(a) Why does the size of the atoms progressively become smaller when we move from sodium (Na) to chlorine (Cl) in the third period of the periodic table?
(b) Helium and neon are unreactive gases. What, if anything, do their atoms have in common?

(a) The size of the atoms progressively becomes smaller when we move from sodium (Na) to chlorine (Cl) in the third period of the periodic table because of the increase in the atomic number of the elements as the number of electrons and protons in the atoms also increases. As the positive charge increases on the nucleus, the outermost electrons are held even tighter, and therefore the size of the atom decreases.

(b) The atoms of helium and neon have completely filled valence shells. The number of valence electrons is 8. Hence, they do not show any reactivity.