Chlorine

The halogen of table salt, bleach and swimming pools: a greenish, choking gas as an element, and a harmless ion in the sea.

In nature it is never aloneCl₂Read about the molecule

At a glance

Symbol: Cl
Atomic number: 17
Average atomic mass: 35.45
Group: 17
Period: 3
Family: halogen
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁵, or [Ne] 3s² 3p⁵
Valence electrons: 7
Electronegativity: 3.16
Common ions: Cl⁻ (chloride), as in NaCl and HCl
Isotopes: ³⁵Cl (75.8%) and ³⁷Cl (24.2%), which is why the average mass is 35.45 and not a whole number

On the table

Period 3, group 17: the second rung down the stem of the 7. The other halogens are tinted, and every cell opens its own article.

Why it pairs up

Seven valence electrons, one short of eight. Two chlorine atoms share one pair, a single bond.

Properties

State at 25 °C: gas
What it looks like: yellow-green gas made of Cl₂ molecules
Melting point: −101.5 °C
Boiling point: −34.0 °C
Density: 3.21 g/L at 0 °C and 1 atm

Where it is found

About 1.9% of seawater by mass is chloride, and beds of rock salt (halite, NaCl) mark the places where ancient seas dried up. It is about 0.015% of the crust, and never free in nature.

Discovery and name

Scheele made the gas in 1774 from hydrochloric acid and manganese dioxide, but thought it contained oxygen. Humphry Davy showed in 1810 that it was an element and named it from the Greek chloros, pale green.

What it is used for

Chlorine gas disinfects drinking water and pools and makes bleach. Chlorine compounds make PVC pipe and flooring, solvents, pesticides and many medicines, and the chloride in table salt is essential to life.

In compounds

NaClsodium chloride, table saltHClhydrochloric acid, stomach acidNaOClsodium hypochlorite, bleachCaCl₂calcium chloride, road salt(C₂H₃Cl)ₙPVC plasticCHCl₃chloroform