Explain why the second ionisation energy of sodium is significantly greater than its first ionisation energy.

IB DP Chemistry Standard Level (2023 syllabus) — S1.3 Electron configurations · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The first ionisation energy removes the single 3s electron, which is far from the nucleus and heavily shielded by the inner 1s² and 2s²2p⁶ electrons.
After this removal the ion is Na⁺ with the configuration 1s²2s²2p⁶ (neon).
The second ionisation energy must remove an electron from the 2s/2p shell, which is closer to the nucleus and experiences a higher effective nuclear charge because there is less shielding.
Consequently, much more energy is required to overcome the stronger attraction, so the second ionisation energy is significantly greater.

Examiner tips

  • Mention the 3s electron is far and shielded; state Na⁺ has a noble‑gas core; explain the increased effective nuclear charge; link this to the higher energy required.
  • Use the term ‘effective nuclear charge’ and ‘shielding’ – these are key phrases in the mark scheme.

Common mistakes

  • Saying the second electron is removed from the same 3s shell; it is actually from the 2s/2p shell.
  • Forgetting to mention the Na⁺ configuration (neon) or the role of shielding/effective nuclear charge.

Mark scheme (4 marks)

  1. The first ionisation energy involves removal of the outermost electron from the third shell (3s¹), which is far from the nucleus / well shielded.
  2. After the first ionisation, the sodium ion (Na⁺) has the electron configuration of neon / a full second shell (2s²2p⁶).
  3. The second electron must be removed from the second shell, which is closer to the nucleus and experiences less shielding / greater effective nuclear charge.
  4. Therefore, considerably more energy is required to overcome the stronger attraction between the nucleus and this inner-shell electron, making the second ionisation energy much larger.

Key terms in this question

ionisation energy

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