How do atomic radius vary in a period and in a group? How do you explain the variation.

Explanation

Within a group atomic radius increases down the group.
Reason: This is due to continuous increases in the number of electronic shells or orbit numbers in the structure of atoms of the elements down a group.
Variation across period:
Atomic radii: From left to right across a period atomic radii generally decreases due to increase in effective nuclear charge from left to right across a period.