Chlorine (Cl)

Chlorine (symbol Cl) is the chemical element with atomic number 17: a halogen, a gas at 0 °C, with an atomic mass of 35.45 u. Discovered in 1774 (Carl Wilhelm Scheele).

Chlorine is a greenish, choking, toxic gas from the halogen family. Bonded to sodium, it forms table salt, perfectly harmless. Its ability to kill germs made it one of the great advances in public health: chlorinating drinking water drove back cholera and typhoid. It is also the basis of PVC and many medicines.

Key facts

Atomic number: 17
Symbol: Cl
French name: Chlore
Atomic mass: 35.45 u
Molar mass: 35.45 g/mol
Family: Halogens
Position: period 3, group 17, p-block
State at 0 °C: Gas
Melting point: -101.5 °C
Boiling point: -34 °C
Density: 0.00321 g/cm³
Electronegativity (Pauling): 3.16
First ionization energy: 1,251 kJ/mol
Atomic radius: 100 pm
Electron configuration: [Ne] 3s² 3p⁵
Abundance in Earth's crust: 145 ppm (19th most abundant element)
Indicative price: 0.08 $/kg (order of magnitude)
Radioactive: no
Discovery: 1774, Carl Wilhelm Scheele

Origin of the name

From the Greek chlōros, pale green.

What does chlorine look like?

A pale yellow-green gas, heavier than air, with a sharp, choking smell.

Where is chlorine found?

About 0.02% of Earth's crust, but it is the most abundant dissolved element in the oceans: about 19 g of chloride per liter. The human body contains about 95 g, as chloride ions.

Natural sources

Halite (rock salt)
Sylvite
Carnallite
Seawater

How is chlorine produced?

It is produced by electrolysis of salt water (the chlor-alkali process), which also yields caustic soda and hydrogen. World production is around 90 to 100 million tonnes a year.

Top producers

China
United States
Germany
Japan
India

What is chlorine used for?

Making PVC (pipes, windows, cables).
Disinfecting drinking water and swimming pools.
Bleach and household cleaners.
Synthesis of medicines and pesticides.
Production of titanium and ultra-pure silicon.

In everyday life

Household bleach
Tap water
The public swimming pool
PVC drainpipes
Table salt (sodium chloride)

Role in living things

The chloride ion is essential: it balances electrical charges in body fluids and forms the hydrochloric acid in the stomach, which digests food and kills germs. Some white blood cells even produce hypochlorous acid, the active ingredient in bleach, to destroy bacteria.

Hazards of chlorine

Chlorine gas attacks the lungs and can be fatal. Mixing bleach with an acid (descaler) or with ammonia releases toxic gases: a common household accident. Some chlorinated compounds (PCBs, CFCs, dioxins) are persistent and harmful to the environment.

Cosmic origin

Supernovae: Forged in the cores of massive stars, then scattered when they explode.
White dwarfs: Produced by the thermonuclear explosion of a white dwarf (type Ia supernova).

History of chlorine

The Swede Carl Wilhelm Scheele produced it in 1774 but thought it was a compound containing oxygen. In 1810, Humphry Davy showed it was an element and named it after the Greek khlôros, “pale green.” In 1789, Claude Louis Berthollet invented a bleaching liquid made in the Javel district of Paris, which is why bleach is still called “eau de Javel” in French.

Did you know?

On April 22, 1915, near Ypres, the German army released clouds of chlorine: the first large-scale chemical attack in history.
The “chlorine” smell of swimming pools comes mostly from chloramines, formed when chlorine reacts with swimmers' sweat and urine.
CFCs, chlorinated compounds once used in fridges and aerosol sprays, tore the hole in the ozone layer.

Isotopes of chlorine

Two stable isotopes, in a ratio of roughly three to one. Radioactive chlorine-36 was produced in large amounts by nuclear tests at sea and is used to date groundwater.

chlorine-35: 75.76% of natural chlorine, stable
chlorine-36: radioactive, half-life 301,000 years. Dates very old groundwater and rock exposure.
chlorine-37: 24.24% of natural chlorine, stable. A 600-tonne tank of chlorine-rich liquid detected neutrinos from the Sun (the Davis experiment).

Common compounds

Table salt (NaCl) · Bleach (NaClO) · Freon (CFC-12) (CCl₂F₂)

Related elements

Same family (halogens): Fluorine (F), Bromine (Br), Iodine (I), Astatine (At).

Same column (group 17): Fluorine (F), Bromine (Br), Iodine (I), Astatine (At), Tennessine (Ts).