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
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
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.