This molecular buildup is tabulated
above. The molecule-ion
has only one electron, which occupies the bonding
orbital by itself. This electron spends most of its time between
the nuclei, shielding them from one another and holding them together
like half of a Lewis electron-pair bond. Two electrons are present
in H and they
completely fill the bonding MO. This is the ordinary electron-pair
bond discussed in Chapter 4.
has three electrons, and since two of them fill the MO,
the third is forced into the antibonding orbital.
The molecule has two electrons holding the nuclei together, and
one pulling them apart, for a net of one bonding electron. The antibonding
electron weakens the molecule. He
would have two electrons in the bonding orbital and two in the antibonding
orbital. The push and pull of these electrons cancel, and no net
bonding is left in the He
Molecule, which therefore does not exist.