Guides
Atomic Radius Trends Across Periods and Groups
Atomic radius reflects the size of an atom and varies with nuclear charge and number of electron shells, giving a clear periodic trend.
The trends
- Across a period (left → right): decreases. Same number of shells but higher nuclear charge pulls electrons in.
- Down a group (top → bottom): increases. More electron shells.
So the radius is largest at the bottom-left (Cs, Fr) and smallest at the top-right (excluding noble gases).
Ionic radius
- A cation is smaller than its atom (loses the outer shell).
- An anion is larger than its atom (gains electrons, more repulsion).
Relation to other properties
Smaller radius usually means higher ionization energy and electronegativity and stronger non-metallic character; larger radius means electrons are lost more easily and stronger metallic character.
See it visually in the data visualizations (covalent-radius heatmap/trend, data per Cordero 2008), alongside electronegativity trends.