Question

A helium atom has a smaller atomic radius than the hydrogen atom. Explain why.

Answer 1

Hydrogen has one proton and one electron, whereas Helium has two (protons and electrons). But they are both in the same period (energy level) so their average distance from the nucleus is the same. As you travel along a period from left to right, the atomic number increases, but the valence electrons remain in the same shell. Therefore due to the increasing nuclear charge (pulling electrons closer to the nucleus) the atomic radius decreases from left to right. Therefore He has a smaller atomic radius than H.