# Atomic and ionic radius

Atomic radius shrinks across a period because Zeff grows, and grows down a group because new shells are added. A cation is smaller than its atom. An anion is larger than its atom.

Chemistry (university) · Part 01: Atomic structure and periodic trends · Card 9 of 10 · Last updated 2026-10-19

## Analogy

Like a balloon pulled tighter by a stronger magnet in the middle, or given an extra layer of wrapping.

## Example

**Isoelectronic ions**

O²⁻, F⁻, Na⁺, Mg²⁺ all have 10 electrons.
More protons pull harder.
Size: O²⁻ > F⁻ > Na⁺ > Mg²⁺

## In practice

Radius trends help predict which ion fits in a crystal and why Na⁺ is much smaller than K⁺.

## Practice

### Flashcards

- **Which is larger, Cl or Cl⁻?** Cl⁻. Extra electrons repel each other and the cloud expands.

### Interview questions

**Explain why atomic radius decreases across a period but increases down a group.**

- Across a period: protons increase while electrons go into the same shell
- Zeff rises, so the pull on outer electrons grows
- Down a group: extra shells are added
- Outer electrons are further away and more shielded

## Questions people ask

### Why is a cation smaller than its neutral atom?

Losing electrons often removes the whole outer shell, and the remaining electrons are pulled in by the same nuclear charge. With fewer electrons repelling each other, the pull per electron is stronger. The ion shrinks, sometimes by more than half the atom's radius.

### Why is an anion larger than its neutral atom?

An anion has extra electrons but the same number of protons. The added electrons repel the others, and the nucleus's pull is shared across more electrons. The cloud spreads out, so the ion is bigger than the atom it came from.

## Related concepts

- [Effective nuclear charge](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-1/chemistry-university-part-1-concept-8)
- [Ionization energy](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-1/chemistry-university-part-1-concept-10)

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