potassium potassium potassium is the chemical element with the symbol k (from neo-latin kalium) and atomic number 19. it is a silvery white metal that is soft enough to easily cut with a knife. potassium metal reacts rapidly with atmospheric oxygen to form flaky white potassium peroxide in only seconds of exposure. it was first isolated from potash, the ashes of plants, from which its name derives. in the periodic table, potassium is one of the alkali metals, all of which have a single valence electron in the outer electron shell, which is easily removed to create an ion with a positive charge (which combines with anions to form salts). in nature, potassium occurs only in ionic salts. elemental potassium reacts vigorously with water, generating sufficient heat to ignite hydrogen emitted in the reaction, and burning with a lilac-colored flame. it is found dissolved in seawater (which is 0.04% potassium by weight), and occurs in many minerals such as orthoclase, a common constituent of granites and other igneous rocks. this article is about the chemical element. for the use of potassium as a medication, see potassium chloride (medical use). for the use of potassium in biology, see potassium in biology. potassium, 19kpotassium pearls (in paraffin oil, ~5 mm each)potassiumpronunciation/pəˈtæsiəm/ ​(pə-tass-ee-əm)appearancesilvery white, faint bluish-purple hue when exposed to airstandard atomic weight ar°(k)39.0983±0.000139.098±0.001 (abridged) potassium in the periodic table hydrogen helium lithium beryllium boron carbon nitrogen oxygen fluorine neon sodium magnesium aluminium silicon phosphorus sulfur chlorine argon potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon caesium barium lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury (element) thallium lead bismuth polonium astatine radon francium radium actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson na↑k↓rb argon ← potassium → calcium atomic number (z)19groupgroup 1: hydrogen and alkali metalsperiodperiod 4block s-blockelectron configuration 4s1electrons per shell2, 8, 8, 1physical propertiesphase at stpsolidmelting point336.7 k ​(63.5 °c, ​146.3 °f) boiling point1030.793 k ​(757.643 °c, ​1395.757 °f) density (near r.t.)0.89 g/cm3when liquid (at m.p.)0.82948 g/cm3 critical point2223 k, 16 mpa heat of fusion2.33 kj/mol heat of vaporization76.9 kj/mol molar heat capacity29.6 j/(mol·k) atomic propertiesoxidation states−1, +1 (a strongly basic oxide)electronegativitypauling scale: 0.82 ionization energies1st: 418.8 kj/mol 2nd: 3052 kj/mol 3rd: 4420 kj/mol (more) atomic radiusempirical: 227 pm covalent radius203±12 pm van der waals radius275 pm spectral lines of potassiumother propertiesnatural occurrenceprimordialcrystal structure ​body-centered cubic (bcc)speed of sound thin rod2000 m/s (at 20 °c) thermal expansion83.3 µm/(m⋅k) (at 25 °c) thermal conductivity102.5 w/(m⋅k) electrical resistivity72 nω⋅m (at 20 °c) magnetic orderingparamagnetic molar magnetic susceptibility+20.8×10−6 cm3/mol (298 k)young's modulus3.53 gpa shear modulus1.3 gpa bulk modulus3.1 gpa mohs hardness0.4 brinell hardness0.363 mpa cas number7440-09-7 historydiscovery and first isolationhumphry davy (1807)symbol"k": from new latin kaliumisotopes of potassium main isotopes decay abun­dance half-life (t1/2) mode pro­duct 39k 93.3% stable 40k 0.0120% 1.248×109 y β− 40ca ε 40ar β+ 40ar 41k 6.73% stable potassium is chemically very similar to sodium, the previous element in group 1 of the periodic table. they have a similar first ionization energy, which allows for each atom to give up its sole outer electron. it was suspected in 1702 that they were distinct elements that combine with the same anions to make similar salts, and this was proven in 1807 through using electrolysis. naturally occurring potassium is composed of three isotopes, of which 40k is radioactive. traces of 40k are found in all potassium, and it is the most common radioisotope in the human body. potassium ions are vital for the functioning of all living cells. the transfer of potassium ions across nerve cell membranes is necessary for normal nerve transmission; potassium deficiency and excess can each result in numerous signs and symptoms, including an abnormal heart rhythm and various electrocardiographic abnormalities. fresh fruits and vegetables are good dietary sources of potassium. the body responds to the influx of dietary potassium, which raises serum potassium levels, by shifting potassium from outside to inside cells and increasing potassium excretion by the kidneys. most industrial applications of potassium exploit the high solubility of its compounds in water, such as saltwater soap. heavy crop production rapidly depletes the soil of potassium, and this can be remedied with agricultural fertilizers containing potassium, accounting for 95% of global potassium chemical production. etymology the english name for the element potassium comes from the word potash, which refers to an early method of extracting various potassium salts: placing in a pot the ash of burnt wood or tree leaves, adding water, heating, and evaporating the solution. when humphry davy first isolated the pure element using electrolysis in 1807, he named it potassium, which he derived from the word potash. the symbol k stems from kali, itself from the root word alkali, which in turn comes from arabic: القَلْيَه al-qalyah 'plant ashes'. in 1797, the german chemist martin klaproth discovered "potash" in the minerals leucite and lepidolite, and realized that "potash" was not a product of plant growth but actually contained a new element, which he proposed calling kali. in 1807, humphry davy produced the element via electrolysis: in 1809, ludwig wilhelm gilbert proposed the name kalium for davy's "potassium". in 1814, the swedish chemist berzelius advocated the name kalium for potassium, with the chemical symbol k. the english and french-speaking countries adopted davy and gay-lussac/thénard's name potassium, whereas the germanic countries adopted gilbert/klaproth's name kalium. the "gold book" of the international union of pure and applied chemistry has designated the official chemical symbol as k. properties physical the flame test of potassium. potassium is the second least dense metal after lithium. it is a soft solid with a low melting point, and can be easily cut with a knife. potassium is silvery in appearance, but it begins to tarnish toward gray immediately on exposure to air. in a flame test, potassium and its compounds emit a lilac color with a peak emission wavelength of 766.5 nanometers. neutral potassium atoms have 19 electrons, one more than the configuration of the noble gas argon. because of its low first ionization energy of 418.8 kj/mol, the potassium atom is much more likely to lose the last electron and acquire a positive charge, although negatively charged alkalide k− ions are not impossible. in contrast, the second ionization energy is very high (3052 kj/mol). chemical potassium reacts with oxygen, water, and carbon dioxide components in air. with oxygen it forms potassium peroxide. with water potassium forms potassium hydroxide (koh). the reaction of potassium with water can be violently exothermic, especially since the coproduced hydrogen gas can ignite. because of this, potassium and the liquid sodium-potassium (nak) alloy are potent desiccants, although they are no longer used as such. compounds structure of solid potassium superoxide (ko2). four oxides of potassium are well studied: potassium oxide (k2o), potassium peroxide (k2o2), potassium superoxide (ko2) and potassium ozonide (ko3). the binary potassium-oxygen compounds react with water forming koh. koh is a strong base. illustrating its hydrophilic character, as much as 1.21 kg of koh can dissolve in a single liter of water. anhydrous koh is rarely encountered. koh reacts readily with carbon dioxide (co2) to produce potassium carbonate (k2co3), and in principle could be used to remove traces of the gas from air. like the closely related sodium hydroxide, koh reacts with fats to produce soaps. in general, potassium compounds are ionic and, owing to the high hydration energy of the k+ ion, have excellent water solubility. the main species in water solution are the aquo complexes + where n = 6 and 7. potassium heptafluorotantalate (k2) is an intermediate in the purification of tantalum from the otherwise persistent contaminant of niobium. organopotassium compounds illustrate nonionic compounds of potassium. they feature highly polar covalent k–c bonds. examples include benzyl potassium kch2c6h5. potassium intercalates into graphite to give a variety of graphite intercalation compounds, including kc8. isotopes there are 25 known isotopes of potassium, three of which occur naturally: 39k (93.3%), 40k (0.0117%), and 41k (6.7%) (by mole fraction). naturally occurring 40k has a half-life of 1.250×109 years. it decays to stable 40ar by electron capture or positron emission (11.2%) or to stable 40ca by beta decay (88.8%). the decay of 40k to 40ar is the basis of a common method for dating rocks. the conventional k-ar dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (40ar) was quantitatively retained. minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40ar that has accumulated. the minerals best suited for dating include biotite, muscovite, metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered. apart from dating, potassium isotopes have been used as tracers in studies of weathering and for nutrient cycling studies because potassium is a macronutrient required for life on earth. 40k occurs in natural potassium (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations. 40k is the radioisotope with the largest abundance in the human body. in healthy animals and people, 40k represents the largest source of radioactivity, greater even than 14c. in a human body of 70 kg, about 4,400 nuclei of 40k decay per second. the activity of natural potassium is 31 bq/g. cosmic formation and distribution potassium in feldspar potassium is formed in supernovae by nucleosynthesis from lighter atoms. potassium is principally created in type ii supernovae via an explosive oxygen-burning process. (these are nuclear fusion reactions, not to be confused with chemical burning of potassium in oxygen.) 40k is also formed in s-process nucleosynthesis and the neon burning process. potassium is the 20th most abundant element in the solar system and the 17th most abundant element by weight in the earth. it makes up about 2.6% of the weight of the earth's crust and is the seventh most abundant element in the crust. the potassium concentration in seawater is 0.39 g/l (0.039 wt/v%), about one twenty-seventh the concentration of sodium. potash potash is primarily a mixture of potassium salts because plants have little or no sodium content, and the rest of a plant's major mineral content consists of calcium salts of relatively low solubility in water. while potash has been used since ancient times, its composition was not understood. georg ernst stahl obtained experimental evidence that led him to suggest the fundamental difference of sodium and potassium salts in 1702, and henri louis duhamel du monceau was able to prove this difference in 1736. the exact chemical composition of potassium and sodium compounds, and the status as chemical element of potassium and sodium, was not known then, and thus antoine lavoisier did not include the alkali in his list of chemical elements in 1789. for a long time the only significant applications for potash were the production of glass, bleach, soap and gunpowder as potassium nitrate. potassium soaps from animal fats and vegetable oils were especially prized because they tend to be more water-soluble and of softer texture, and are therefore known as soft soaps. the discovery by justus liebig in 1840 that potassium is a necessary element for plants and that most types of soil lack potassium caused a steep rise in demand for potassium salts. wood-ash from fir trees was initially used as a potassium salt source for fertilizer, but, with the discovery in 1868 of mineral deposits containing potassium chloride near staßfurt, germany, the production of potassium-containing fertilizers began at an industrial scale. other potash deposits were discovered, and by the 1960s canada became the dominant producer. metal sir humphry davy pieces of potassium metal potassium metal was first isolated in 1807 by humphry davy, who derived it by electrolysis of molten koh with the newly discovered voltaic pile. potassium was the first metal that was isolated by electrolysis. later in the same year, davy reported extraction of the metal