Group VII properties

O Level / IGCSE Chemistry · The Periodic Table. 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

Chlorine, bromine and iodine are the halogens of Group VII. They are non-metals that exist as diatomic molecules: Cl2, Br2 and I2. Each atom has seven outer-shell electrons and gains one more to form a 1− halide ion.

Going down the group the colour gets darker, the density increases, the melting and boiling points rise and the reactivity decreases. The reactivity trend is the opposite of the trend in Group I.

A more reactive halogen takes the place of a less reactive one in a solution of its halide. This is a displacement reaction. It is also a redox reaction, because electrons pass from the halide ions to the halogen.

Rules to remember

Common mistake

Students write that chlorine displaces bromide, or name the product as a halide. The halogen element displaces the other halogen element: chlorine displaces bromine from potassium bromide.

Worked example

Bromine water is added to aqueous potassium chloride and, separately, to aqueous potassium iodide. In which tube is there a reaction, and what is seen?

  1. Reactivity order is chlorine > bromine > iodine.
  2. Bromine is less reactive than chlorine, so it cannot displace chlorine: no reaction with potassium chloride.
  3. Bromine is more reactive than iodine, so it displaces iodine: Br2 + 2I− → 2Br− + I2.

Answer: Only the potassium iodide tube reacts. The solution turns brown as iodine forms. The potassium chloride tube just stays the orange colour of the bromine water.

Practice questions

Try each one, then open the answer.

1. A halogen melts at −101 °C and boils at −34 °C. What is its appearance at room temperature and pressure?

  1. A
    a red-brown liquid with an irritating vapour
  2. B
    a grey-black solid that gives a purple vapour
  3. C
    a colourless gas that is very unreactive
  4. D
    a pale yellow-green gas
Show answer

Answer: D. At room temperature (about 25 °C) the halogen is above its boiling point, so it is a gas. The halogen that is a pale yellow-green gas is chlorine. Bromine is a red-brown liquid and iodine is a grey-black solid.

2. Which mixture would react?

  1. A
    chlorine and aqueous potassium iodide
  2. B
    bromine and aqueous potassium chloride
  3. C
    iodine and aqueous sodium chloride
  4. D
    iodine and aqueous potassium bromide
Show answer

Answer: A. A halogen displaces a less reactive halogen from its halide. Chlorine is more reactive than iodine, so it displaces it; the other halogens are less reactive than the halide present.

3. The boiling points of chlorine, bromine and iodine are −34 °C, 59 °C and 184 °C. Fluorine is above chlorine in Group VII. What is the most likely boiling point of fluorine?

  1. A
    −188 °C
  2. B
    −34 °C
  3. C
    +25 °C
  4. D
    +100 °C
Show answer

Answer: A. Boiling point increases down the group, so fluorine must boil well below −34 °C. Only −188 °C fits.

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