An s and a p orbital as in (c)
cannot be combined, because they have different bond-axis symmetries.
Any positive overlap between the s and the upper lobe of the p would
be canceled exactly by negative overlap between s and the bottom
lobe, and nothing would result. If the p orbital is turned 90 degrees
so one lobe points toward s along the bond axis as in (d), then
both AO's have s symmetry along the bond axis. They can be combined
without canceling, provided the similar-energy requirement is met.
With these three principles, we
are now ready to build MO's from the 2s, 2p,
orbitals of two atoms. The two 2s orbitals are added and subtracted
to produce bonding and antibonding MO's in the way seen previously