Question

The size of an atom is increases down a group and decrease along the period

Answer 1

The number of energy levels increases as you move down a group as the number of electrons increases. Each subsequent energy level is further from the nucleus than the last. Therefore, the atomic radius increases as the group and energy levels increase.

In other words, the number of shells increases as we go down the group. The outermost electrons are repelled by the inner shell electrons (Screening effect) and hence, the atomic size increases.