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 oxidising agent: 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.

© Wikipedia | CC-by-SA-3.0 | Read more …

Atomic properties

Standard atomic weight35.45 ±0.01 [35.446 … 35.457]
Atomic mass35.4535 u

Atomic radii

Radius (empirical)100 pm
Radius (calculated)79 pm
Covalent radius102 ±4 pm
Van der Waals radius175 pm

Atomic shell

Electron configurationNe 3s2 3p5
Ionization energy(1st) 12.967632 eV
(2nd) 23.81364 eV
(3rd) 39.8 eV
(4th) 53.24 eV
(5th) 67.68 eV
(6th) 96.94 eV
(7th) 114.2013 eV
Shell model

Physical properties

Density3.215 g·cm−3 (273 K)
Molar volumesolid: 1.739·10-5 m3·mol−1
Speed of sound206 m·s−1


Melting point171.6 K
Boiling point239.1 K
Liquid range67.51 K
Critical point416.9 K @ 7.991 MPa


Melting enthalpy3.2 kJ·mol-1
Enthalpy of vaporization10.2 kJ·mol-1
Binding energy122 kJ·mol-1

Heat and conductivity

Specific heat capacity480 J·kg−1·K−1
Thermal conductivity0.0089 W·m-1·K-1

Elastic properties

Bulk modulus1.1 GPa

Electrical properties

Resistance1,000 Ωm


Magnetic susceptibility-4.051·10-5 cm3·mol−1

Optical properties

Refractive index1

Chemical properties

Basicitystrongly acidic
Oxidation state±1, 3, 4, 5, 6, 7
Standard potential1.36 V (Cl + e- → Cl-)


Pauling scale3.16
Sanderson scale3.48
Allred-Rochow scale2.83
Mulliken scale3.1
Allen scale2.869
Ghosh-Gupta scale7.292 eV
Boyd-Edgecombe scale3.05
Nagle scale2.82
Pearson absolute negativity8.3 eV

Other properties

Natural occurrenceprimordial
Crystal structureOrthorhombic
Goldschmidt Classificationlithophile
Superconductorwithout transition tempperature
Price/kg0.082 USD

Natural abundances

1,000 ppb ≈ 1·1013 M☉
8,000 ppb ≈ 1.59·1013 Mt
380,000 ppb ≈ 380 g
Earth’s crust
170,000 ppb ≈ 4.7·106 Mt
1.987·107 ppb ≈ 27,200 Mt
Flowing water
8,000 ppb ≈ 128 Gt
Human body
1.2·106 ppb ≈ 83.9 g