These
look so much like slightly larger versions of the H
orbitals that it is unnecessary to draw them again. If the bond
axis is chosen as the z axis, then the two 2p
orbitals can be combined, as in the left drawing, to yield a bonding
orbital
and an antibonding orbital.
The MO's are both s, orbitals because
they are symmetrical about the bond axis. Each of the original p
orbitals had one nodal plane of zero electron probability, and these
nodes are preserved in the resulting bonding MO. When we bring density
lobes of opposite signs together in making the antibonding MO, we
introduce another change of sign, so the antibonding orbital has
three nodal surfaces, as shown. The bonding is
like the in
concentrating most of the electron probability between the nuclei,
where it helps to hold the atoms together. The antibonding resembles
the in
having most of the density on the outside of the nuclei, where it
tends to pull the atoms apart.
The 2p
and 2p
atomic orbitals are at right angles to the bond line, and can be
paired to make bonding and antibonding MO's as in the lower figure
on the preceding page. Only p
orbitals are shown, but the p
orbitals look just like them, at right angles out of the page. These
AO's lead to p MO's, with a change of
sign upon 180' rotation around the bond axis. The bonding and
orbitals preserve the nodal surface present in the original p
and porbitals,
and the antibonding
and
add an extra node that is introduced when the p's are combined with
opposite signs. }