Chlorine, bromine and iodine are all elements in Group 7 of the periodic table. Explain why these three elements have similar chemical properties, and describe how their reactivity changes going down the group.

Eduqas A-Level Chemistry — 2.2 Group 1, 2 and 7 · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Elements in the same group have the same number of electrons in their outer shell. The outer‑shell electrons determine the chemical properties and reactivity of the element. Therefore chlorine, bromine and iodine all have similar chemical behaviour. Reactivity decreases down the group because the atoms have more electron shells. The outer shell is further from the nucleus, so it is harder for the atom to attract an additional electron, making the atom less reactive.

Examiner tips

  • Use the term ‘outer‑shell electrons’ to link to the mark scheme.
  • State that reactivity decreases down the group and give the reason – more shells, outer electron further from nucleus.

Common mistakes

  • Failing to mention that the outer‑shell electrons are the same in the group.
  • Not explaining why reactivity decreases – e.g. ignoring the increased distance from the nucleus or shielding effect.

Mark scheme (5 marks)

  1. Elements in the same group have the same number of electrons in their outer shell
  2. It is the outer shell electrons that determine chemical properties / reactivity
  3. Reactivity decreases going down Group 7 (from chlorine to iodine)
  4. Going down the group, atoms have more electron shells
  5. The outer shell is further from the nucleus so it is harder to attract an additional electron, making the atom less reactive

Key terms in this question

Group 7 · reactivity · periodic table

Related

More Group 1, 2 and 7 questions

▶ Try answering this question with AI marking (free) →