CHEM 121 glossary
Ionic radius
The size of an ion in an ionic solid. Cations are smaller than their parent atoms (they usually lose a whole shell) and anions are larger (extra repulsion, no extra protons); in an isoelectronic series, more protons means a smaller ion.
Ten electrons each; radius falls as the proton count rises from 8 to 12.
Example
Sodium shrinks from 186 pm to 102 pm on becoming ; chlorine grows from 99 pm to 181 pm on becoming .