Monatomic cations and anions each have a different number of electrons than the corresponding atom. Their size relative to the atom depends on whether they have more or fewer electrons.
Monatomic cations are smaller than the corresponding atom.
Monatomic anions are larger than the corresponding atom.
For a given row in the periodic table,
Monatomic cations are smaller than the corresponding atom.
Cations have fewer electrons than the corresponding atom . Each cation in the diagram has one fewer electron shells than the corresponding atom.
Monatomic anions are larger than the corresponding atom.
Anions have more electrons in their outer shells than the corresponding atom. The increased size is due to additional repulsive forces between the outer electrons.
The sizes of monatomic ions relative to one another depend both on the number of occupied electron shells and their nuclear charge.For a given row in the periodic table,
the cations are smaller than the anions.
For a given group in the periodic tablethe size of its monatomic ions increases going down the group because each successive member of the group has more electron shells occupied.