Explain why the first ionisation energy of calcium is greater than that of potassium, yet the second ionisation energy of potassium is greater than the second ionisation energy of calcium.

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)

1. Calcium has a greater nuclear charge (20 p⁺) than potassium (19 p⁺), so the outermost 4s electron in Ca experiences a larger effective nuclear charge.
2. Both Ca and K have their outermost electrons in the 4s subshell, so the shielding from inner electrons is essentially the same.
3. The larger effective nuclear charge in Ca therefore makes the first ionisation energy of Ca higher than that of K.
4. The second ionisation energy of K is very large because the second electron removed is from the 3p subshell, a full inner shell that is closer to the nucleus and more strongly attracted.
5. In Ca the second electron removed is also from the 4s subshell, so it is at the same energy level and shielded similarly; hence the second IE of Ca is only slightly higher than its first IE.

Examiner tips

  • Use the term ‘effective nuclear charge’ when comparing first IE. Explain that both elements have their outer electrons in 4s to justify similar shielding. Highlight that the second IE of K involves removing an electron from a filled 3p shell. Show the contrast that Ca’s second IE removes a 4s electron, not a 3p electron.

Common mistakes

  • Confusing the order of electron removal (e.g. saying K’s second IE removes a 4s electron). Failing to mention that both elements have their outer electrons in the same shell for the first IE. Using vague terms like ‘more attraction’ without specifying effective nuclear charge or shielding.

Mark scheme (4 marks)

  1. Calcium has a greater nuclear charge / more protons than potassium, so its outermost electron experiences a greater effective nuclear charge / attraction to the nucleus
  2. Both outermost electrons are in the same shell (4s), so shielding is similar, meaning the increased nuclear charge in calcium results in a higher first ionisation energy
  3. The second ionisation energy of potassium is very large because the second electron is removed from a full inner shell (3p), which is at a lower energy level and much closer to / more strongly attracted by the nucleus
  4. Calcium's second electron is also removed from the 4s sub-shell, so it is at the same energy level and similarly shielded, meaning the second IE of calcium is only slightly higher than its first IE

Key terms in this question

first ionisation energy · second ionisation energy

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