Explain why the third ionisation energy of magnesium is significantly greater than its second ionisation energy, whilst the successive ionisation energies from the first to the second are relatively close together.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The first and second ionisation energies involve removal of electrons from the same outer (3s) subshell, so the increase is relatively small.
The third electron must be removed from a full inner shell (2p subshell, n=2), which is closer to the nucleus.
Electrons in a lower principal quantum level (n=2) experience less shielding from inner electrons and are more strongly attracted to the nucleus.
Therefore significantly more energy is required to remove the third electron, causing the large jump in ionisation energy.
The third electron must be removed from a full inner shell (2p subshell, n=2), which is closer to the nucleus.
Electrons in a lower principal quantum level (n=2) experience less shielding from inner electrons and are more strongly attracted to the nucleus.
Therefore significantly more energy is required to remove the third electron, causing the large jump in ionisation energy.
Examiner tips
- Mention the same outer subshell for the first two ionisations
- Explain the inner shell removal and shielding effect
- Use the term ‘large jump’ to highlight the significant increase
Common mistakes
- Confusing the third ionisation with the removal of a 3s electron
- Ignoring the shielding effect of inner electrons
- Using vague terms like ‘more energy’ without specifying the shell change
Mark scheme (4 marks)
- The first and second ionisation energies involve removal of electrons from the same outer (3s) subshell/shell, so the increase is relatively small.
- The third electron must be removed from a full inner shell (2p subshell / n = 2 shell), which is closer to the nucleus.
- Electrons in a lower principal quantum level (n = 2) experience less shielding from inner electrons / are more strongly attracted to the nucleus.
- Therefore significantly more energy is required to remove the third electron, causing the large jump in ionisation energy.
Key terms in this question
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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