Section 1.6 An Introduction to Molecular Orbital Theory 23nodesnode nodeσ∗ antibonding molecular orbitalσ bonding molecular orbital2 p atomic
orbital2 p atomic
orbitalEnergy>Figure 1.5
End-on overlap of two porbitals to
form a bonding molecular orbital
and a antibonding molecular
orbital.s*sSide-to-side overlap of two patomic or-
bitals forms a bond. All other cova-
lent bonds in organic molecules are
bonds.SPformed. The antibonding molecular orbital has threenodes. (Notice that after each
node, the phase of the molecular orbital changes.)
Unlike the bond formed as a result of end-on overlap, side-to-side overlap of two
patomic orbitals forms a pi bond(Figure 1.6). Side-to-side overlap of two in-
phase patomic orbitals forms a bonding molecular orbital, whereas side-to-side
overlap of two out-of-phase porbitals forms a antibonding molecular orbital. The
bonding molecular orbital has one node—a nodal plane that passes through both nu-
clei. The antibonding molecular orbital has two nodal planes. Notice that bonds
are cylindrically symmetrical, but bonds are not.
The extent of overlap is greater when porbitals overlap end-on than when they
overlap side-to-side. This means that a bond formed by the end-on overlap of por-
bitals is stronger than a bond formed by the side-to-side overlap of porbitals. It also
means that a bonding molecular orbital is more stable than a bonding molecular
orbital because the stronger the bond, the more stable it is. Figure 1.7 shows a molec-
ular orbital diagram of two identical atoms using their three degenerate atomic orbitals
to form three bonds—one bond and two bonds.s p
s ppspp* sp
p*p1 P 2ss*A bond is stronger than a S Pbond.π∗ antibonding molecular orbitalπ bonding molecular orbital2 p atomic
orbital2 p atomic
orbitalnodal planenodal planenodal planeEnergy>Figure 1.6
Side-to-side overlap of two parallel
porbitals to form a bonding
molecular orbital and a
antibonding molecular orbital.p*p