These two concise equations
can be translated into words as
I: A sigma bonding MO from s atomic orbitals is obtained by adding
the 1s AO's from atoms A and B.
II: A sigma antibonding MO from s atomic orbitals is obtained by
subtracting the 1s orbitals, or combining the wave functions with
Together they can hold a maximum of four electrons. By adding one
electron at a time we can build up the four molecules or molecule-ions
The energy levels of these two MO's are shown schematically. The
bonding MO is more stable than the original 1s atomic orbitals,
and the antibonding MO is less stable; so in effect the original
energy level of the 1s AO's is split into two levels, one lower
and one higher. These energy levels now can be occupied by as many
electrons as are available in a molecule, just as the atomic energy
levels were occupied in Chapter 8.