Question

Explain why cation is smaller and anion is larger than their parent atoms

Answer 1

The effective nuclear charge is more in cation and less in anion when compared to parent atom.
Size of atom (or) α (ion) α 1/ effective nuclear charge
so the size of cation is smaller and anion is larger when compared to neutral atom.

Answer 2

A cation is formed by loss of electron(s).Hence after losing its electon the cation has more protons than electrons i.e., it has more positive charge.Due to this excess positive charge the electrons of the outer orbital are pulled towards the nucleus leading to decrease in size. In case of anions, due to gain of electrons negative charge is more than the positive charge due to which the size of the ion increases. Hope I helped you :)