Explain why the first ionisation energy of magnesium is greater than that of both sodium and aluminium.

IB DP Chemistry Higher Level (2023 syllabus) — S3.1 The periodic table: classification of elements · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Magnesium has a greater nuclear charge than sodium, so the outer electron in magnesium is attracted more strongly to the nucleus.

Sodium and magnesium have the same number of inner shells/shielding, so the effective nuclear charge experienced by the outer electron is greater in magnesium.

Aluminium’s outer electron is in a 3p subshell, which is at higher energy/further from the nucleus than magnesium’s 3s subshell.

Therefore, less energy is required to remove the 3p electron from aluminium than to remove a 3s electron from magnesium.

Examiner tips

  • Use the term ‘effective nuclear charge’ and explain shielding. Include the subshell difference for Al. Keep the answer concise and point‑wise.

Common mistakes

  • Confusing ionisation energy with ionisation potential. Forgetting to mention the 3p vs 3s subshell difference for aluminium.

Mark scheme (4 marks)

  1. Magnesium has a greater nuclear charge than sodium, so the outer electron in magnesium is attracted more strongly to the nucleus.
  2. Sodium and magnesium have the same number of inner shells / shielding, so the effective nuclear charge experienced by the outer electron is greater in magnesium.
  3. Aluminium's outer electron is in a 3p subshell, which is at higher energy / further from the nucleus than magnesium's 3s subshell.
  4. Therefore, less energy is required to remove the 3p electron from aluminium than to remove a 3s electron from magnesium.

Key terms in this question

first ionisation energy

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