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

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)

Magnesium has the configuration [Ne]3s², so its outermost electron is in a 3s orbital. Aluminium is [Ne]3s²3p¹, so its outermost electron is in a 3p orbital.

The 3p subshell lies higher in energy and further from the nucleus than the 3s subshell. Consequently the 3p electron in aluminium is more shielded by the two 3s electrons.

Thus the effective nuclear charge felt by the outer electron is lower in aluminium than in magnesium.

Because a lower effective nuclear charge means the outer electron is held less tightly, less energy is required to remove it from aluminium. Therefore magnesium’s first ionisation energy is greater than aluminium’s.

Examiner tips

  • State the electron configurations and identify the subshell of the outer electron. Explain the shielding effect and effective nuclear charge. Relate this to the ionisation energy. Use correct terminology (subshell, shielding, effective nuclear charge).
  • Show the logical sequence: subshell → shielding → effective charge → ionisation energy.
  • Use the exact wording from the mark scheme where possible.

Common mistakes

  • Confusing the order of removal (removing a 3s electron from Al instead of 3p).
  • Failing to mention shielding or effective nuclear charge.

Mark scheme (4 marks)

  1. Magnesium's outer electron is removed from a 3s subshell/orbital, whereas aluminium's outer electron is removed from a 3p subshell/orbital.
  2. The 3p subshell is higher in energy / further from the nucleus than the 3s subshell, so the 3p electron in aluminium experiences greater shielding (from the 3s electrons).
  3. Therefore the effective nuclear charge experienced by the outermost electron is lower in aluminium than in magnesium.
  4. Consequently, less energy is required to remove the outermost electron from aluminium than from magnesium, so Al has a lower first ionisation energy.

Key terms in this question

first ionisation energy

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