# 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

## 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

- [first ionisation energy](https://www.gradenine.co.uk/glossary/first-ionisation-energy)
- [second ionisation energy](https://www.gradenine.co.uk/glossary/second-ionisation-energy)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-first-ionisation-energy-21a13424) · Published by Druglandscape Ltd.