Open Science Wiki
p block
Group 17 (halogens)
period 3
Atomic Information
Atomic Number
Atomic Radius
175 pm
35.45 Ar
reactive non-metal
Standard state(298 K)
gas (at STP)
Electronic Configuration
[Ne] 3s2 3p5
Electronegativity (Pauling)
348.575 kJ/mol
First ionisation energy
1251.2 kJ/mol
Physical Properties
Melting Point
171.6 K
Boiling Point
239.11 K
Density of solid
3.2 g/L (at STP)
1.56 g/cm3 (when liquid)
Heat Properties
Enthalpy of fusion
6.406 kJ/mol
Enthalpy of atomisation
33.949 J/(mol·K)
Enthalpy of vaporisation
20.41 kJ/mol

Chlorine is one of the most abundant chemical elements on Earth. It is ubiquitous in soils, minerals, plants and animals. Seawater is a huge reservoir of dissolved chlorine weathered from the continents and transported to the oceans by Earth's rivers.


Hydrochloric acid (HCl) was known to the alchemists. The gaseous element itself was first produced in 1774 by Carl Wilhelm Scheele at Uppsala, Sweden, by heating hydrochloric acid with the mineral pyrolusite which is naturally occuring manganese dioxide, MnO2. A dense, greenish-yellow gas was evolved which he recorded as having a choking smell and which dissolved in water to give an acid solution. He noted that it bleached litmus paper, and decolourised leaves and flowers.

Humphry Davy investigated it in 1807 and eventually concluded not only that it was a simple substance, but that it was truly an element. He announced this in 1810 and yet it took another ten years for some chemists finally to accept that chlorine really was an element.(


Chlorine is not found uncombined in nature. Halite (sodium chloride or ‘common salt’) is the main mineral that is mined for chlorine. Sodium chloride is a very soluble salt that has been leached into the oceans over the lifetime of the Earth. Several salt beds, or ‘lakes’ are found where ancient seas have evaporated, and these can be mined for chloride.(

Chlorine is also found in the minerals carnallite (magnesium potassium chloride) and sylvite (potassium chloride).(

40 million tonnes of chlorine gas are made each year from the electrolysis of brine (sodium chloride solution). This process also produces useful sodium hydroxide.(


Chlorine is produced industrially from the compound sodium chloride, one of the many salts found in geologic deposits formed from the slow evaporation of ancient seawater. When electricity is applied to a brine solution of sodium chloride, chlorine gas (Cl2), caustic soda (NaOH) and hydrogen gas (H2) are generated according to the following reaction:


Chlorine appears as a greenish yellow gas with a pungent suffocating odor. Toxic by inhalation. Slightly soluble in water. Liquefies at -35°C and room pressure. It is readily liquefied by pressure applied at room temperature. Contact with unconfined liquid can cause frostbite by evaporative cooling. It does not burn but, like oxygen, supports combustion. Long-term inhalation of low concentrations or short-term inhalation of high concentrations has ill effects. Vapors are much heavier than air and tend to settle in low areas.