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

(N/A) Atomic radius generally decreases from left to right across a period. This is because within a period,the outer electrons are present in the same valence shell and the atomic number increases from left to right,resulting in an increased effective nuclear charge. As a result,the attraction of electrons to the nucleus increases.
On the other hand,the atomic radius generally increases down a group. This is because down a group,the principal quantum number $(n)$ increases,which results in an increase of the distance between the nucleus and valence electrons.