Atomic Radius – IGCSE Chemistry Definition
IGCSE Chemistry definition of atomic radius: the size of an atom. Covers trends across periods and down groups, with explanations for Cambridge 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Atomic radius trends are a Supplement topic that links directly to reactivity trends in Groups I and VII. If you understand why atoms get bigger down a group and smaller across a period, you can explain virtually every periodic trend the examiner asks about.
The 0620 definition
Atomic radius is a measure of the size of an atom, typically defined as half the distance between the nuclei of two bonded atoms of the same element.
Trend across a period
Moving left to right across a period (e.g. Period 3: Na → Ar):
Atomic radius decreases.
| Element | Na | Mg | Al | Si | P | S | Cl | Ar |
|---|---|---|---|---|---|---|---|---|
| Protons | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
| Electron shells | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
All elements in Period 3 have 3 electron shells. But the number of protons increases, pulling all electrons more tightly toward the nucleus. More protons, same shielding, stronger pull — smaller atom.
Trend down a group
Moving down a group (e.g. Group I: Li → Cs):
Atomic radius increases.
| Element | Li | Na | K |
|---|---|---|---|
| Electron shells | 2 | 3 | 4 |
| Protons | 3 | 11 | 19 |
Each element has one more electron shell than the one above. Although the nuclear charge also increases, the additional shell is further from the nucleus and is shielded by inner shells. The atom is physically larger.
Why this matters for reactivity
- Group I metals (reactivity increases down): the outer electron is further from the nucleus and easier to lose
- Group VII halogens (reactivity decreases down): the outer shell is further from the nucleus and it is harder to attract an incoming electron
Atomic radius is the underlying reason for both trends.
Worked exam question
Explain why the atomic radius of sodium is larger than that of chlorine, even though both are in Period 3. [2]
Mark scheme
Sodium and chlorine have the same number of electron shells [1]; chlorine has more protons / greater nuclear charge, so the electrons are pulled closer to the nucleus / stronger attraction [1]
Common exam mistakes
- Saying atoms get bigger across a period because they have more electrons — they do have more electrons, but these are in the same shell, and the increased nuclear charge pulls them in.
- Confusing atomic radius with ionic radius. Na has a larger atomic radius than Cl, but Na⁺ is smaller than Cl⁻.
- Forgetting to mention shielding when explaining why atomic radius increases down a group.
Studying this yourself? Tutoring arrangements are normally made by a parent or guardian. Message us for the details to share with them, or send them this page.