chlorine chlorine chlorine is a chemical element with the symbol cl and atomic number 17. the second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. chlorine is a yellow-green gas at room temperature. it is an extremely reactive element and a strong oxidizing chlorine, 17clchlorinepronunciation/ˈklɔːriːn, -aɪn/ ​(klor-een, -⁠eyen)appearancepale yellow-green gasstandard atomic weight ar°(cl)35.45±0.01 (abridged) chlorine 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 f↑cl↓br sulfur ← chlorine → argon atomic number (z)17groupgroup 17 (halogens)periodperiod 3block p-blockelectron configuration 3s2 3p5electrons per shell2, 8, 7physical propertiesphase at stpgasmelting point(cl2) 171.6 k ​(−101.5 °c, ​−150.7 °f) boiling point(cl2) 239.11 k ​(−34.04 °c, ​−29.27 °f) density (at stp)3.2 g/lwhen liquid (at b.p.)1.5625 g/cm3 critical point416.9 k, 7.991 mpa heat of fusion(cl2) 6.406 kj/mol heat of vaporisation(cl2) 20.41 kj/mol molar heat capacity(cl2)33.949 j/(mol·k) vapour pressure p (pa) 1 10 100 1 k 10 k 100 k at t (k) 128 139 153 170 197 239 atomic propertiesoxidation states−1, +1, +2, +3, +4, +5, +6, +7 (a strongly acidic oxide)electronegativitypauling scale: 3.16 ionisation energies1st: 1251.2 kj/mol 2nd: 2298 kj/mol 3rd: 3822 kj/mol (more) covalent radius102±4 pm van der waals radius175 pm spectral lines of chlorineother propertiesnatural occurrenceprimordialcrystal structure ​orthorhombicspeed of sound206 m/s (gas, at 0 °c)thermal conductivity8.9×10−3 w/(m⋅k) electrical resistivity>10 ω⋅m (at 20 °c) magnetic orderingdiamagnetic molar magnetic susceptibility−40.5×10−6 cm3/molcas numbercl2: 7782-50-5 historydiscovery and first isolationcarl wilhelm scheele (1774)recognized as an element byhumphry davy (1808)isotopes of chlorine main isotopes decay abun­dance half-life (t1/2) mode pro­duct 35cl 76% stable 36cl trace 3.01×105 y β− 36ar ε 36s 37cl 24% stable age: : among the elements, it has the highest electron affinity and the third-highest electronegativity on the revised pauling scale, behind only oxygen and fluorine. chlorine played an important role in the experiments conducted by medieval alchemists, which commonly involved the heating of chloride salts like ammonium chloride (sal ammoniac) and sodium chloride (common salt), producing various chemical substances containing chlorine such as hydrogen chloride, mercury(ii) chloride (corrosive sublimate), and hydrochloric acid (in the form of aqua regia). however, the nature of free chlorine gas as a separate substance was only recognised around 1630 by jan baptist van helmont. carl wilhelm scheele wrote a description of chlorine gas in 1774, supposing it to be an oxide of a new element. in 1809, chemists suggested that the gas might be a pure element, and this was confirmed by sir humphry davy in 1810, who named it after the ancient greek χλωρός (khlōrós, "pale green") because of its colour. because of its great reactivity, all chlorine in the earth's crust is in the form of ionic chloride compounds, which includes table salt. it is the second-most abundant halogen (after fluorine) and twenty-first most abundant chemical element in earth's crust. these crustal deposits are nevertheless dwarfed by the huge reserves of chloride in seawater. elemental chlorine is commercially produced from brine by electrolysis, predominantly in the chlor-alkali process. the high oxidising potential of elemental chlorine led to the development of commercial bleaches and disinfectants, and a reagent for many processes in the chemical industry. chlorine is used in the manufacture of a wide range of consumer products, about two-thirds of them organic chemicals such as polyvinyl chloride (pvc), many intermediates for the production of plastics, and other end products which do not contain the element. as a common disinfectant, elemental chlorine and chlorine-generating compounds are used more directly in swimming pools to keep them sanitary. elemental chlorine at high concentration is extremely dangerous, and poisonous to most living organisms. as a chemical warfare agent, chlorine was first used in world war i as a poison gas weapon. in the form of chloride ions, chlorine is necessary to all known species of life. other types of chlorine compounds are rare in living organisms, and artificially produced chlorinated organics range from inert to toxic. in the upper atmosphere, chlorine-containing organic molecules such as chlorofluorocarbons have been implicated in ozone depletion. small quantities of elemental chlorine are generated by oxidation of chloride ions in neutrophils as part of an immune system response against bacteria. history the most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as 3000 bc and brine as early as 6000 bc. early discoveries around 900, the authors of the arabic writings attributed to jabir ibn hayyan (latin: geber) and the persian physician and alchemist abu bakr al-razi (c. 865–925, latin: rhazes) were experimenting with sal ammoniac (ammonium chloride), which when it was distilled together with vitriol (hydrated sulfates of various metals) produced hydrogen chloride. however, it appears that in these early experiments with chloride salts, the gaseous products were discarded, and hydrogen chloride may have been produced many times before it was discovered that it can be put to chemical use. one of the first such uses was the synthesis of mercury(ii) chloride (corrosive sublimate), whose production from the heating of mercury either with alum and ammonium chloride or with vitriol and sodium chloride was first described in the de aluminibus et salibus ("on alums and salts", an eleventh- or twelfth century arabic text falsely attributed to abu bakr al-razi and translated into latin in the second half of the twelfth century by gerard of cremona, 1144–1187). another important development was the discovery by pseudo-geber (in the de inventione veritatis, "on the discovery of truth", after c. 1300) that by adding ammonium chloride to nitric acid, a strong solvent capable of dissolving gold (i.e., aqua regia) could be produced. although aqua regia is an unstable mixture that continually gives off fumes containing free chlorine gas, this chlorine gas appears to have been ignored until c. 1630, when its nature as a separate gaseous substance was recognised by the brabantian chemist and physician jan baptist van helmont. carl wilhelm scheele, discoverer of chlorine isolation the element was first studied in detail in 1774 by swedish chemist carl wilhelm scheele, and he is credited with the discovery. scheele produced chlorine by reacting mno2 (as the mineral pyrolusite) with hcl: 4 hcl + mno2 → mncl2 + 2 h2o + cl2 scheele observed several of the properties of chlorine: the bleaching effect on litmus, the deadly effect on insects, the yellow-green color, and the smell similar to aqua regia. he called it "dephlogisticated muriatic acid air" since it is a gas (then called "airs") and it came from hydrochloric acid (then known as "muriatic acid"). he failed to establish chlorine as an element. common chemical theory at that time held that an acid is a compound that contains oxygen (remnants of this survive in the german and dutch names of oxygen: sauerstoff or zuurstof, both translating into english as acid substance), so a number of chemists, including claude berthollet, suggested that scheele's dephlogisticated muriatic acid air must be a combination of oxygen and the yet undiscovered element, muriaticum. in 1809, joseph louis gay-lussac and louis-jacques thénard tried to decompose dephlogisticated muriatic acid air by reacting it with charcoal to release the free element muriaticum (and carbon dioxide). they did not succeed and published a report in which they considered the possibility that dephlogisticated muriatic acid air is an element, but were not convinced. in 1810, sir humphry davy tried the same experiment again, and concluded that the substance was an element, and not a compound. he announced his results to the royal society on 15 november that year. at that time, he named this new element "chlorine", from the greek word χλωρος (chlōros, "green-yellow"), in reference to its color. the name "halogen", meaning "salt producer", was originally used for chlorine in 1811 by johann salomo christoph schweigger. this term was later used as a generic term to describe all the elements in the chlorine family (fluorine, bromine, iodine), after a suggestion by jöns jakob berzelius in 1826. in 1823, michael faraday liquefied chlorine for the first time, and demonstrated that what was then known as "solid chlorine" had a structure of chlorine hydrate (cl2·h2o). later uses chlorine gas was first used by french chemist claude berthollet to bleach textiles in 1785. modern bleaches resulted from further work by berthollet, who first produced sodium hypochlorite in 1789 in his laboratory in the town of javel (now part of paris, france), by passing chlorine gas through a solution of sodium carbonate. the resulting liquid, known as "eau de javel" ("javel water"), was a weak solution of sodium hypochlorite. this process was not very efficient, and alternative production methods were sought. scottish chemist and industrialist charles tennant first produced a solution of calcium hypochlorite ("chlorinated lime"), then solid calcium hypochlorite (bleaching powder). these compounds produced low levels of elemental chlorine and could be more efficiently transported than sodium hypochlorite, which remained as dilute solutions because when purified to eliminate water, it became a dangerously powerful and unstable oxidizer. near the end of the nineteenth century, e. s. smith patented a method of sodium hypochlorite production involving electrolysis of brine to produce sodium hydroxide and chlorine gas, which then mixed to form sodium hypochlorite. this is known as the chloralkali process, first introduced on an industrial scale in 1892, and now the source of most elemental chlorine and sodium hydroxide. in 1884 chemischen fabrik griesheim of germany developed another chloralkali process which entered commercial production in 1888. elemental chlorine solutions dissolved in chemically basic water (sodium and calcium hypochlorite) were first used as anti-putrefaction agents and disinfectants in the 1820s, in france, long before the establishment of the germ theory of disease. this practice was pioneered by antoine-germain labarraque, who adapted berthollet's "javel water" bleach and other chlorine preparations (for a more complete history, see below). elemental chlorine has since served a continuous function in topical antisepsis (wound irrigation solutions and the like) and public sanitation, particularly in swimming and drinking water. chlorine gas was first used as a weapon on april 22, 1915, at ypres by the german army. the effect on the allies was devastating because the existing gas masks were difficult to deploy and had not been broadly distributed. properties chlorine, liquefied under a pressure of 7.4 bar at room temperature, displayed in a quartz ampule embedded in acrylic glass solid chlorine at −150 °c chlorine is the second halogen, being a nonmetal in group 17 of the periodic table. its properties are thus similar to fluorine, bromine, and iodine, and are largely intermediate between those of the first two. chlorine has the electron configuration 3s23p5, with the seven electrons in the third and outermost shell acting as its valence electrons. like all halogens, it is thus one electron short of a full octet, and is hence a strong oxidising agent, reacting with many elements in order to complete its outer shell. corresponding to periodic trends, it is intermediate in electronegativity between fluorine and bromine (f: 3.98, cl: 3.16, br: 2.96, i: 2.66), and is less reactive than fluorine and more reactive than bromine. it is also a weaker oxidising agent than fluorine, but a stronger one than bromine. conversely, the chloride ion is a weaker reducing agent than bromide, but a stronger one than fluoride. it is intermediate in atomic radius between fluorine and bromine, and this leads to many of its atomic properties similarly continuing the trend from iodine to bromine upward, such as first ionisation energy, electron affinity, enthalpy of dissociation of the x2 molecule (x = cl, br, i), ionic radius, and x–x bond length. (fluorine is anomalous due to its small size.) all four stable halogens experience intermolecular van der waals forces of