Chlorine (Cl₂) — Properties, Uses & Safety

Overview

Chlorine (Cl2) is a yellow-green diatomic gas with a pungent, suffocating odor and a molar mass of 70.91 g/mol. It is a halogen element (Group 17) and one of the most reactive nonmetals, with a high electronegativity of 3.16 on the Pauling scale. At standard conditions, chlorine exists as a dense gas approximately 2.5 times heavier than air. Chlorine was first produced by Carl Wilhelm Scheele in 1774 and recognized as an element by Humphry Davy in 1810. It carries extensive GHS hazard codes: H270 (may cause or intensify fire as an oxidizer), H280 (gas under pressure), H314 (causes severe skin burns and eye damage), H330 (fatal if inhaled), and H400 (very toxic to aquatic life). Chlorine gas was infamously used as a chemical weapon in World War I, causing severe respiratory damage. Despite its toxicity, chlorine is one of the most important industrial chemicals. The chlor-alkali process (electrolysis of brine: 2 NaCl + 2 H2O -> Cl2 + 2 NaOH + H2) produces chlorine gas, sodium hydroxide, and hydrogen simultaneously. Chlorine's most beneficial application is water disinfection — chlorination of drinking water and swimming pools has prevented countless waterborne disease outbreaks and is considered one of the greatest public health achievements of the 20th century. Chlorine is also used to manufacture PVC plastic, solvents, pesticides, pharmaceuticals, and bleach (sodium hypochlorite, NaClO). For chemistry students, chlorine connects to numerous fundamental topics: halogen chemistry and periodic trends, electronegativity and electron affinity, oxidation-reduction reactions, electrolysis and electrochemistry, and the formation of both ionic compounds (NaCl) and covalent compounds (HCl, CCl4). The chlor-alkali process is a classic industrial application of electrolysis. Chlorine's role in the ozone layer depletion story (via CFCs) connects chemistry to environmental science and policy.

IUPAC Name

molecular chlorine

CAS Number

7782-50-5

PubChem CID

24526

Molecular Formula

Cl₂

Molar Mass

70.906 g/mol

State at STP

gas

Appearance

Yellow-green gas with pungent odor