The Halogens - Group 17 (7)

General Properties

• Non-metals, existing as separate diatomic molecules.
• Electronic configuration: ... ns2 np5.

Trends in Properties

Appearance

• Fluorine - Yellow (Gas at RTP)
• Chlorine - Green (Gas at RTP)
• Bromine - Red-Brown (Liquid at RTP)
• Iodine - Grey (Solid at RTP)

Boiling Point

• Increases down the group.
• Values:
  • Fluorine: -188°C
  • Chlorine: -34°C
  • Bromine: 58°C
  • Iodine: 183°C
• Increased size leads to stronger Van der Waals interactions, requiring more energy to separate molecules.

Electronegativity

• Decreases down the group.
• Values:
  • Fluorine: 4.0
  • Chlorine: 3.0
  • Bromine: 2.8
  • Iodine: 2.5
• Despite increasing nuclear charge, increasing number of shells and atomic radius reduce effective electron attraction.

Atomic and Ionic Size

• Both increase down the group.
• Covalent Radius (nm):
  • Fluorine: 0.064
  • Chlorine: 0.099
  • Bromine: 0.111
  • Iodine: 0.128
• Ionic Radius (nm):
  • Fluorine: 0.136
  • Chlorine: 0.181
  • Bromine: 0.195
  • Iodine: 0.216
• Larger ions due to electron repulsion expanding radius.

Oxidising Power

• Halogens are oxidising agents needing an electron for octet completion.
• Oxidising power decreases down the group due to:
  • Increased shielding
  • Increased atomic radius
• Example reactions:
  • Chlorine oxidises bromide and iodide.
  • Bromine oxidises iodide.

Reactions with Water and Alkalis

• Halogen reactivity decreases down the group.
• Chlorine in water turns litmus red, then decolourises.
• Example reaction: Cl2(g) + H2O(l) → HCl(aq) + HOCl(aq)
• Reacts with NaOH:
  • Cold, dilute: 2NaOH(aq) + Cl2(g) → NaCl(aq) + NaOCl(aq) + H2O(l)

Halide Ions

Reducing Ability

• Halide ions act as reducing agents, donating electrons.
• Reducing power increases down the group.
• Example: Reaction with concentrated sulphuric acid.

Testing for Halide Ions

• Use silver nitrate after acidifying with dilute nitric acid.
• Precipitate formation:
  • Chloride: White, soluble in ammonia.
  • Bromide: Cream, insoluble in dilute ammonia, soluble in concentrated ammonia.
  • Iodide: Yellow, insoluble in ammonia.

Hydrogen Halides

Boiling Points

• Vary, with HF having higher due to hydrogen bonding.
• Boiling points:
  • HF: 20°C
  • HCl: -85°C
  • HBr: -69°C
  • HI: -35°C

Preparation

• Displacement and direct combination methods.

Uses of Halogens and Halides

• Chlorine: Water purification, bleach, solvents, polymers (PVC), CFCs.
• Fluorine: CFCs, PTFE polymers for non-stick surfaces and insulation.
• Fluoride: Added to water and toothpaste to prevent tooth decay.
• Hydrogen fluoride: Glass etching.
• Silver bromide: Used in photographic film.

Environmental and Health Concerns

• Debate on chlorine use in water purification due to toxic by-products.
• Fluoride addition to water is controversial due to potential dental and liver issues; personal choice debated.

These notes summarize the key points discussed regarding the properties and reactions of the halogens and halide ions, as well as their uses and related environmental and health considerations.