Question

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

Answer 1

While going along the period the number of electrons increases for the same number of shells so the effective nuclear charge increases in the elements and as a result the outermost electron will be more strongly attached to the central nucleus. This decreases the radius of the elements that go from left to right.
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 in the distance between the nucleus and valence electrons.