ElementsWhat is chlorine?
Chlorine is a yellow-green gas with a sharp smell that is poisonous to breathe. In nature it is found as salts, most of all as the sodium chloride of sea water.
What is chlorine used for?
Making drinking water and swimming pools safe, bleach, PVC plastic for pipes, and many medicines and other chemicals.
Where the name comes from
From the Greek word chloros, pale green, the colour of the gas.
Key facts
- Symbol
- Cl
- Atomic number
- 17
- Atomic mass
- 35.45 u
- Family
- Halogen
- Period
- 3
- Group
- 17
- Block
- p
- State at room temperature
- Gas
- Melting point
- -101.5 °C (171.65 K, -151 °F)
- Boiling point
- -34 °C (239.11 K, -29 °F)
- Density
- 0.003214 g/cm³ (as a gas at 0 °C)
- Electrons per shell
- 2, 8, 7
- Electron configuration
- [Ne]3s2 3p5
- Common oxidation states
- +7, +5, +1, -1
- Electronegativity (Pauling)
- 3.16
- First ionisation energy
- 12.968 eV
- Electron affinity
- 3.617 eV
- Atomic radius (van der Waals)
- 175 pm
- Discovered
- 1774
Place in the periodic table
Chlorine is element number 17. It sits in period 3 (row 3) of the periodic table, in group 17 (column 17), and belongs to the p-block. Just before it comes sulfur (16), and just after it comes argon (18). In the same column, fluorine is above it and bromine is below it. Elements in one column tend to behave alike.
Family: Halogens.
Physical properties
At room temperature chlorine is a gas. It melts at -101.5 °C and boils at -34 °C, so it stays liquid over a range of 67 degrees. One litre of the gas weighs about 3.214 grams at 0 °C and normal pressure. That is heavier than air, which weighs about 1.29 grams a litre. It has the 10th lowest melting point of all elements.
Inside the atom
An atom of chlorine has 17 protons in its nucleus and, when neutral, 17 electrons around it. The electrons are arranged in 3 shells: 2, 8, 7. The outermost shell holds 7 electrons. Written in full, the electron configuration is [Ne]3s2 3p5.
Its electronegativity on the Pauling scale is 3.16. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 12.968 electronvolts (about 1,251 kilojoules per mole). One mole of chlorine weighs 35.45 grams, so a single gram contains about 1.70 × 1022 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains chlorine with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.