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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.
Summary of halogen appearances and trends down Group VII with chlorine, bromine and iodine displacement reactions
Diagram 1: Group VII trends and halogen displacement reactions

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).

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