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Cambridge IGCSE Chemistry · 0620

Chapter 8: The Periodic Table — Part 2

Topic 8.3–8.4 · Halogens and transition elements

Group VII: The Halogens

  • Halogens (chlorine, bromine, and iodine) are diatomic non-metals, meaning they exist as molecules consisting of two atoms (Cl2, Br2, I2).
  • Appearance at r.t.p.:
    • Chlorine (Cl2): A pale yellow-green gas.
    • Bromine (Br2): A red-brown liquid.
    • Iodine (I2): A grey-black solid.
  • Trends down the group:
    • Increasing density.
    • Increasing melting and boiling points: As molecules get larger, there are stronger intermolecular forces to overcome.
    • Decreasing reactivity: Atomic radius and electron shielding increase down the group, decreasing the attraction between the nucleus and outer electrons, making it harder for the atom to gain an electron.

Exam Traps

  • Astatine is a solid at r.t.p. and less reactive than iodine — apply the trend; do not assume it is a gas.

Displacement Reactions of Halogens

  • A more reactive halogen will displace a less reactive halogen from its ionic compound in aqueous solution.
  • Observations:
    • Chlorine will displace bromine from sodium bromide: Cl2 + 2NaBr ? 2NaCl + Br2. The solution changes from colourless to orange.
    • Chlorine will displace iodine from potassium iodide: Cl2 + 2KI ? 2KCl + I2. The solution changes from colourless to brown.
  • These observations allow for the prediction of properties and reactivities of other Group VII elements like fluorine or astatine.

Exam Traps

  • Orange (bromine) and brown (iodine) are distinct observations — do not swap them in displacement questions.

Transition Elements

  • Located in the middle of the Periodic Table between Groups II and III.
  • General Properties: Transition elements are metals with high densities, high melting points, and the ability to form coloured compounds. They often act as catalysts both as elements and in compounds.
  • Variable Oxidation Numbers: Transition elements form ions with variable oxidation numbers.
    • Example: Iron can form Iron(II) (Fe2+) and Iron(III) (Fe3+) ions. The specific state is indicated by a Roman numeral in the compound name (e.g., Iron(II) sulfate).

Exam Traps

  • Do not confuse Cu2+ (blue) or Fe3+ (brown/yellow) solution colours with halogen displacement colours.

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