In many molecules, the choice of which atoms are connected by multiple bonds is
arbitrary. When several choices exist, all of them should be drawn. For example, as
shown in Figure 3-1, three drawings (resonance structures) of CO3
2-
are -needed to show
the double bond in each of the three possible C-0 positions. In fact, experimental
evidence shows that all the C-0 bonds are identical, with bond lengths (129 pm) between
double-bond and single-bond distances (116 pm and 143 pm respectively); none of the
drawings alone is adequate to describe the molecular structure, which is a combination of
all three, not an equilibrium between them. This is called resonance to signify that there
is more than one possible way in which the valence electrons can be placed in a Lewis
structure. Note that in resonance structures, such as those shown for CO3
2- in Figure 3-1,
the electrons are drawn in different places but the atomic nuclei remain in fixed positions.