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, 2pand 2patomic orbitals of two atoms. The two 2s orbitals are added and subtracted to produce bonding and antibonding MO's in the way seen previously for H

=2s+2s           =2s-2s