Where do we go from here? How do we combine 1s, 2s, and the different 2p atomic orbitals into MO's in larger molecules? The principles remain the same:

1. Combine the available AO's properly to produce the same number of MO's of different energies, some bonding, others antibonding.
2. Fill the MO's from the lowest energy upward with all the available electrons, two per orbital.

The immediate problem is the meaning of the word "properly" in the first step above. How are the atomic orbitals combined?
You can go a long way in MO theory with common sense and three combination principles:

1. Two AO's must be close enough in space to overlap appreciably before they can be combined into an MO.
2. The combining AO's must be of similar energy.
3. The orbitals must have the same symmetry around the bond axis.