Question

Why do metals have large atomic radii as compared to non metals?

Answer 1

Considering the ionic charges of metals they all have +ve charges in ion form. This means, they tend to lose electrons,wich in turn means that they have electrons so far from the nucleus that its preferable to lose some rather than gain some. So electrons far from nucleus means the atomic radii will be large for metals.

Non metals on the other hand tend to have a-ve charge in ion form. Theyare hungry for some electron(which also explains the increasing electronegativity) So much so that ,they even form compounds among themselves just to share some electrons.

This can also be explained as each row in the periodic table corresponds to electrons filling orbitals with approximately same energy as each other. However as we go across each row atomic number or number of protons increases with the number of electrons.
Protons are positively charged and so exert an attractive force on the electrons. More positive charge induces greater attraction of any electrons in orbitals of similar level. And a bigger force of attraction is able to pull the electrons in closer to the nucleus which reduces radii.

Atomic radii changes in a periodic table

Atomic radii decreases as one moves from left to right,across the periodic table. Each next element has one more electron and one more proton. These new electron are at the similar distances to the nucleus. They are also withdrawed to the nucleus because of the postive charge of it. As a result atomic radii are smaller. E.g Calcium has smaller atomic radius than potassium.

As one moves from top to bottom of periodic table atomic radii increases because new electron orbitals are added E.g Potassium has a bigger atomic radius than sodium.