• When two hydrogen 1s orbitals overlap out of phase with each other, an σ* antibonding molecular orbital results. When the two 1s orbitals have opposite signs, they tend to cancel out where they overlap. The result is a node that separates the two atoms (Figure 3).

• The relative energies of the atomic orbitals and the molecular orbitals of the system are shown in the molecular orbital diagram (Figure 4). In the MO diagram, the energies are represented as horizontal lines; the bottom line is the lowest energy level, the top line the highest energy level. When the 1s orbitals are in phase, the resulting molecular orbital is an σ -bonding molecular orbital which is lower in energy than that of a 1s atomic orbital.

Figure 3

Figure 4