Question

how does the variation in atomic size occurs in a group and in a period

Answer 1

The atomic size increases from the top to the bottom in any group as a result of increases in all of the three factors. (As the number of energy levels increases, the size must increase.) Going across a period (from left to right), the number of protons increases and therefore the nuclear charge increases

Answer 2

In a period, the atomic size decreases as we move from left to right

While,

In a group, the atomic size increases as we move from top to bottom.

Hope it helps you.