The innermost shell in any atom can hold a maximum of only two electrons,
and the second shell can hold eight. We will defer the reasons for
this to Chapter 8, but can use the conclusions now.
Each hydrogen atom lacks one electron of having a closed inner shell,
so when the two atoms combine to form an
molecule, each atom gains an electron and satisfies its deficiency.
Helium atoms do not combine because they already have their shells
filled with two electrons. If two helium atoms were forced together,
they would have four electrons in the vicinity of the nuclei (right).
Two would be located between the nuclei and would hold the atoms
together as in .
The other two would be forced to the outside of the
molecule, away from the first two. Not only would these contribute
no screening and bonding, they would attract the nuclei and pull
them away from one another. With two electrons pulling together
and two pulling apart, there would be no net bonding, and the two
He atoms would separate.
The two electrons that would tend to hold the molecule together
are called bonding electrons, and the two that would tend
to pull the nuclei apart and rupture the molecule are antibonding