Chlorine (Cl2)

Summary: Chlorine (Cl₂) is a diatomic molecule in which two chlorine atoms (2,8,7) share one pair of electrons, leaving three lone pairs on each atom; a pale yellow-green toxic gas that bleaches damp litmus paper and is used to treat drinking water. Tags: igcse chemistry Created: 2026-07-18

Chlorine (Cl₂) is a diatomic Molecule of the Element chlorine, a member of Group 7 Halogens in the Periodic Table. Each chlorine atom has the electronic configuration 2,8,7, so it is one electron short of the stable octet of argon (see Group 0 Noble Gases). Rather than each gaining an electron to form an Ion as it does in ionic compounds, two chlorine atoms achieve full outer shells by sharing a single pair of electrons, forming one covalent bond and leaving three non-bonding lone pairs on each atom. The molecule is a simple molecular substance: the Cl—Cl bond within each molecule is strong, but only a weak Intermolecular Force acts between molecules, so chlorine is a gas at room temperature. Chlorine is a pale yellow-green, dense, toxic gas with a choking smell, and its powerful oxidising and bleaching behaviour underlies both its laboratory test and its use in Water Treatment.

Bonding: Dot-and-Cross Description

In the Dot-and-Cross Diagram of Cl₂, the outer-shell electrons of one atom are drawn as dots and those of the other as crosses (only the 7 valence electrons of each atom are shown):

• Each Cl atom contributes 1 electron to the shared region between the atoms, giving 1 shared (bonding) pair — one dot and one cross.
• The remaining 6 electrons on each atom form 3 lone pairs (three pairs of dots on one atom, three pairs of crosses on the other).
• Counting the shared pair, each Cl atom now has 8 outer electrons — the octet of argon.
• Total in the diagram: 1 bonding pair + 6 lone pairs = 14 outer electrons.

Displayed formula: Cl—Cl (a single covalent bond).

Key Data

Physical Properties

• Pale yellow-green, dense, toxic gas with a sharp, choking smell — colour darkens down Group 7 Halogens (fluorine pale yellow → chlorine yellow-green → bromine red-brown → iodine grey-black).
• Simple molecular substance: low melting and boiling points because boiling only overcomes weak forces between molecules.
• Slightly soluble in water, forming a solution (chlorine water) that is acidic and bleaching.
• Does not conduct electricity in any state — no free ions or delocalised electrons.

Test for Chlorine

Hold damp blue litmus paper in the gas. Chlorine turns it red and then bleaches it white. (The brief red stage occurs because chlorine dissolves in the water on the paper to form an acidic solution; the bleaching then destroys the dye.) See Chemical Tests.

Chemical Behaviour and Uses

• A reactive oxidising agent: a more reactive halogen displaces a less reactive one from a solution of its halide (e.g. Cl₂ + 2KBr → 2KCl + Br₂) — the classic reactivity evidence for Group 7 Halogens.
• Water treatment: small amounts of chlorine are added to drinking water and swimming pools to kill bacteria (see Water Treatment).
• Manufacture of bleach and disinfectants.
• Making hydrochloric acid via hydrogen chloride and chlorinated organic compounds such as PVC (see Polymers).
• Chlorine is produced industrially at the anode during the Electrolysis of concentrated aqueous sodium chloride (brine) — see Electrolysis of Aqueous Solutions.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.5, Cambridge Assessment International Education

Common Misconceptions