How does the ionic radius of a nonmetal compare with its atomic radius?

The ionic radius of a nonmetal is greater than its atomic radius.

When you add an electron to a nonmetal, you are changing two properties:

1. The added electron is slightly more shielded by the electrons already there.

For example, the effective nuclear charge (##Z_"eff"##) experienced by a ##"2p"## electron in ##"F"## is +5.2.

For a ##"2p"## electron in ##"F"^"-"##, ##Z_"eff" = 4.85##.

This decreased attraction should cause an increase in the ionic radius.

2. The added electron increases electron repulsions.

These should also increase the ionic radius.

Which is the bigger effect?

The atomic radius of ##"F"## is 72 pm and the ionic radius of ##"F"^"-"## is 133 pm.

(From www.chem.uwec.edu)

The near-doubling of the radius cannot be explained by the relatively small increase in nuclear shielding.

Rather, the increased size results from the increased electron repulsions caused by the added electron.