Why is the radius of cation less than that of parent atom?
A cation is formed by the loss of one or more electrons from the neutral gaseous atom. With the removal of one or more electrons from an atom, a magnitude of nuclear charge remains the same as that of the parent atom while the number of electron decreases. As a result, the same nuclear charge acts on the lesser number of electrons. In other words, the effective nuclear charge per electron increases and therefore, electrons are more strongly attracted and pulled towards the nucleus. This causes the decrease in the size of positive ion.
For example.
Atomic radius of Na = 154 pm (Nuclear charge = + 11 ; e = 11)
Ionic radius of Na++ = 95 pm (Nuclear charge = + 11 ; e = 10)
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