Radii of anion is more than neutral element. Here, O−2 has 10e− and F has 10e− for isoelectronic species radius increases as
e−
p
ratio increases. ∵O−2
e
p
=
10
8
∴F−
e
p
=
10
9
So, radius order is O2−>F−. On moving left to right in a period, effective nuclear charge increases as radius decreases. So, order of size is O2−>F−>O>F