133
(a)
FIGURE 9-19
A correct structure for CH,O.
I
In general, you should always follow the procedure just described when the
central atom does not obey the octet rule and the ligands are able to form
multiple bonds (see p 121).
PROPERTIES OF MOLECULAR ORBITALS
Resonance
A surprising thing is noted when we examine interatomic distances for certain
ions and molecules, such as COj", NO^T, or C 6 H 6 . If we draw the usual
electron-dot formulas for these, we get the structures shown in Figure 9-20.
Using Table 9-1, we would say that in CO§~ there are one C=O bond of length
1.22 A and two C-O bonds of length 1.43 A; in NO^T, one N-O bond of length
1.36 A and one N=O bond of length 1.15 A; in C 6 H 6 , three C-C bonds of length
1.54 A and three C=C bonds of length 1.34 A. In actual fact, all the bond lengths
in any one of these three molecules are the same; for C0§~ all lengths are 1.30
A; for NO^ they are 1.24 A; and for CsH 6 they are 1.40 A. These values are very
close to the weighted averages of single and double bond lengths. It appears
that, in such molecules or ions, there is no real preference for single or double
N
0
.c. .c.
H
C
H
H
FIGURE 9-20
Electron-dot formulas.