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

(N/A) The atomic radius decreases as we move from left to right in a period. This is because, as we move along a period, the nuclear charge increases while the electrons are added to the same principal energy shell. This leads to an increase in the effective nuclear charge, which pulls the electrons closer to the nucleus, thereby decreasing the atomic size.
Within a group, the atomic radius increases as we move down the group. This is because a new principal energy shell is added at each succeeding element, which increases the distance between the nucleus and the outermost electrons, outweighing the effect of increased nuclear charge.

Atom (Period $II$)	$Li, Be, B, C, N, O, F$
Atomic radius $(pm)$	$152, 111, 88, 77, 74, 66, 64$
Atom (Period $III$)	$Na, Mg, Al, Si, P, S, Cl$
Atomic radius $(pm)$	$186, 160, 143, 117, 110, 104, 99$