Now we extend these ideas to other homonuclear diatomic molecules of
Period 2 elements. The first step is to build up the molecular orbital energy-level
diagram from the valence-shell atomic orbitals provided by the atoms. Because
Period 2 atoms have 2s- and 2p-orbitals in their valence shells, we form molecular
orbitals from the overlap of these atomic orbitals. There are a total of eight atomic
orbitals (one 2s- and three 2p-orbitals on each atom), so we can expect to build
eight molecular orbitals. The two 2s-orbitals overlap to form two -orbitals, one
bonding (the 2s-orbital) and the other antibonding (the 2s*-orbital); these orbitals
resemble the 1s- and 1s*-orbitals in H2. The six 2p-orbitals (three on each
neighboring atom) form the remaining six molecular orbitals. They can overlap in
two distinct ways. The two 2p-orbitals that are directed toward each other along
the internuclear axis form a bonding -orbital (2p) and an antibonding *-orbital
(2p*) where they overlap (FIG. 4.29). The two 2p-orbitals on each atom that are
perpendicular to the internuclear axis overlap side by side to form bonding and
antibonding “-orbitals” (FIG. 4.30). A -orbital is a molecular orbital with one
nodal plane that contains the internuclear axis. There are two 2p-orbitals on each
atom perpendicular to the internuclear axis, and so four molecular orbitals—two
bonding 2p-orbitals and two antibonding 2p*-orbitals—are formed by their
overlap.