Energy continues to decrease
as the atoms come closer and the screening of nuclear charges by
electrons increases. If the process is carried too far, however,
the electrons are "squeezed out" from between the nuclei, which
have come so close together that the repulsion between their positive
charges becomes quite strong. The molecule is made less stable.
At some intermediate point, screening by electrons and repulsion
of nuclei will balance: The H-H molecule will have the lowest energy
and will be most stable. If the nuclei are pushed any closer, nuclear
repulsion pushes them back again; if they are pulled apart, electron-pair
screening is lost.
This lowest-energy separation, ,
is the bond length of the H-H bond, and the energy required
to pull the molecule apart into isolated atoms again, ,
is the bond dissociation energy or bond energy. The atoms
in a molecule oscillate about this minimum-energy position; thus
is the average
bond length. In the H-H or ,
molecule this distance is 0.74 .
G. N. Lewis symbolized an electron-pair bond by two dots for the
electrons. It is more common today to represent the bond by a single
line connecting the bonded atoms, but you should remember that each
such bond consists of a pair of electrons.