Atomic number 17

Chlorine

Cl
+ phase
− phase

Shells Orbitals 3s 3p

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17 electrons in 3 shells: 2, 8, 7.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Halogen
Period: 3
Group: 17
Block: p
Standard atomic weight: 35.45(1) Da
Electron configuration: [Ne] 3s² 3p⁵
Phase at 25 °C: gas
Melting point: −101.5 °C
Boiling point: −34.04 °C
Density: 3.214 g/L
Electronegativity (Pauling): 3.16
Discovered: 1774, Carl Wilhelm Scheele

Neighbours in the table

S Ar

Chlorine’s outermost shell holds seven electrons, two in the 3s orbital and five in the 3p. In the atom model, the 17 electrons fall into shells of 2, 8 and 7, one fewer outside than in the shell below. An atom that takes up one more electron becomes the chloride ion, chlorine in the 1− oxidation state. Its electronegativity, the pull an atom exerts on the electrons in a bond it shares, is 3.16 on the Pauling scale, behind only fluorine and oxygen.

That pull makes chlorine an oxidising agent: it takes electrons from nearly every element it meets. From a metal it takes them outright, and molten sodium ignites in chlorine to give the ionic lattice of sodium chloride. With nonmetals it shares instead, forming covalent compounds, although carbon, nitrogen and oxygen are notable exceptions. Left to itself, chlorine pairs its atoms into the molecule Cl₂, a greenish-yellow gas that gave the element its name.

It boils at −34.04 °C and freezes at −101.50 °C; pressure at room temperature liquefies it readily, so it ships as a liquid under pressure. A litre of it weighs 3.214 g, and the PubChem safety summary warns that the gas, much heavier than air, settles in low places. Chlorine does not burn, but like oxygen it keeps other things burning.

Where it is found

Chlorine combines directly with nearly every element, so it is never found free in nature. The halogens are all too reactive to occur uncombined, and chlorides are the most abundant of their compounds. Chlorine occurs combined chiefly with sodium, as common salt, and in the minerals carnallite and sylvite.

Seawater carries 0.54 mol of chloride ions per litre. PubChem puts the oceans at about 19 grams of chlorine per litre, against 145 milligrams in every kilogram of crustal rock. Chloride also concentrates in salt lakes such as the Dead Sea and the Great Salt Lake, and in extensive beds of NaCl, KCl or MgCl₂. Chlorine’s standard atomic weight, 35.45 Da, is a conventional value; in nature the weight is an interval rather than one number.

Part of the chlorine in your own body is hydrochloric acid, a component of stomach acid. Los Alamos records an odder address: the skin of some tree frogs carries a chlorine compound with very strong painkilling action. Neither is elemental chlorine; both hold the element bound into a compound.

Why it matters

Most commercial chlorine comes from passing a current through a solution of sodium chloride, the chlor-alkali process, which oxidises chloride ions back to the gas. More arrives as a by-product when sodium, calcium and magnesium are won by electrolysing their molten chlorides. Mined rock-salt deposits feeding those cells supply the sodium and chlorine in almost every other compound of the two.

Dissolved chlorine does not all stay as Cl₂. In cold water it splits two ways: some atoms are oxidised to hypochlorous acid, and an equal number are reduced to chloride ions. Warm water also holds less of the gas: one volume of water takes up 3.10 volumes at 10 °C but only 1.77 volumes at 30 °C.

That hypochlorous acid turns coloured substances colourless, which is how chlorine bleaches wood pulp and cotton. Chlorine also oxidises and so destroys organic and biological material in water, and kills the bacteria in community supplies. Los Alamos notes that chlorination now reaches even the smallest water supplies.

Large quantities go into compounds. Chlorine replaces hydrogen in hydrocarbons to give substances such as chloroform, and goes into the polymer PVC. Hydrogen chloride dissolves in water to make hydrochloric acid. Being a stronger oxidising agent than bromine, chlorine pushes it out of bromide solutions, which is how bromine is produced.

Properties

Properties of chlorine Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	17
Standard atomic weight	35.45(1) Da	Da	IUPAC/CIAAW 2024
Melting point	−101.5 °C	°C	Los Alamos: −101.5 °C
Boiling point	−34.04 °C	°C	Los Alamos: −34.04 °C
Density	3.214 g/L	g/L
Electronegativity (Pauling)	3.16
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁵
Phase at 25 °C	gas

The standard atomic weight of this element is an interval in nature, from 35.446 to 35.457 Da; the value shown is IUPAC's conventional abridged value.

Safety

Elemental chlorine is the hazardous form of this element; the chloride ion in table salt is not. Breathing the gas harms the airways, and contact irritates the eyes and the skin; it is also classed as very toxic to aquatic life. That concerns above all the people who handle or transport the gas at work. The PubChem Laboratory Chemical Safety Summary for chlorine and the CDC/NIOSH Pocket Guide entry are the authoritative sheets.

Questions

Is chlorine gas dangerous?

Yes. PubChem's record carries the GHS signal word Danger, and the harmonised European entry classes chlorine as an oxidising gas that is toxic if inhaled and may intensify a fire. NIOSH gives the exposure routes as inhalation and contact with the skin or eyes, and names the eyes, skin and respiratory system as the target organs.

Why does a swimming pool smell like bleach?

Chlorine is what treats the water: PubChem's element page lists treating swimming pools, alongside making drinking water safe, among the element's uses. What you smell, though, is probably not chlorine. The CDC says that a 'chlorine' smell at a pool is most likely chloramines, irritants that form when chlorine combines with sweat, urine, dirt and skin cells washed off swimmers.

Is the chlorine in table salt the same as chlorine gas?

No. Chlorine gas is pairs of neutral atoms that take electrons from almost anything they meet. In salt each chlorine atom has already taken one and become a chloride ion, held in an ionic lattice beside sodium ions. That is why the one is a toxic gas and the other seasons food, and why the sodium chloride page shows a formula unit of a lattice rather than a molecule.

Who discovered chlorine and when?

Carl Wilhelm Scheele, a Swedish pharmacist and chemist, obtained the gas in 1774. He took his greenish-yellow gas for a compound containing oxygen. Humphry Davy, the English chemist, showed in 1810 that it was an element in its own right, and the name he gave it comes from chloros, the Greek for its pale greenish-yellow colour.

Compounds containing chlorine

HCl Hydrogen chloride 2 atoms, 1 single bond.
KCl Potassium chloride 2 atoms in one formula unit of an ionic lattice.
NaCl Sodium chloride 2 atoms in one formula unit of an ionic lattice.
CaCl₂ Calcium chloride 3 atoms in one formula unit of an ionic lattice.
MgCl₂ Magnesium chloride 3 atoms in one formula unit of an ionic lattice.
CCl₄ Carbon tetrachloride 5 atoms, 4 single bonds, tetrahedral.
CHCl₃ Chloroform 5 atoms, 4 single bonds, tetrahedral.
CCl₂F₂ Dichlorodifluoromethane 5 atoms, 4 single bonds, tetrahedral.

All 11 compounds containing chlorine

CH₂Cl₂ Dichloromethane 5 atoms, 4 single bonds, tetrahedral.
NH₄Cl Ammonium chloride 6 atoms in one formula unit of an ionic lattice.
C₂H₃Cl Vinyl chloride 6 atoms, 5 bonds.

Appears in these articles

What holds atoms together: ionic, covalent and metallic bonds Table salt has no molecules in it; hydrogen chloride gas is nothing but molecules.
Intermolecular forces: why water boils at 100 °C and methane at −161 °C Methane's molar mass is 16 g/mol and water's 18, yet methane boils at −161 °C and water at 100 °C.