1.1.7—Trends: the variations in atomic radius

Atomic radius describes the size of an atom, while ionic radius describes the size after electrons have been gained or lost. Both are controlled by the attraction between the nucleus and the outer electron region.

Across a period, proton number increases while electrons are added to the same principal shell. Nuclear attraction therefore becomes stronger overall, so atomic radius generally decreases across the period.

Down a group, each step adds an occupied electron shell. The outer electrons are farther from the nucleus and more shielded by inner shells, so atomic radius generally increases down the group.

A positive ion is usually smaller than its atom because electron loss reduces the outer electron region; a negative ion is usually larger because added electrons increase electron–electron repulsion. Use the stated species and a consistent radius definition when comparing values.