We still have not answered the question as to why hydrogen atoms form molecules but helium atoms do not. From what has been said so far, you might expect that helium atoms would share two electron pairs to make two bonds per molecule, or perhaps to make long -He-He-He-He- chains or rings of atoms. Why doesn't this happen? To answer this question we must introduce another idea from quantum mechanics, that of electron shells.

Electrons in atoms behave as though they were grouped into levels or shells, with all electrons in one shell having approximately the same energies, but with large energy differences between shells. Each shell can hold only a certain maximum number of electrons. If one shell is filled, then an additional electron will be forced to go into a higher-energy, less-stable shell, and this electron will be lost easily during chemical reactions.

Conversely, if an atom lacks only one or two electrons to complete a shell, the atom will have a strong attraction for electrons, and can take them away from the type of atom mentioned previously. A completely filled electron shell, with no vacancies and no extra electrons outside it, is a particularly stable situation for an atom.

Not only can atoms gain and lose electrons, they can share them in covalent bonds. When they do, all the shared electrons contribute toward filling vacancies in the outer electron shell of each atom.