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9701 · 1.4

Ionisation energy — practice questions

Practice and worked examples for 9701 Ionisation energy. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

Explain why the first ionisation energy of magnesium (12Mg) is greater than that of sodium (11Na).

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  1. Identify electron configurations: Na is [Ne] 3s¹ and Mg is [Ne] 3s². Both elements are in Period 3.
  2. Compare factors: Magnesium has a greater nuclear charge (+12) than sodium (+11).
  3. Shielding and Radius: Both elements have their outermost electron in the n=3 shell, so shielding from inner shells is similar. The increased nuclear charge in Mg pulls the 3s subshell slightly closer, resulting in a smaller atomic radius than Na.
  4. Conclusion: The outer electron in magnesium experiences a stronger effective nuclear charge due to more protons in the nucleus and a smaller atomic radius. This results in a stronger electrostatic attraction to the nucleus, requiring more energy to remove the electron. Therefore, Mg has a higher first ionisation energy than Na.

Worked example 2

The first five successive ionisation energies of an element X are 738, 1451, 7733, 10543, and 13630 kJ mol⁻¹. To which group of the periodic table does element X belong? Explain your answer.

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  1. Analyse the data: Look for large jumps between consecutive values.
    • From 1st to 2nd IE: 1451 - 738 = 713 kJ mol⁻¹
    • From 2nd to 3rd IE: 7733 - 1451 = 6282 kJ mol⁻¹
    • From 3rd to 4th IE: 10543 - 7733 = 2810 kJ mol⁻¹
  2. Identify the large jump: There is a very large increase in energy required between the 2nd and 3rd ionisation energies (from 1451 to 7733 kJ mol⁻¹).
  3. Interpret the jump: This large jump indicates that the third electron is being removed from a principal quantum shell that is closer to the nucleus and experiences less shielding. This means the first two electrons were in the outermost (valence) shell.
  4. Conclusion: Since there are two valence electrons, the element X belongs to Group 2 of the periodic table.
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