Why is the correct option O^2-, aren't all of them equal since they all have the same number of energy shells and same number of electrons in them
F-: There is only one electron added, there is inter-electronic repulsion due to which the radius increases
Mg2+: Two electrons are removed, but the number of protons remain the same, so nucleus pulls the electrons towards itself, and the radius decreases slightly
Na+: An electron is removed, so there is more effective nuclear charge, and pulls electrons towards nucleus, so again radius decreases.
O2-: Two electrons are added there is more inter-electronic repulsion, so the radius increases (and it is more than F-, because here two electrons are added but in F- only one electron was added)
In these questions where we compare the radius of isoelectronic species, i.e. species with same number of electrons, we compare the ratio of Nuclear charge (Z) to the number of electrons(e) ratio.
If Z/e ratio increases the radius decreases,and if ratio decreases radius increases.