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.