Explain why elements in the same group of the periodic table have similar chemical properties, and describe how the reactivity of Group 1 metals changes as you go 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).

The periodic table arranges all known elements in order of increasing atomic number. Elements in the same group share certain characteristics that make them behave in chemically similar ways.

Model answer (5 marks)

Elements in the same group have the same number of electrons in their outer shell, which determines how an element reacts and what type of bonds it forms. The outer‑shell electrons are the ones that are lost or gained in reactions. In Group 1 the outer electron is in the ns¹ configuration. As you go down the group the outer electron is in a shell that is further from the nucleus and is shielded by more inner‑shell electrons. Consequently the attraction between the nucleus and the outer electron decreases, so the outer electron is lost more easily. Therefore the reactivity of Group 1 metals increases down the group.

Examiner tips

  • Use the term ‘outer‑shell electrons’ and ‘ns¹ configuration’ to show understanding of electronic structure.
  • Explain the shielding effect and distance from nucleus when describing why reactivity increases.
  • Link the loss of the outer electron to the formation of +1 ions and high reactivity.
  • Use the word ‘increases’ to match the mark scheme wording.

Common mistakes

  • Confusing the trend for reactivity with that of the halogens or other groups.
  • Failing to mention the shielding effect or the distance of the outer electron from the nucleus.
  • Using vague phrases like ‘they are similar’ without explaining the electronic reason.

Mark scheme (5 marks)

  1. Elements in the same group have the same number of electrons in their outer shell
  2. The number of outer shell electrons determines how an element reacts / what type of bonds it forms
  3. Reactivity of Group 1 metals increases going down the group
  4. As you go down Group 1, the outer electron is in a shell that is further from the nucleus
  5. The outer electron is lost more easily because the attraction between the nucleus and the outer electron decreases / the outer electron is less strongly held

Key terms in this question

group · reactivity · periodic table

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