Group VII Properties

1. General Characteristics of Group VII Halogens

• Position in the Periodic Table
  • Group VII elements are known as halogens.
  • Located in Group 17 (modern numbering), on the right-hand side of the Periodic Table, before the noble gases.
  • They are non-metallic elements with 7 electrons in their outer shell (ns²np⁵ configuration).
  • Exist as diatomic molecules (X₂) under standard conditions (Cl₂, Br₂, I₂).
• General trends down the group
  • Density: Increases down the group because the mass of the atoms increases faster than the increase in volume.
  • Reactivity: Decreases down the group due to increased atomic radius and greater shielding, making it harder for the nucleus to attract an additional electron.

2. Physical Properties at Room Temperature and Pressure (r.t.p.)

• Chlorine (Cl₂): Pale yellow-green gas, pungent odour, toxic.
• Bromine (Br₂): Red-brown liquid, evaporates easily to form an orange-brown vapour, corrosive, toxic.
• Iodine (I₂): Grey-black crystalline solid, sublimates to form a purple vapour when heated.
• Trend in state down the group:
  • From gas → liquid → solid at r.t.p.
  • Caused by increasing molecular mass leading to stronger Van der Waals forces between molecules.

3. Electronic Configuration

• General outer electron configuration: ns²np⁵.
• Halogens are one electron short of a stable noble gas configuration.
• They gain one electron during reactions to form halide ions (X⁻) with a –1 charge.

4. Reactivity Trend

• Decreases down the group:
  • As you go down, the outermost shell is further from the nucleus.
  • Increased shielding by inner electron shells reduces nuclear attraction for the incoming electron.
  • The ability to oxidise other substances decreases down the group.
• Fluorine (above chlorine) is the most reactive halogen.
• Chlorine reacts more vigorously than bromine, and bromine reacts more vigorously than iodine.

5. Chemical Properties

(a) Oxidising Power

• Halogens are strong oxidising agents because they accept electrons easily to form halide ions.
• Oxidising ability decreases down the group.
• Order of oxidising power: Cl₂ > Br₂ > I₂.

(b) Displacement Reactions with Halide Ions

• A more reactive halogen can displace a less reactive halogen from its halide salt in solution.
• General equation:
  X₂ + 2Y⁻ → 2X⁻ + Y₂
  where X₂ is the more reactive halogen, and Y⁻ is the halide ion of the less reactive halogen.

Examples:

1. Chlorine + potassium bromide solution
   Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
   • Chlorine displaces bromine (solution changes from colourless to orange-brown).
2. Chlorine + potassium iodide solution
   Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)
   • Solution changes from colourless to brown (due to iodine).
3. Bromine + potassium iodide solution
   Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)
   • Solution changes from colourless to brown.

6. Reactions with Metals

• Halogens react with metals to form metal halides.
• Example:
  • 2Na(s) + Cl₂(g) → 2NaCl(s)
    (White crystalline ionic solid formed)

7. Predicting Properties of Other Halogens

• Astatine (At₂) (below iodine):
  • Expected to be a solid at r.t.p., dark in colour.
  • Least reactive halogen.
• Tennessine (Ts) (synthetic, below astatine):
  • Likely to be metallic in nature (trend towards metallic character down the group).
  • Predicted to have very low reactivity compared to chlorine.

8. Summary Table – Group VII Trends