# Explain why the atomic radius of potassium is greater than that of both sodium and chlorine, with reference to nuclear charge and electron shielding.

> IB DP Chemistry Higher Level (2023 syllabus) — S3.1 The periodic table: classification of elements · Explain · 4 marks

## Mark scheme (4 marks)

1. Potassium has more electron shells (energy levels) than sodium, so valence electrons are further from the nucleus.
2. Greater shielding by inner electrons in potassium reduces the effective nuclear charge experienced by the valence electron, increasing atomic radius compared to sodium.
3. Potassium and chlorine are in the same period (period 3 / period 4 comparison is not relevant here — both K and Cl are compared: chlorine has a greater nuclear charge than potassium for the same principal quantum shell would be incorrect; the correct point is that chlorine has a much higher nuclear charge than potassium while both have valence electrons in period 3 — actually K is period 4 and Cl is period 3, so K is larger due to the additional shell, but comparing within-period: chlorine has more protons than sodium with the same number of shells, so greater nuclear charge pulls electrons closer).

Corrected point: Chlorine has a higher nuclear charge (17 protons) than potassium's valence shell shielding scenario — the key point is that chlorine's valence electrons are in the same shell (n = 3) as sodium but experience a greater effective nuclear charge (more protons, similar shielding), pulling electrons closer and reducing radius.
4. Therefore potassium has the largest atomic radius of the three because the additional electron shell effect outweighs nuclear charge, while chlorine's high effective nuclear charge contracts its electron cloud more than sodium's.

## Key terms

- [atomic radius](https://www.gradenine.co.uk/glossary/atomic-radius)
- [nuclear charge](https://www.gradenine.co.uk/glossary/nuclear-charge)
- [electron shielding](https://www.gradenine.co.uk/glossary/electron-shielding)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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