Question

how and why does the atomic size vary as you go:
1. from left and right along a period ?
2. down a group?

Answer 1

accross the period-nuclear charge increases
down the group-no of shells increase

Answer 2

When we move fron left to right in periodic table no. of shells remain same but the no. of electron increases due to which the size of an atom decreases.

When we move down a group the no. of shells goes on increases and hence the size of atom also increases