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 opposite signs.

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 mentioned previously.

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.