The atomic radius is the distance between the nucleus to the boundary of the surrounding cloud of electrons.

Atomic radii vary in a predictable and explicable manner across the periodic table. The radius increases moving down a group due to the addition of a new energy level or shell.

The atomic radius generally decrease along each period of the table, because although more electrons are being added to atoms, they are at similar distances to the nucleus (same shell), and the increasing nuclear charge “pulls” the electron clouds inwards, making the atomic radii smaller.

The noble gases have full valence electron shells, corresponding to an electron configuration s2 p6, making them very stable and not following the exact behavior than other atoms in the neighborhood.