3.16 Bond Lengths
A bond length is the distance between the centers of two atoms joined by a covalent
bond. It corresponds to the internuclear distance at the potential-energy minimum
for the two atoms (see Fig. 3.15). Bond lengths affect the overall size and shape of
a molecule. The transmission of hereditary information in DNA, for instance,
depends on bond lengths because the two strands of the double helix must fit together like pieces of a jigsaw puzzle (Section 20.15). Bond lengths are also crucial
to the action of enzymes because only a molecule of the right size and shape will
fit into the active site of the enzyme molecule (Section 15.16). As TABLE 3.5 shows,
the lengths of bonds between Period 2 elements typically lie in the range from 100 pm
to 150 pm. Bond lengths are determined experimentally by using either spectroscopy
or x-ray diffraction (BOX 3.2).
Bonds between heavy atoms tend to be longer than those between light atoms
because heavier atoms have larger radii than lighter ones (FIG. 3.20). Multiple bonds
are shorter than single bonds between the same two elements, because the additional
bonding electrons attract the nuclei more strongly and pull the atoms closer together;
compare the lengths of the various carbon–carbon bonds in Table 3.5. The averaging
effect of resonance can also be seen: the length of the carbon–carbon bond in
benzene is intermediate between the lengths of the single and double bonds of a
Kekulé structure (but closer to that of a double bond). For bonds between atoms of
the same two elements, the stronger the bond, the shorter it is. Thus, a CΩC triple
bond is both stronger and shorter than a C£C double bond. Similarly, a C£O
double bond is both stronger and shorter than a CßO single bond.
Each atom makes a characteristic contribution, called its covalent radius, to the
length of a bond (FIG. 3.21). A bond length is approximately the sum of the covalent
radii of the two atoms (36). The OßH bond length in ethanol, for example, is
the sum of the covalent radii of H and O, 37 66 pm 103 pm. Figure 3.21 also
shows that the covalent radius of an atom taking part in a multiple bond is smaller
than that for a single bond of the same atom.