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) On moving from sodium to chlorine in the third period, the size of the atoms decreases because of the increase in the atomic number of the elements. 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 size of the atom decreases.

(b) The atoms of helium and neon have their valence shell completely filled with electrons. The number of valence electron is 8. So they do not show any reactivity.