Question

In a period from sodium to chlorine which element has the largest atomic radius

Answer 1

From left to right across a period, more protons are being added to the nucleus, but the electrons which are being added are being added to the valence shell, not to the lower energy levels.  As more protons are added to the nucleus, the electrons in the valence shell feel a higher effective nuclear charge — the sum of the charges on the protons in the nucleus and the charges on the inner, core electrons.  (See figure below.)  The valence electrons are therefore held more tightly, and the size of the atom contracts across a period.

Hence Sodium will have large radius than chlorine.