Fluorine, chlorine, bromine, and iodine are all non-metals found in Group 7 of the periodic table. As you move down Group 7, the elements change from gases to liquids to solids at room temperature, and their reactivity decreases. Explain, in terms of atomic structure and electron arrangement, why the reactivity of the Group 7 elements decreases going down the group.

OCR A-Level Chemistry B: Salters (H433) — 3.1 The periodic table and periodicity · Explain · 5 marks · View as Markdown

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

Group 7 elements react by gaining one electron to form a negatively charged ion with a stable electron arrangement. Fluorine is the most reactive halogen, and reactivity decreases going down the group from fluorine to iodine.

Model answer (5 marks)

As you move down Group 7 each element has an extra electron shell.
The valence electron that is gained is therefore further from the nucleus.
Inner shells shield the nucleus more strongly, reducing the effective nuclear charge on the outer electron.
The attractive force between the nucleus and an incoming electron therefore decreases.
Because the atom finds it harder to gain an electron, the reactivity of the Group 7 elements decreases down the group.

Examiner tips

  • Use the word ‘shielding’ and ‘effective nuclear charge’ to show understanding of atomic structure.
  • Show the chain of reasoning: more shells → further distance → more shielding → weaker attraction → less reactivity.
  • Mention the change in physical state to illustrate the trend, but keep the focus on electron arrangement.

Common mistakes

  • Confusing the trend for metals (reactivity increases down a group).
  • Forgetting to mention the increase in shielding or the decrease in effective nuclear charge.
  • Writing that reactivity increases because the outer electron is further away, which is the opposite of the correct explanation.

Mark scheme (5 marks)

  1. Going down Group 7, each successive element has one more electron shell (energy level)
  2. The outer shell (where the electron is gained) is further from the nucleus going down the group
  3. There is greater shielding of the nucleus by inner electron shells going down the group
  4. The attractive force of the nucleus on an incoming electron decreases going down the group
  5. Therefore the atom finds it harder to gain an electron, so reactivity decreases going down the group

Key terms in this question

Group 7 · reactivity

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