Question

Why the size of cation is smaller than parent atom?and size of anion is larger than parent atom?

Answer 1

A cation is always formed by the loss of an electron in its outer most shell. But the number of protons in the nucleus stays the same. Thus the force of attraction on the outer most shell increases which leads to a decrease in size.

On the other hand, an anion is formed by the gain of electrons but again, the number of protons stays the same, because of this the force of attraction between the nucleus and the outer most shell decreases. This leads to an increase in size.