Chlorine (Cl) — Atomic Number 17

Chlorine is a chemical element with the symbol Cl and atomic number 17. It is a toxic green-yellow diatomic gas and a powerful oxidising agent. It is used to disinfect drinking water and produce PVC plastic. The name comes from Greek chloros (“pale green”).

At a Glance

Discovery

Discovered in 1774 by Carl Wilhelm Scheele by reacting MnO₂ with HCl. Humphry Davy proved it was an element in 1810.

Chemical Properties

Chlorine gains 1 electron to form Cl⁻ (configuration 2,8,8, like argon).

Key Reactions

1. With sodium: $2\text{Na}+{\text{Cl}}_2\to 2\text{NaCl}$ (yellow flame, white smoke)
2. With water: ${\text{Cl}}_2+{\text{H}}_2\text{O}\rightleftharpoons \text{HCl}+\text{HOCl}$ (disproportionation)
3. With cold NaOH: ${\text{Cl}}_2+2\text{NaOH}\to \text{NaCl}+\text{NaClO}+{\text{H}}_2\text{O}$ (bleach)
4. With hydrogen: ${\text{H}}_2+{\text{Cl}}_2\to 2\text{HCl}$ (explodes in bright light)

Characteristic Tests

• Bleaches damp blue litmus paper — turns red then white
• Distinctive green-yellow colour and choking swimming-pool smell

Industrial Production

Electrolysis of brine (concentrated NaCl): $2\text{NaCl}+2{\text{H}}_2\text{O}\to 2\text{NaOH}+{\text{Cl}}_2+{\text{H}}_2$

Compounds & Uses

• Water disinfection (kills bacteria)
• Bleach and cleaning products

IGCSE Essentials

1. Diatomic Cl₂ — green-yellow gas, toxic
2. Bleaches damp litmus paper — key test
3. Test for Cl⁻: AgNO₃ → white precipitate, soluble in dilute NH₃
4. Produced by electrolysis of brine
5. Reactivity: F₂ > Cl₂ > Br₂ > I₂

Sources

• Chlorine — Wikipedia
• Chlorine — Ptable.com