9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     Larger Diatomic Molecules

This principle can be used to draw an energy-level diagram for the molecular orbitals. The first consideration is the relative energies of the AO's from which the MO's are made. Then, among MO's built from AO's of the same energy, the relative order is dictated by the increasing number of nodes. Both the bonding and antibonding MO's from the 2s AO's are lower in energy than any of the 2p-derived orbitals, but among these latter, the node-counting rule is useful to determine relative energies. The and bonding orbitals have a single node and the lowest energy, followed by the bonding orbital, which has two nodes. All of these orbitals are more stable than the 2p AO's that led to them. Among the antibonding orbitals, the and which have two nodes, are next, and the , which has three nodes, has the highest energy of all. These energies are diagramed at the left, with each two-electron orbital being represented by a colored circle. It is worth emphasizing here that "bonding" and "antibonding" only imply that the MO is more, or less, stable than the AO's from which it arose, not that it is of low energy or high energy on an absolute basis. The antibonding orbital has lower energy than the bonding orbitals derived from 2p; but it still is antibonding because if the atoms were pulled apart, the electrons in would drop down to the less energetic 2s atomic orbitals.

  Page 15 of 67 HomeGlossary