Question

Why cations are always smaller than their parent atoms whereas anions are larger than their parent atom expiain with example?

Answer 1

in case of cations they have comparatively less electrons than their parent atoms keeping the nuclear change same... therefore the electrons are held more tightly by the protons in the nucleus nd their atomic radii become smaller than their parent atoms.....
in case of anions they have comparatively more atoms than their parent atoms keeping the nuclear charge same ..and therefore the electrons r held more loosely by the protons in the nucleus and that's why the atomic radii is comparatively larger than the parent atoms...

hope it helps... :)

Answer 2

Cation is formed by lose of electron, but the charge on nucleus is the same, therefore the effective nuclear charge is greater therefore the size decreases. Anion is formed by addition of electron and the force of attraction of nucleus on the electrons will be lesser, therefore the size increases than parent atom.