134
Sizes and Shapes of Molecules
bonds to be located between any specific atoms. How should this situation be
described? Pauling suggested that the actual state of the molecule is a resonance
hybrid of all the separate forms that can be written in the classical way. The
principal resonance forms for our three examples would be those shown in
Figure 9-21.
It must be emphasized that none of these resonance forms actually exists, but
the superposition of all of them for a given molecule serves as a good representation, and shows that each bond possesses both single-bond and double-bond
characteristics.
c = o
c —o
C—0
N02
0
0
\ . #' •
N
CJH,
FIGURE 9-21
Resonance forms.
A Molecular Orbital Description (Sigma Bonds)
So far we have looked at molecular orbitals in a simplified way as electron pairs
that try to seek locations of minimum potential energy. Now that double bonds
are under consideration, it will pay us to examine in greater detail the characteristics of both atomic and molecular orbitals. You recall that an atomic orbital
is a volume element oriented with respect to the nucleus of the atom where
there is a high probability of finding (at the most) two electrons that are identical in quantum numbers except for direction of spin. The orbital of each type of
electron (s, p, d, etc.) has a characteristic shape and orientation.
One important type of molecular orbital is regarded as being formed from the
"end-overlap" of two atomic orbitals (one from each atom). If two electrons (of
opposite spin) fill this molecular orbital (one from each atom, or two from one
Sizes and Shapes of Molecules
bonds to be located between any specific atoms. How should this situation be
described? Pauling suggested that the actual state of the molecule is a resonance
hybrid of all the separate forms that can be written in the classical way. The
principal resonance forms for our three examples would be those shown in
Figure 9-21.
It must be emphasized that none of these resonance forms actually exists, but
the superposition of all of them for a given molecule serves as a good representation, and shows that each bond possesses both single-bond and double-bond
characteristics.
c = o
c —o
C—0
N02
0
0
\ . #' •
N
CJH,
FIGURE 9-21
Resonance forms.
A Molecular Orbital Description (Sigma Bonds)
So far we have looked at molecular orbitals in a simplified way as electron pairs
that try to seek locations of minimum potential energy. Now that double bonds
are under consideration, it will pay us to examine in greater detail the characteristics of both atomic and molecular orbitals. You recall that an atomic orbital
is a volume element oriented with respect to the nucleus of the atom where
there is a high probability of finding (at the most) two electrons that are identical in quantum numbers except for direction of spin. The orbital of each type of
electron (s, p, d, etc.) has a characteristic shape and orientation.
One important type of molecular orbital is regarded as being formed from the
"end-overlap" of two atomic orbitals (one from each atom). If two electrons (of
opposite spin) fill this molecular orbital (one from each atom, or two from one