Physical properties of the Group 17 elements

AS Level Chemistry · Group 17. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The halogens exist as diatomic molecules, X2. The molecules are non-polar, so the only forces between them are instantaneous dipole–induced dipole (id-id) forces. Down the group each molecule has more electrons, so these forces get stronger. More energy is needed to separate the molecules, and volatility decreases: chlorine is a gas, bromine a liquid and iodine a solid at room temperature.

Bond strength is a different idea. It is about the covalent bond inside one molecule. From chlorine to iodine the atoms get larger, so the shared pair of electrons is further from both nuclei and more shielded. The X–X bond becomes longer and weaker.

Rules to remember

Common mistake

Students often explain boiling points using bond strength. Boiling separates whole X2 molecules, so the explanation must use the forces between molecules, not the covalent bond inside them.

Worked example

The Br–Br bond is weaker than the Cl–Cl bond, yet bromine has a higher boiling point than chlorine. Explain why.

  1. Boiling does not break the X–X bond; it separates X2 molecules from each other.
  2. Br2 has 2 × 35 = 70 electrons, but Cl2 has only 2 × 17 = 34 electrons.
  3. More electrons give larger temporary dipoles, so the id-id forces between Br2 molecules are stronger.
  4. More energy is needed to overcome these forces, so the boiling point is higher.

Answer: Bromine has stronger id-id forces between its molecules because each molecule has more electrons; the bond energy does not affect the boiling point.

Practice questions

Try each one, then open the answer.

1. Which row correctly gives the appearance of the halogens at room temperature?

  1. A
    chlorine: yellow-green gas; bromine: red-brown gas; iodine: purple liquid
  2. B
    chlorine: yellow-green liquid; bromine: red-brown liquid; iodine: grey-black solid
  3. C
    chlorine: colourless gas; bromine: red-brown liquid; iodine: purple solid
  4. D
    chlorine: yellow-green gas; bromine: red-brown liquid; iodine: grey-black solid
Show answer

Answer: D. Volatility decreases down the group: gas, liquid, solid. Iodine is a grey-black solid that gives a purple vapour on warming; the vapour, not the solid, is purple.

2. Why does fluorine have a lower boiling point than chlorine?

  1. A
    the F–F bond is weaker than the Cl–Cl bond
  2. B
    F2 molecules are polar
  3. C
    F2 molecules have fewer electrons, so their instantaneous dipole–induced dipole forces are weaker
  4. D
    fluorine is more electronegative than chlorine
Show answer

Answer: C. Boiling separates molecules, not atoms. F2 has 18 electrons compared with 34 in Cl2, so the temporary dipoles, and hence the id-id forces, are smaller.

3. How does volatility change from chlorine to bromine to iodine?

  1. A
    it decreases
  2. B
    it increases
  3. C
    it stays the same
  4. D
    it increases, then decreases
Show answer

Answer: A. Chlorine is a gas, bromine a liquid and iodine a solid at room temperature, so volatility decreases down the group as the intermolecular forces get stronger.

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