Question

Atomic size decreases across a period but increases down the group of a periodic table

Answer 1

Atomic size decreases across a period since the metallic nature of the element decreases and hence the nuclear pull increases and therefore the electrons are closely held to the centre of the atom which makes its size smaller.

It increases down a group since metallic nature increases and it can easily give electrons so the nuclear pull here is less, making the atom bigger down the group.