This
bonding-antibonding arithmetic is summarized in the table on the
previous page. We can calculate the number of electron-pair bonds
by counting the net number of bonding electrons and dividing by
two. If we refer to an electron-pair bond as a "single bond" then
H has a single
bond, and
have
one half bonds, and He
has no bond at all. The number of bonds between two atoms in a molecule
is termed its bond order: and
have
bond order 1/2, H
has bond order 1, and the C-C double bond in ethylene has bond order
2. Bond order is related to bond length and bond energy. The measured
experimental bond lengths, r, and bond energies, BE, are given below
the filling diagrams. The single bond in H
is 0.74 A long, and 103 kcal moleof
energy is required to break the bonds and turn a mole of H
molecules into H atoms. The half bonds of the two molecule-ions
re- quire approximately half as much energy to break them, and are
weak enough to allow the bond length to stretch from 0.74 Å
to a little over I Å. Simple MO theory is correct in predicting
trends in bond lengths and bond energies for these small molecules,
and this should give us confidence that the basic approach to bonding
is correct.
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