As you go down the group, the number of electron shells increases. As the number of shells increases, the radius increases.

Additionally, there is increased shielding from the extra electron shells between the outer shell and the nucleus. Although the nuclear charge (the number of protons) increases down the group, which should pull electrons in more strongly, the combined effect of adding an entire new electron shell and the increased shielding is far more significant.

The number of shells doesn’t change (and therefore neither does the shielding) but the number of protons in the nuclei increases. This increases the strength of the attraction between the nucleus and the outer shell electrons, decreasing the radius.

As you are adding electrons to an inner shell (3d rather than 4s), the shielding increases as you go across the period. The increase in shielding counteracts the increase in the number of protons in the nuclei.

This results in the radius not changing much without a clear pattern.

(As always with this sort of question there is a more complex answer out there – please do feel free to look it up)

Na⁺ has one fewer electron than Na. It has one fewer shell than Na, resulting in a smaller radius.

Li has a larger radius than oxygen. Oxygen has more protons in its nucleus but the same number of shells. The attraction between the nucleus and the outer shell electrons is stronger in oxygen.

Al³⁺ and Na⁺ both have the same electron configuration. They therefore have the same number of shells and amount of shielding. Al³⁺ has more protons in its nucleus, resulting in an increased electrostatic attraction between the nucleus and the outer shell electrons, resulting in a smaller radius.

H⁺ has no electrons whereas H^– has two electrons. H^– has a larger radius.

S^2- and Cl^– have the same electron configuration. They therefore have the same number of shells and amount of shielding. Cl^– has more protons in its nucleus, resulting in an increased electrostatic attraction between the nucleus and the outer shell electrons, resulting in a smaller radius.

F^– and Na⁺ have the same electron configuration. However, Na⁺ has more protons in its nucleus, resulting in an increased electrostatic attraction between the nucleus and the outer shell electrons, resulting in a smaller radius.