Explain why the successive ionisation energies of aluminium show two large increases: one between the third and fourth ionisation energies, and another between the tenth and eleventh ionisation energies.

IB DP Chemistry Higher 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 three electrons are removed from the outermost (n = 3) shell, which is farthest from the nucleus and heavily shielded, so the ionisation energies are relatively low.

The large jump between the 3rd and 4th ionisation energies occurs when the 4th electron is removed from the n = 2 shell. This shell is closer to the nucleus and experiences much less shielding, so a much larger amount of energy is required.

The second large increase, between the 10th and 11th ionisation energies, happens when the 11th electron is removed from the n = 1 shell. This shell is the closest to the nucleus and has essentially no shielding by inner electrons, so the energy required rises sharply.

These changes match aluminium’s configuration 1s² 2s² 2p⁶ 3s² 3p¹ (2, 8, 3), showing that successive ionisation energies reveal the existence of distinct electron shells.

Examiner tips

  • Use the command word ‘Explain’ – describe the process and give the reason for each increase.
  • Mention the shell numbers (n = 3, 2, 1) and the effect of shielding.
  • Show the link to the electron configuration 2, 8, 3.
  • Keep the answer concise – 4 points only.

Common mistakes

  • Confusing the jump between the 3rd and 4th with the 4th and 5th ionisation energy.
  • Failing to mention the shielding effect or the shell number.
  • Writing a generic statement about ‘more energy needed’ without linking to the specific shells.

Mark scheme (4 marks)

  1. The first three electrons are removed from the outer (third) shell / n = 3 shell, which is further from the nucleus and more shielded, so less energy is required.
  2. The large increase between the 3rd and 4th ionisation energies occurs because the 4th electron must be removed from the second shell (n = 2), which is closer to the nucleus and experiences less shielding, requiring significantly more energy.
  3. The second large increase, between the 10th and 11th ionisation energies, occurs because the 11th electron must be removed from the first shell (n = 1), which is closest to the nucleus and has essentially no shielding by inner electrons.
  4. The pattern is consistent with aluminium's electron configuration of 2, 8, 3 (1s² 2s² 2p⁶ 3s² 3p¹), confirming that successive ionisation energies provide evidence for the existence of distinct electron shells / energy levels.

Key terms in this question

successive ionisation energies

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