Group 7: The Halogens

Halogens with metals: metal halides

• Halogens react with many metals to form ionic metal halide salts
• Each halogen atom gains one electron to form a halide ion with a 1− charge (F⁻, Cl⁻, Br⁻, I⁻)
• The formula of the metal halide depends on the charge on the metal ion:
  • Group 1 metals form M⁺ ions, giving compounds like NaCl
    • 2 Na(s) + Cl₂(g) → 2 NaCl(s)
  • Group 2 metals form M²⁺ ions, giving compounds like CaBr₂
    • Ca(s) + Br₂(l) → CaBr₂(s)
  • Iron forms FeX₃ with chlorine and bromine
    • 2 Fe(s) + 3 Cl₂(g) → 2 FeCl₃(s)
• Reactivity falls down the group, so the same metal reacts more slowly with bromine or iodine than with chlorine

Halogens with non-metals: hydrogen halides

• With non-metal partners, the halogens form simple molecular covalent compounds
• With hydrogen, every halogen gives a hydrogen halide:
  • H₂(g) + F₂(g) → 2 HF(g) (very vigorous, even in the dark and cold)
  • H₂(g) + Cl₂(g) → 2 HCl(g) (needs UV light or heat)
  • H₂(g) + Br₂(g) → 2 HBr(g) (slower, needs heat)
  • H₂(g) + I₂(g) → 2 HI(g) (slow, partial reaction)
• The vigour of the reaction falls down the group — the same pattern as for the metals

Displacement reactions of the halogens

• A displacement reaction happens when a more reactive halogen pushes a less reactive halogen out of an aqueous solution of its halide salt
• The reactivity order in Group 7 is: chlorine > bromine > iodine
• Chlorine in water (pale green) displaces both bromine and iodine:
  • Cl₂(aq) + 2 KBr(aq) → 2 KCl(aq) + Br₂(aq) — solution turns orange
  • Cl₂(aq) + 2 KI(aq) → 2 KCl(aq) + I₂(aq) — solution turns brown
• Bromine in water (orange) displaces iodine:
  • Br₂(aq) + 2 KI(aq) → 2 KBr(aq) + I₂(aq) — solution turns brown
• Iodine cannot displace either chlorine or bromine because it is the least reactive
• These reactions are sometimes called single replacement reactions