds. Ionic and molecular compounds are named using somewhat-different methods. Binary ionic compounds typically consist of a metal and a nonmetal. The name of the metal is written first, followed by the name of the nonmetal with its ending changed to – ide . For example, K 2 O is called potassium oxide. If the metal can form ions with different charges, a Roman numeral in parentheses follows the name of the metal to specify its charge. Thus, FeCl 2 is iron(II) chloride and FeCl 3 is iron(III) chloride. Some compounds contain polyatomic ions; the names of common polyatomic ions should be memorized. Molecular compounds can form compounds with different ratios of their elements, so prefixes are used to specify the numbers of atoms of each element in a molecule of the compound. Examples include SF 6 , sulfur hexafluoride, and N 2 O 4 , dinitrogen tetroxide. Acids are an important class of compounds containing hydrogen and having special nomenclature rules. Binary acids are named using the prefix hydro- , changing the – ide suffix to – ic , and adding “acid;” HCl is hydrochloric acid. Oxyacids are named by changing the ending of the anion ( -ate to – ic , and -ite to -ous ) and adding “acid;” H 2 CO 3 is carbonic acid.
Glossary
- binary acid
- compound that contains hydrogen and one other element, bonded in a way that imparts acidic properties to the compound (ability to release H + ions when dissolved in water)
- binary compound
- compound containing two different elements.
- covalent bond
- attractive force between the nuclei of a molecule’s atoms and pairs of electrons between the atoms
- covalent compound
- (also, molecular compound) composed of molecules formed by atoms of two or more different elements
- empirical formula
- formula showing the composition of a compound given as the simplest whole-number ratio of atoms
- isomers
- compounds with the same chemical formula but different structures
- ionic bond
- electrostatic forces of attraction between the oppositely charged ions of an ionic compound
- ionic compound
- compound composed of cations and anions combined in ratios, yielding an electrically neutral substance
- molecular compound
- (also, covalent compound) composed of molecules formed by atoms of two or more different elements
- molecular formula
- formula indicating the composition of a molecule of a compound and giving the actual number of atoms of each element in a molecule of the compound.
- monatomic ion
- ion composed of a single atom
- nomenclature
- system of rules for naming objects of interest
- oxyacid
- compound that contains hydrogen, oxygen, and one other element, bonded in a way that imparts acidic properties to the compound (ability to release H + ions when dissolved in water)
- oxyanion
- polyatomic anion composed of a central atom bonded to oxygen atoms
- polyatomic ion
- ion composed of more than one atom
- spatial isomers
- compounds in which the relative orientations of the atoms in space differ
- structural isomer
- one of two substances that have the same molecular formula but different physical and chemical properties because their atoms are bonded differently
- structural formula
- shows the atoms in a molecule and how they are connected
Contributors and Attributions
-
Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at<EMAIL_ADDRESS>).
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Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at<EMAIL_ADDRESS>).
-
Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at<EMAIL_ADDRESS>).


12.8: Partial Pressure and Dalton's Law
- Explain Dalton’s law of partial pressure and apply the partial pressure concept in respiration.
Dalton's law of Partial Pressures
Dalton's law applies to a mixture of two or more gases. This law states that in a mixture of gases, the total pressure is the sum of the partial pressures of all the components. If there are three different gases in a container then the law is given by the following mathematical equation.
P total = P 1 + P 2 + P 3
The partial pressure of a gas is the pressure that gas would exert if it occupied the container by itself. Partial pressure is represented by the letter P .
Figure 12.8.1 below demonstrates the concept of partial pressure in more concrete terms, showing the pressure of each gas alone in a container and then showing the gases combined pressure once mixed.
Figure \(\PageIndex{1}\): Alone, Hydrogen has a pressure of 0.4 atm, while Nitrogen has a pressure of 0.6 atm. When placed in the same container, the total pressure is 1 atm, with each gas contributing its partial pressure. Image credits: OpenStax College
Air is a mixture of 78.0 % nitrogen, 21.1 % oxygen, and 0.9 % of other gases such as argon, helium, carbon dioxide, water vapors, methane, and hydrogen. The atmospheric pressure is the sum of the partial pressures of components of air. At sea level the atmospheric pressure is 760 torr.

Mount Everest: P total = P atm = 200 torr = P N2 + P O2 + P others = 156 torr + 42.2. torr + 2 torr
Therefore as altitude increases, and atmospheric pressure decreases, the partial pressure of oxygen decreases from 160 torr at sea level to 42.2 torr at Mount Everest. So climbers have to carry oxygen supplies. Otherwise climbers face the risk of Acute Mountain Sickness . Some of the symptoms are headaches, nausea, and shortness of breath.
Some runners take advantage of high altitude training. Why? P O2 is low at high altitude and so training at high altitudes increases aerobic capacity. The person tends to make more red blood cells to increase the intake of oxygen.
The concept of partial pressures also explains why some people with respiratory problems are uncomfortable in low pressure air that is very moist. When the atmospheric pressure is low and the air is moist the P water vapor is high and so the P O2 is low.
Basic Principles of Gas Exchange during Respiration
Gas exchange during respiration occurs primarily through diffusion. Diffusion is a process in which transport is driven by a concentration gradient. Gas molecules move from a region of high concentration to a region of low concentration. Blood that is low in oxygen concentration and high in carbon dioxide concentration undergoes gas exchange with air in the lungs. The air in the lungs has a higher concentration of oxygen and lower in concentration of carbon dioxide than that of oxygen-depleted blood. This concentration gradient allows for gas exchange during respiration.
Partial pressure is a measure of the concentration of the individual components in a mixture of gases. The partial pressure of a gas above a liquid is directly related to the amount of gas dissolved in the liquid. The rate of diffusion of a gas is proportional to its partial pressure within the total gas mixture.
Partial pressure of oxygen in ambient air is 160 mm Hg. In the arterial blood, the P O2 = 100 mm Hg. The venous blood has a P O2 = 40 mm Hg. The P O2 in the remote tissues is less than 40 torr. This pressure gradient drives the oxygen into the tissue cells via diffusion. See figure 12.8.2 for visual connection.
At the same time, the venous blood has a P CO2 = 46 mm Hg. The arterial blood has a P CO2 = 40 mm Hg and the ambient air has a P CO2 = 0.2 mm Hg A net movement of CO 2 takes place. The pressure gradient drives CO 2 out of tissue cells. CO 2 (g) is a bi-product of metabolism.
Figure \(\PageIndex{2}\): The partial pressures of oxygen and carbon dioxide change as blood moves through the body.
In short, the changes in partial pressure of gases drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in partial pressures result in the movement of carbon dioxide out of the blood into the lungs, and oxygen into the blood.
Summary
The partial pressure of a gas is the pressure that gas would exert if it occupied the container by itself. Dalton's law applies to a mixture of two or more gases. This law states that in a mixture of gases, the total pressure is the sum of the partial pressures of all the components. Gas exchange during respiration occurs primarily through diffusion. Gas molecules move from a region of high concentration to a region of low concentration.


10.2: Music as Political Advocacy
Campaign Songs
The use of music to rally support for a political cause has a long history. Music has been used around the world to display the power of monarchies, to urge soldiers into battle, and to inspire patriotic support for all manner of governments. Here, we will focus on the role of music in American politics, with special attention given to specific songs that have been used by candidates seeking election to the presidency. Although we are focusing on campaign songs, those are not the only musical tools that have been employed by American political candidates. Before the rise of mass media, campaigning was conducted in person at events such as rallies, parades, and whistle stops, often to the accompaniment of a local brass band playing popular and patriotic tunes. Today, we are more likely to hear campaign music via television, whether we are watching a televised campaign event or a paid political advertisement. At events, candidates are likely to use popular songs that convey a relevant message, set the right mood, or speak to a desired voting base. In advertisements, we are more likely to hear underscoring similar to that used in television shows and movies. Such music attracts less attention than a popular song, but it plays an equally important role in communicating the candidate’s message.
In this 2016 campaign ad for Georgia gubernatorial candidate Casey Cagle, the music is bouncy and cheerful.
To see underscoring at work, we might examine two recent television advertisements from a 2016 Republican candidate for governor of Georgia, Casey Cagle. In a spot from early in the campaign, titled “Difference,”1 Casey touts his humble beginnings, strong personal ethic, and political accomplishments. The music behind his voiceover is bouncy and cheerful. The major mode tells us that this is a message of optimism, while the mid-tempo drumbeat and repetitive, staccato melody notes are reassuring and uplifting. Later in the campaign, however, Cagle— in response to a strong showing by his rival—attempted to construct a new public persona for himself, one that emphasized strength and determination. These qualities are reflected in the television advertisement “About,”2 which features music that would be equally at home in an action film. The minor mode and suspenseful harmonies keep the listener on edge, while periodic percussive strikes suggest danger and unrest. As musicologist Naomi Graber has observed, the music builds to a climax in parallel with Cagle’s rousing speech, suggesting that Cagle himself is the superhero in this narrative.
In this ad from later in Cagle’s campaign, the music is suspenseful.
Candidates are sometimes musicians themselves and are therefore able to exploit their own musical skills to win the sympathy and support of the public. In 1886, for example, two brothers—Alf and Bob Taylor—ran against one another in a bid to become governor of Tennessee. Both men were excellent fiddlers, and they not only performed frequently throughout the campaign but also used their love of folk music to establish a down-home identity and connect with rural voters. More than a century later, President Bill Clinton advertised himself as fun-loving and cool by playing saxophone on The Arsenio Hall Show , while Democratic presidential candidate Martin O’Malley garnered attention by playing guitar and singing Taylor Swift’s “Bad Blood” on The View . Although the genres (and skill levels) of these candidate-performers varied widely, all used music to paint themselves as good-humored and relatable.
When it comes to conveying a specific political message through music, song is clearly the best choice, and it has been in use since the beginning of American electoral politics. In the earliest years, election songs were usually parodies . As early as George Washington’s reelection campaign in 1792, supporters advocated their candidates’ causes by writing new words to recent popular songs. This technique could be very effective. Because the melody was already familiar, parody lyrics were quickly learned, sung, and spread. The success of a popular song could even contribute to the success of the campaign that chose to parody that song. In 1840, William Harrison’s campaign song “Tippecanoe and Tyler Too,” which parodied the familiar “Three Little Pigs,” became so popular that it was widely credited with winning him the presidency.
There is also a long history of presidential candidates relying on endorsements by celebrity musicians to boost their popularity. The earliest example of this came in 1860, when the famous abolitionist singer Jesse Hutchinson wrote a parody song in support of Abraham Lincoln. The song—”Lincoln and Liberty, Too,” set to the tune of “Rosin the Beau”—was a hit, but Hutchinson’s popularity probably contributed just as much to Lincoln’s victory.
New songs have also been written in support of presidential candidates. While this practice dates to the early 20th century, it is particularly widespread today, when anyone can write a song, record a performance, and upload their contribution to social media. Technology has always influenced the role of music in presidential elections. Beginning in the 1930s, the prevalence of radio (and later television) led campaigns to employ songs that were already popular—an approach that proved more effective than the creation of a new song that might or might not prove a hit. In this chapter, we will consider three such songs. These songs are all quite different in style and message, but they share one important thing in common: None of them were written with any political end in mind. In each case, the presidential campaign coopted the song because it supported the campaign’s political message.
Milton Ager and Jack Yellen, “Happy Days Are Here Again”
The first presidential candidate to coopt a popular song for his campaign was Franklin Delano Roosevelt, who in 1932 adopted the recent hit song “Happy Days Are Here Again”3 to represent his political agenda. Although the song was not created for his campaign and did not communicate an explicitly political message, Roosevelt found that it perfectly captured the spirit of his presidential bid.
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“Happy Days Are Here Again” 3. Composers: Milton Ager and Jack Yellen Performance: Annette Hanshaw (1930) |
It is no surprise that Roosevelt should have been the first presidential candidate to use a popular song in his campaign. The 1920s and 1930s were a time of extraordinary growth in the music recording and broadcast industries, with the result that popular songs were more influential than ever before. The first commercial license was issued to a radio station in 1921, while 1925 saw the invention of the electrical microphone and an accompanying improvement in the quality of recorded and broadcast sound. Due in part to the economic downturn of 1929, radio soon eclipsed the phonograph in popularity. The most successful programming in that era consisted of live musical performances. Roosevelt was the first presidential candidate (and later president) to take full advantage of the radio as a platform for connecting with Americans and spreading his message.
The decision to play “Happy Days Are Here Again” at the 1932 Democratic National Convention was made spontaneously after the man who introduced Roosevelt gave a lifeless and dour speech. Roosevelt’s campaign managers had originally intended to brand him with “Anchors Aweigh” (a nod to his Navy service), but they felt that the occasion called for something more lively and selected “Happy Days Are Here Again” at the last moment. The song proved effective and was used for the remainder of the campaign. In fact, it was such a success that “Happy Days Are Here Again” was associated with the Democratic Party for many years to come.
In 1932, a song like “Happy Days Are Here Again” would have had particular resonance with Americans, who were experiencing the worst effects of the Great Depression. The 1930s saw a public clamor for escapist films, theater, and songs. Almost no popular songs from this era reference the difficulties that plagued Americans following the stock market crash of October 1929. Instead, they celebrate good times, affluence, and carefree living.
The authors of “Happy Days Are Here Again”— composer Milton Ager and lyricist Jack Yellen— were themselves the beneficiaries of fortunate timing, for they wrote their song just before the onset of the Great Depression. “Happy Days” was commissioned by MGM for inclusion in an early sound film, Chasing Rainbows (1930). While waiting for the film to be released, however, Ager and Yellen provided their new song to bandleader
George Olsen, who performed regularly at Hotel Pennsylvania in New York City. Olsen’s band premiered “Happy Days” on October 24, 1929—the day of the crash. Olsen supposedly called on his soloists to “sing it for the corpses,” a reference to the morose diners who had just lost their life savings. As Yellen recalled, “After a couple of choruses, the corpses joined in, sardonically, hysterically. Before the night
was over, the hotel lobby resounded with what had become the theme song of ruined stock speculators as they leaped from hotel windows.” “Happy Days Are Here Again” went on to become a major hit. It appealed to listeners who reveled in the irony of the lyrics (“happy days” were certainly not anywhere to be seen), but it also embodied the American spirit of optimism and perseverance in the face of extreme difficulty. The lyrics—like those of most successful songs—are not about any particular event or situation. Instead, they speak in broad terms, bidding farewell to “sad times” and “bad times” before celebrating the fact that “the skies above are clear again” and calling on all to “tell the world” that “happy days are here again.”
“Happy Days Are Here Again” was a project of a songwriting industry known as Tin Pan Alley . The main product of Tin Pan Alley song publishers was sheet music, which consumers would purchase so as to be able to perform hit songs in their own homes. By 1929, sheet music sales had already been eclipsed by phonograph sales, and radio was further eating into the popular music market share. Still, songwriting teams, usually in the employment of Tin Pan Alley publishers, continued to crank out potential hits into the 1940s. (The name “Tin Pan Alley” refers to the discordant sounds of cheap pianos being played in the many publishing houses located on a single block in Manhattan.)
During the Tin Pan Alley era, songs were not often associated with individual performers. Instead, hit songs were recorded by all of the prominent singers, each of whom was expected to perform a common repertoire. “Happy Days Are Here Again” was first recorded in November 1929 by Leo Reisman and His Orchestra, but this was only the first in a parade of renditions by a variety of popular singers. We will consider the 1930 recording by Ben Selvin and His Orchestra, with Annette Hanshaw on vocals.
Ben Selvin (1898-1980) holds the honor as the most productive recording artist in American history, having recorded as many as 20,000 individual tunes. It is difficult to make an exact count because Selvin recorded under dozens of different names for all of the major record
labels of his era. This was a typical practice, since performing artists were required to sign exclusive contracts with recording companies. The use of pseudonyms allowed an artist to triple or quadruple their income by recording with a variety of labels. As a producer at Columbia Records, Selvin also oversaw the creation of recordings by other artists. His catalog includes some of the finest records made in the early 1930s.
Annette Hanshaw (1901-1985) was among the countless singers who performed and recorded with Selvin’s band. She was also very famous in her own right, being voted the best female popular singer in a 1934 Radio Stars magazine poll. Hanshaw, however, later confessed that she detested performing and could not stand her own records. She loved music but became very nervous in front of a microphone and was dissatisfied with her abilities as a singer.
Selvin and Hanshaw’s recording of “Happy Days Are Here Again” is typical of 1920s-era sweet jazz . This type of music was intended primarily for dancing, and it could be heard in upscale urban hotels, ballrooms, and on the radio. Selvin’s band contains instruments that we associate with jazz today, such as saxophones, trumpets, trombones, and piano, but—in keeping with the standard of the era—it also contains a violin section, and the bass line is played on the tuba instead of the string bass. The tempo is lively and the beat is clearly articulated by the rhythm section (piano and tuba).
Ager and Yellen’s jazz-influenced tune is highly syncopated , meaning that melody notes often fall between beats. We can hear this in the chorus: The word “happy” falls on the beat, but the words “days” and “are” come after the beat. (Try clapping the pulse while reciting the lyrics in rhythm to see how this works.) This pattern is repeated with the next phrase, in which “here” falls on the beat but the second syllable of “again” come off the beat. Selvin’s band makes this pattern sound even more exciting by shortening the melody notes that fall on the beat.
The form of this recording is also typical of the era. Although the song “Happy Days Are Here Again” technically begins with a verse , Selvin chooses to open his rendition with the much catchier chorus . After singing through the chorus once, Hanshaw sings the verse, which is much shorter than the chorus and also in the minor mode. This provides welcome contrast, but the change in mood doesn’t last long. Next, we hear a rendition of the chorus by an anonymous trio of male singers. Finally, we get an instrumental version, with the male singers joining in for the closing phrase. In short, most of the recording is taken up by three complete turns through the chorus—which, by the end of the three minutes, is firmly lodged in the listener’s head.
This approach to the production of popular music—extreme repetition, softened by a brief excursion into contrasting material—is still with us today. Although the forms, sounds, and lyrics have all changed, record producers continue to foreground the most memorable musical material while finding opportunities to introduce contrast. This delicate balance between too much repetition (which would make a song annoying) and too little (which would prevent it from being memorable) characterizes the eternal formula for successful popular music.
Paul Simon, “Bridge Over Troubled Water”
Successful campaign songs always combine a relevant text with music that sets an appropriate emotional tone. It is easy to see how a song like “Happy Days Are Here Again” could serve this purpose well: It is bright and cheerful, and exhibits unchecked optimism about the future. The 1970 Simon & Garfunkel hit “Bridge Over Troubled Water”4 is a less obvious choice for a political campaign. While the song’s lyrics touch on themes that a candidate might want to emphasize, the music lacks the energy of a typical campaign song. We will begin, therefore, by examining the song itself. Then we will consider why George McGovern, the Democratic challenger to President Richard Nixon, might have selected it for his 1972 campaign.
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“Bridge Over Troubled Water” 4. Composer: Paul Simon Performance: Simon and Garfunkel (1970) |
In 1969, Paul Simon and Art Garfunkel were two of the biggest stars in American popular music. Their recent albums Sounds of Silence (1966) and Parsley, Sage, Rosemary and Thyme (1966) had both proved major hits, and they were at the forefront of the folk rock genre. Although both men sang and made contributions to the duo’s arrangements , Simon was the lead songwriter—a fact that caused some friction, as Simon increasingly felt that Garfunkel was not making an equal contribution to the partnership. The duo broke up in 1970 just as their fourth album, Bridge over Troubled Water , was rapidly climbing the charts.
Simon wrote “Bridge over Troubled Water” very quickly in 1969. He later recounted that the basic idea took him about twenty minutes to work out, while the whole song was finished in under two hours. The title lyric was inspired by a line from the 1958 Swan Silvertones song “Mary Don’t You Weep,” in which Claude Jeter sings, “I’ll be your bridge over deep water if you trust in me.” Simon’s source ended up shaping not only the lyrics but the sound of his new song. The Swan Silvertones were an African American gospel group, and their music drew from the rich tradition of black church music. Although Simon and Garfunkel did not attempt a wholesale imitation of this style, they wanted their recording to reflect the influence of gospel (a style discussed in Chapter 11).
“Bridge over Troubled Water” contains three verses, the last of which was added at the request of Garfunkel to support the song’s dramatic musical climax. Simon wrote the verse, but never cared for it—his message was communicated in its entirety in the first two verses. These vow to the listener that, although they may be “weary, feeling small,” “down and out,” or “on the street,” the singer will always “take your part” and “lay me down / Like a bridge over troubled water.” In short, the song promises solace and support for anyone who is experiencing difficulties in life.
Although Simon wrote the song using his favored instrument, the guitar, he chose to make the recording with a piano played in the gospel style. The instrumental introduction, with its grandiose chords and syncopated rhythms, could indeed preface the entrance of a gospel singer. Instead, we hear the voice of Garfunkel, which Simon thought was better suited to the tune than his own. The first verse is accompanied only by piano. The second verse is additionally supported by vibraphone, which adds warmth and resonance without changing the character of the song.
At the conclusion of the second verse, however, the music takes a new direction. First, a drum set crashes onto the soundscape. Strings enter next, followed by an electric bass. The full instrumental forces grow in energy throughout the third and final verse, which also features Simon singing a harmony part. In the final seconds of the song, the tempo slows as the strings ascend to their highest note, producing a triumphant conclusion.
Despite the drama that Simon and Garfunkel were able to produce in their studio recording of “Bridge over Troubled Water,” the song itself is not particularly energetic. It features a slow tempo, a sustained vocal line, and lyrics that encourage the listener to feel calm and resolute. It’s not a song one would play to rile up a crowd or to encourage enthusiastic support for a cause.
All the same, George McGovern chose “Bridge over Troubled Water” as the theme song for his 1972 presidential bid, and he played it at his rallies throughout the campaign. The song certainly embodied many of McGovern’s values. He ran on a platform of national unification and healing, advertising his ability to reach across the aisle and bring opposing factions together. The preceding years had indeed been characterized by devastating internal conflict, including the 1968 assassination of Martin Luther King, Jr., the 1969 Stonewall riots, and growing unrest concerning the war in Vietnam—unrest that culminated in the 1970 Kent State massacre in which National Guardsmen killed four student protesters. McGovern positioned himself as a staunchly anti-war candidate who would bring peace to the nation. For this reason, “Bridge over Troubled Water” was uniquely suited to his campaign. At one point, McGovern even quoted the song directly, stating, “I want, indeed, to become a bridge over troubled waters.”
McGovern was able to use “Bridge over Troubled Water” in another way as well. He invited Simon and Garfunkel to participate in a benefit concert that took place in Madison Square Garden on June 14, 1972. (Simon actively supported McGovern’s candidacy; Garfunkel was less enthusiastic but eager to perform before a large crowd.) McGovern naturally benefitted from having star performers take the stage, just as he profited from associating himself with a song that was heard constantly on the radio. However, McGovern was also able to polish his image by bringing the sundered duo back together for the first time since their partnership had ended. He, after all, was the candidate of reconciliation and peace making. McGovern’s ability to restore the Simon and Garfunkel partnership therefore attested to his potential skills as a national leader.
“Bridge over Troubled Water,” therefore, was perhaps not as bizarre a choice for a campaign song as one might think. Unfortunately for McGovern, the song does not seem to have done him any good: He was soundly defeated in the 1972 election by incumbent Richard Nixon, who won over 60% of the vote and took every state except Massachusetts and the District of Columbia.
Tom Petty and Jeff Lynne, “I Won’t Back Down”
In contrast with Simon and Garfunkel’s soulful ballad, Tom Petty’s 1989 hit “I Won’t Back Down”5 is an obvious choice for a political candidate. In fact, it is so well-suited to the campaign trail that half a dozen candidates have employed it in just the first two decades of the 21st century. It was also used to voice a non- partisan political message following the September 11 terrorist attacks. At the same time, at least one candidate’s attempt to coopt the message and popularity of “I Won’t Back Down” backfired. We will try to understand why this song has been so useful in the context of political discourse and consider the risks of coopting the creative work of a celebrity for partisan purposes—especially when that celebrity is politically active.
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“I Won’t Back Down” 5. Composer: Tom Petty and Jeff Lynne Performance: Tom Petty and the Heartbreakers (1989) |
By the time he released “I Won’t Back Down,” Tom Petty (1950-2017) was already a rock music superstar. Petty was born and raised in Gainesville, FL, where he dropped out of high school to join a band. He was inspired by Elvis Presley and The Beatles, whose success proved to him that it was possible for a regular, working-class kid to make it in music. Although Petty only achieved local celebrity in his first decade as a rock musician, the late 1970s saw the meteoric rise of his band Tom Petty and the Heartbreakers, which recorded a string of successful albums between 1976 and 1987. Although the Heartbreakers would reconvene in the early 1990s and continue to release albums until just a few years before Petty’s 2017 death, Petty took a hiatus in 1988 to join George Harrison’s group the Traveling Wilburys.
During this same period, Petty also released his first solo album, Full Moon Fever . The lead single from this album was “I Won’t Back Down.” Half of the songs on Full Moon Fever were cowritten by Tom Petty and Jeff Lynne, an English musician and founding member of the Traveling Wilburys. It is common for popular songwriters to work in pairs. This strategy has proved successful since the early 20th century, when Tin Pan Alley songwriters teamed up to produce hits. Many songwriters find that this approach aids productivity, since the contributors can help one another out of creative difficulties and suggest improvements to the music or lyrics. Lynne also produced the album and is, therefore, in part responsible for the sound of “I Won’t Back Down.”
The success of “I Won’t Back Down”—both as a popular song and as a political soundtrack—can be traced to its straightforward message. Like “Happy Days Are Here Again,” “I Won’t Back Down” does not refer to any particular event or circumstances. We don’t know what the singer is standing up for, or against whom he is resisting, or why he is in a position that requires he stand his ground. All we know is that, despite “a world that keeps on pushin’ me around” and “draggin’ me down,” the singer “won’t back down.”
The music admirably communicates this message of steadfast endurance. “I Won’t Back Down” is set to a medium rock tempo—not so fast as to feel frantic but not so slow as to feel lethargic. The music sounds as if it—like the singer— could carry on this way forever. The sung melody has a protracted melodic rhythm, meaning that the individual note values are long and unhurried. Petty’s unaffected drawl further convinces us that he is not to be moved, while the relaxed guitar solo underscores the point. “I Won’t Back Down” is, naturally enough, in the major mode, but the accompaniment prominently features a minor chord, heard under the word “won’t” in the title line. This tinges the lyric with seriousness of purpose. Although the singer is not overly concerned, he is not taking the threat lightly.
“I Won’t Back Down” first made political headlines in 2000, when George W. Bush used it at events during his presidential campaign. The song seemed an excellent choice, but it became a liability when Petty protested, sending the campaign a cease-and-desist letter and demanding that they quit using his music. Bush’s campaign staff might have known better: Petty had already indicated a penchant for political speech as a musician and had aligned himself with various activist causes. In 1979, Tom Petty and the Heartbreakers performed in a Madison Square Garden concert intended to raise support for the dismantling of nuclear power plants, while in 1985 they took to the stage for the famous Live Aid concert in Philadelphia, which raised funds to combat famine in Ethiopia. And in 1992, Petty had written the song “Peace in L.A.” in response to the Rodney King riots, donating all proceeds to charity. Although none of these were explicitly left-wing causes, Petty was certainly aware of his power as a political agent, and he knew that music could make a difference in a political struggle. After the 2000 election, Petty performed “I Won’t Back Down” at Al Gore’s concession speech, thereby clearly sending the message that he intended to retain control over how his music was used.
Petty, however, did not always interfere when politicians coopted his music. During the 2008 presidential campaign, for example, “I Won’t Back Down” was used by Democratic candidates Hillary Clinton and John Edwards, but also by Republican candidate Ron Paul. In 2012, he again made no comment when “I Won’t Back Down” was played at the Democratic National Convention. That same year, however, he issued another cease-and-desist letter to Representative Michele Bachmann, who used his song “American Girl” to launch her presidential campaign. Petty, of course, was not the first rock star to object to the coopting of his hits—and also not the last. All modern presidential campaigns are accompanied by a chorus of popular recording artists asking that candidates stop using their music. It’s not clear why Petty accepted some uses of his songs and rejected others, but it is entirely clear that he understood the power of his music and celebrity.


9.6: §66. Latin Verbs of the Third I-STEM and Fourth Conjugations
In a course of this kind, where the sole objective is gaining insight into English words, it may not be crucially important to remember Latin verbs by conjugation number; that knowledge is admittedly of less practical value than remembering the declension groups of Latin nouns. However, when we come to look at other Latin verb forms, such as present participles and gerundives, you will probably find it helpful to be able to associate Latin verb vocabulary with these numbered categories.
The Latin 4th conjugation always has an infinitive in – i̅ re , like audí re or vení re . This easily recognized form, therefore, makes it parallel to the 1st in -ā re and the 2nd in – ē re. Once again, unhappily, there is no predictable perfect participle. What is most noticeable about the fourth conjugation is the persistence of that vowel -i- in many of its forms—in audi̠ o (“I hear”), for instance, and the English words aud i̠ enc e , sal i̠ ent , or conven i̠ ence .
There is a small but important subtype of 3rd conjugation verbs that can be described as having an “i-stem,” because they also show that same persistent vowel. [1] To judge them by the evidence of their English derivatives, they appear more closely associated with the 4th conjugation than with the 3rd. For this reason, they are included on Table 9.4 with the 4th conjugation type. They include capere and facere , which may be the most productive of all Latin verbs, from the standpoint of English vocabulary. The “i-stem” deponent verbs gradi and pati are also very important; it may help to remember them with words like gradient, aggression and patient, passion .
| Third Conjugation, I-stem | Third I-stem deponent verbs: | ||
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capere, captus [-cipere, -ceptus] facere, factus [-ficere, -fectus] jacere, jactus [-jicere, -jectus] -spicere, -spectus |
take make, do throw look |
gradi, gressus pati, passus Fourth Conjugation: audire, auditus salire, saltus sentire, sensus venire, ventus |
step, go suffer, allow hear leap, jump feel come |
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If we were learning all four principal parts, the contrast between the regular 3rd conjugation verb and this special subtype would be more apparent. Compare the first principal part of
ago, agere, egi, actus
with that of
capio, capere, cepi, captus,
or
facio, facere, feci, factus
. ↵


11.6: Hydration of Alkenes - Addition of H₂O by Hydroboration
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Objectives
After completing this section, you should be able to
- identify hydroboration (followed by oxidation) as a method for bringing about the (apparently) non-Markovnikov addition of water to an alkene.
- write an equation for the formation of a trialkylborane from an alkene and borane.
- write an equation for the oxidation of a trialkylborane to an alcohol.
- draw the structure of the alcohol produced by the hydroboration, and subsequent oxidation, of a given alkene.
- determine whether a given alcohol should be prepared by oxymercuration-demercuration or by hydroboration-oxidation, and identify the alkene and reagents required to carry out such a synthesis.
- write the detailed mechanism for the addition of borane to an alkene, and explain the stereochemistry and regiochemistry of the reaction.
Key Terms
Make certain that you can define, and use in context, the key term below.
- hydroboration
Study Notes
The two most important factors influencing organic reactions are polar (or electronic) effects and steric effects.
Hydroboration-oxidation is a two step pathway used to produce alcohols. The reaction proceeds in an anti-Markovnikov manner, where the hydrogen (from BH 3 or BHR 2 ) attaches to the more substituted carbon and the boron attaches to the least substituted carbon in the alkene double bond. Furthermore, the borane acts as a Lewis acid by accepting two electrons in its empty p orbital from an alkene that is electron rich. This process allows boron to have an electron octet. A very interesting characteristic of this process is that it does not require any activation by a catalyst. The hydroboration mechanism has the elements of both hydrogenation and electrophilic addition and it is a stereospecific ( syn addition), meaning that the hydroboration takes place on the same face of the double bond, this leads cis stereochemistry.
Introduction
Hydroboration-oxidation of alkenes has been a very valuable laboratory method for the stereoselectivity and regioselectivity of alkenes. An additional feature of this reaction is that it occurs without rearrangement.
First off it is very important to understand little bit about the structure and the properties of the borane molecule. Borane exists naturally as a very toxic gas and it exists as dimer of the general formula B 2 H 6 (diborane). Additionally, the dimer B 2 H 6 ignites spontaneously in air. Borane is commercially available in ether and tetrahydrofuran (THF), in these solutions the borane can exist as a Lewis acid-base complex, which allows boron to have an electron octet. B H 3 → B 2 H 6
The Mechanism
Step 1
- Part 1: Hydroboration of the alkene. In this first step the addition of the borane to the alkene is initiated and proceeds as a concerted reaction because bond breaking and bond formation occurs at the same time. This part consists of the vacant 2p orbital of the boron electrophile pairing with the electron pair of the pi bond of the nucleophile.
Transition state
* Note that a carbocation is not formed. Therefore, no rearrangement takes place .
- Part 2: The Anti Markovnikov addition of Boron. The boron adds to the less substituted carbon of the alkene, which then places the hydrogen on the more substituted carbon. Both, the boron and the hydrogen add simultaneously on the same face of the double bond (syn addition).
Oxidation of the Trialkylborane by Hydrogen Peroxide
Step 2
- Part 1: The first part of this mechanism deals with the donation of a pair of electrons from the hydrogen peroxide ion. the hydrogen peroxide is the nucleophile in this reaction because it is the electron donor to the newly formed trialkylborane that resulted from hydroboration.
- Part 2: In this second part of the mechanism, a rearrangement of an R group with its pair of bonding electrons to an adjacent oxygen results in the removal of a hydroxide ion.
Two more of these reactions with hydroperoxide will occur in order give a trialkylborate
- Part 3: This is the final part of the oxidation process. In this part the trialkylborate reacts with aqueous NaOH to give the alcohol and sodium borate.
If you need additional visuals to aid you in understanding the mechanism, click on the outside links provided here that will take you to other pages and media that are very helpful as well.
Stereochemistry of Hydroboration
The hydroboration reaction is among the few simple addition reactions that proceed cleanly in a syn fashion. As noted above, this is a single-step reaction. Since the bonding of the double bond carbons to boron and hydrogen is concerted, it follows that the geometry of this addition must be syn. Furthermore, rearrangements are unlikely inasmuch as a discrete carbocation intermediate is never formed. These features are illustrated for the hydroboration of α-pinene.
Since the hydroboration procedure is most commonly used to hydrate alkenes in an anti-Markovnikov fashion, we also need to know the stereoselectivity of the second oxidation reaction, which substitutes a hydroxyl group for the boron atom. Independent study has shown this reaction takes place with retention of configuration so the overall addition of water is also syn.
The hydroboration of α-pinene also provides a nice example of steric hindrance control in a chemical reaction. In the less complex alkenes used in earlier examples the plane of the double bond was often a plane of symmetry, and addition reagents could approach with equal ease from either side. In this case, one of the methyl groups bonded to C-6 (colored purple in the equation) covers one face of the double bond, blocking any approach from that side. All reagents that add to this double bond must therefore approach from the side opposite this methyl.
Exercise \(\PageIndex{1}\)
What are the products of this reactions?
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Exercise \(\PageIndex{2}\)
What are the products of this reactions?
- Answer
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Exercise \(\PageIndex{3}\)
What are the products of this reactions?
- Answer
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Exercise \(\PageIndex{4}\)
Draw the structural formulas for the alcohols that result from hydroboration-oxidation of this alkene.
- Answer
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Exercise \(\PageIndex{5}\)
Draw the structural formulas for the alcohols that result from hydroboration-oxidation of (E)-3-methyl-2-pentene. If you need clarification or a reminder on the nomenclature of alkenes refer to the link below on naming the alkenes.
- Answer
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Exercise \(\PageIndex{6}\)
Write out the reagents or products (A–D) shown in the following reaction schemes.
- Answer
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References
- Vollhardt, Peter, and Neil Shore. Organic Chemistry: Structure and Function . 5th. New York: W.H. Freeman and Company, 2007.
- Foote, S. Christopher, and William H. Brown. Organic Chemistry . 5th. Belmont, CA: Brooks/Cole Cengage Learning, 2005.
- Bruice, Paula Yurkanis. Oragnic Chemistry . 5th. CA. Prentice Hall, 2006.
- Bergbreiter E. David , and David P. Rainville. Stereochemistry of hydroboration-oxidation of terminal alkenes. J. Org. Chem. , 1976 , 41 (18), pp 3031–3033
- Ilich, Predrag-Peter; Rickertsen, Lucas S., and Becker Erienne. Polar Addition to C=C Group: Why Is Anti-Markovnikov Hydroboration-Oxidation of Alkenes Not "Anti-"? Journal of Chemical Education., 2006, v83, n11, pg 1681-1685


19.2: Bacterial Cell Division and the Eukaryotic Cell Cycle
The life of actively growing bacteria is not separated into a time for duplicating genes (i.e., DNA synthesis) and one for binary fission (dividing and partitioning the duplicated DNA into new cells). Instead, the single circular chromosmome of a typical bacterium is replicating even before fission is complete, so that the new daughter cells already contained partially duplicated chromosomes. Cell growth, replication and fission are illustrated below
The roughly 30-60 minute life cycle of an actively growing bacterium is not divided into discrete phases. On the other hand, typical eukaryotic cells have a roughly 16-24 hour cell cycle (depending on cell type) that is divided into four separate phases. In the late 1800s, light microscopy revealed that some cells lost their nuclei while forming chromosomes (from chroma , colored; soma , bodies). In mitosis , paired, attached chromosomes ( chromatids ) were seen to separate and to be drawn along spindle fibers to opposite poles of dividing cells. Thus homologous chromosomes were equally partioned to the daughter cells at the end of cell division. Because of the same chromosomal behavior was observed in mitosis in diverse organisms, chromosomes were soon recognized as the stuff of inheritance, the carrier of genes!
The short period of intense mitotic activity was in stark contrast to the much longer ‘ quiet ’ time in the life of the cell, called interphase . The events of mitosis itself were described as occurring in 4 phases occupyiing a short time as shown below
Depending on whom you ask, cytokinesis (the cell movements of actually dividing a cell in two) is not part of mitosis. In that sense, we can think of three stages in the life of a cell: interphase, mitosis and cytokinesis. Of course, it turned out that interphase is not cellular ‘quiet time’ at all!
A. Defining the Phases of the Cell Cycle
Correlation of the inheritance of specific traits with that of chromosomes was demonstrated early in the early 20th century, most elegantly in genetic studies of the fruit fly, Drosophila melanogaster . At that time, chromosomes were assumed to contain the genetic material and that both were duplicated during mitosis. The first clue that this was not so came only after the discovery that DNA was in fact the chemical stuff of genes . The experiment distinguishing the time of chromosome formation from the time of DNA duplication is summarized below.
- Cultured cells were incubated with 3 H-thymine , the radioactive base that cells will incorporate into thymidine triphosphate (dTTP), and then into DNA.
- Cultured cells were incubated with 3 H-thymine, the radioactive base that cells will incorporate into thymidine triphosphate (dTTP), and then into DNA.
- Slides were dipped in a light-sensitive emulsion containing the same light sensitive chemicals as found in the emulsion-side of film.
- After some time to allow the radioactivity on the slide to ‘ expose ’ the emulsion, the slides were developed (in much the same way as developing film).
- The resulting autoradiographs in the microscope revealed images in the form of dark spots created by exposure to hot (i.e., radioactive DNA.
If DNA replicates in chromosomes undergoing mitosis, then when the developed film is placed back over the slide, any dark spots should lie over the cells in mitosis, and not over cells that are not actively dividing. The experimental is illustrated below
Observation of the autoradiographs show that none of the cells in mitosis is radioactively labeled. But some of the cells in interphase were! Therefore, DNA synthesis must take place sometime in interphase, before mitosis and cytokinesis (illustrated below).
340 Experiments that Reveal Replication in Interphase of the Cell Cycle
Next a series of pulse-chase experiments were done to determine when in the cell cycle DNA synthesis actually takes place. Cultured cells given a short pulse (exposure) to 3 H-thymine and then allowed to grow in non-radioactive medium for different times (the chase ). At the end of each chase time, cells were spread on a glass slide and again prepared for autoradiography. Analysis of the autoradiographs identified distinct periods of activity within interphase: Gap1 ( G 1 ), a time of DNA synthesis ( S ) and Gap 2 ( G 2 ). Here are the details of these very creative experiments, performed before it became possible to synchronize cells in culture so that they would all be growing and dividing at the same time.
- Cells were exposed to 3H-thymine for just 5 minutes (the pulse ) and then centrifuged. The radioactive supernatant was then discarded.
- The cells were rinsed and centrifuged again to remove as much labeled precursor as possible.
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The cells were re-suspended in fresh medium containing unlabeled (i.e., nonradioactive) thymine and further incubated for different times (the chase periods).After dipping the slides in light-sensitive emulsion, exposing and developing the film, the autoradiographs were examined, with the following results:
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After a 3-hour (or less) chase period, the slides looked just like they would immediately after the pulse. That is, none of the 7% of the cells that were in mitosis is radioactively labeled, but many interphase cells showed labeled nuclei, as shown below.
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After 4 hours of chase, a few of the 7% of the cells that were in mitosis were labeled, along with others in interphase (below).
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After a 5 hour chase, most cells in mitosis (still about 7% of cells on the slide) were labeled; many fewer cells in interphase were labeled (below).
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After a 20 hour chase, none of the 7% of cells that were in mitosis is labeled. Instead, all of the labeled cells are in interphase (below).
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After a 3-hour (or less) chase period, the slides looked just like they would immediately after the pulse. That is, none of the 7% of the cells that were in mitosis is radioactively labeled, but many interphase cells showed labeled nuclei, as shown below.
- The graph below plots a count of radiolabeled mitotic cells against chase times .
The plot defines the duration of events, or phases of the cell cycle as follows:
- The first phase (interval #1 on the graph) must be the time between the end of DNA synthesis and the start of mitosis, defined as Gap 2 ( G 2 ).
- Cell doubling times are easily measured. Assume that the cells in this experiment doubled every 20 hours. This would be consistent with the time interval of 20 hours between successive peaks in the number of radiolabeled mitotic cells after the pulse (interval #2 ).
- Interval #3 is easy enough to define. It is the time when DNA is synthesized, from start to finish; this is the synthesis , or S phase.
- One period of the cell cycle remains to be defined, but it is not on the graph! That would be the time between the end cell division (i.e., mitosis and cytokinesis) and the beginning of DNA synthesis ( replication ). That interval can be calculated from the graph as the time of the cell cycle (~20 hours) minus the sum of the other defined periods of the cycle. This phase is defined as the Gap 1 ( G 1 ) phase of the cycle.
So at last, here is our cell cycle with a summary of events occurring in each phase.
During all of interphase ( G 1 , S and G 2 ) , the cell grows in size, preparing for the next cell division. Growth in G 1 includes the synthesis of enzymes and other proteins that will be needed for replication.
DNA is replicated during the S phase, along with the synthesis of new histone and other proteins that will be needed to assemble new chromatin . G 2 is the shortest time of interphase and is largely devoted to preparing the cell for the next round of mitosis and cytokinesis. Among the proteins whose synthesis increases in this time are the tubulins and proteins responsible for condensing chromatin into the paired chromatids representing the duplicated chromosomes. Cohesin is a more recently discovered protein made in the run-up to mitosis. It holds centromeres of chromatids together until they are ready to separate.
341 Events in the Phases of the Cell Cycle
In a final note, typical dividing cells have generation times ranging from 16 to 24 hours. Atypical cells, like newly fertilized eggs, might divide every hour or so! In these cells, events that normally take many hours must be completed in just fractions of an hour.
B. When Cells Stop Dividing
Terminally differentiated cells are those that spend the rest of their lives performing a specific function. These cells no longer cycle. Instead, shortly after entering G 1 they are diverted into a phase called G 0 , as shown below.
Referred to as terminally differentiated , these cells normally never divide again. With a few exceptions (e.g., many neurons), most terminally differentiated cells have a finite lifespan, and must be replaced by stem cells. Examples include red blood cells. With a half-life of about 60 days, they are regularly replaced by reticulocytes produced in bone marrow