Atomic Radius and Periodic Trends

Atomic radius describes the effective size of an atom. For main-group elements, it generally decreases across a period as nuclear charge increases while inner-shell shielding changes relatively little. It generally increases down a group as additional occupied electron shells place outer electrons farther from the nucleus.

Comparisons must use the same definition of radius: noble-gas van der Waals radii should not be directly compared with covalent radii. Changes across a transition series are usually smaller than those across the main-group elements.

Key terms

• atomic radius

• effective nuclear charge

• shielding