Question: Use the observed bond lengths to answer each question. (a) Why is bond [1] longer than bond [2] (143 pm versus 136 pm)? (b) Why are bonds [3] and [4] equal in length (127 pm) and shorter than bond [2]?

Bond length is the measurement of the distance between the centers of two chemically bound atoms.

The bond length of an atom depends on the percentage of s character of the atoms. As the s character increases, the bond length decreases.

The order of bond length according to s character is as follows:

$\mathrm{single}\mathrm{bond}>\mathrm{double}\mathrm{bond}>\mathrm{tripple}\mathrm{bond}$

The carbon-oxygen bond [1] is longer than the bond [2] because, in the case of molecule b, the oxygen atom takes part in resonance, thereby generating a partial double bond character on the highlighted carbon-oxygen (C-O) bond.

Structures representing the resonance in [2]

Resonance structures [3]

Bonds [3] and [4] both have double bond characteristics as a result of resonance, and hence they have equal bond length.

The bonds [3] and [4] are shorter than bond [2] as the oxygen atom present in structure 3 is electron-dense (indicated by the negative charge), which shifts to carbon-oxygen during resonance.

As the electron density on the bond increases, the bond length decreases.