Question 9.SE.7: Describe the bonding in the nitrate ion, NO3^-.Does this ion...

Analyze Given the chemical formula for a polyatomic anion, we are asked to describe the bonding and determine whether the ion has delocalized π bonds.

Plan Our first step is to draw Lewis structures. Multiple resonance structures involving the placement of the double bonds in different locations would suggest that the π component of the double bonds is delocalized.

Solve In Section 8.6 we saw that NO$_3^-$ has three resonance structures:

In each structure, the electron-domain geometry at nitrogen is trigonal planar, which implies sp² hybridization of the N atom. It is helpful when considering delocalized π bonding to consider atoms with lone pairs that are bonded to the central atom to be sp² hybridized as well. Thus, we can envision that each of the O atoms in the anion has three sp² hybrid orbitals in the plane of the ion. Each of the four atoms has an unhybridized p orbital oriented perpendicular to the plane of the ion.

The NO$_3^-$ ion has 24 valence electrons. We can first use the sp² hybrid orbitals on the four atoms to construct the three N—O σ bonds. That uses all of the sp² hybrids on the N atom and one sp² hybrid on each O atom. Each of the two remaining sp² hybrids on each O atom is used to hold a nonbonding pair of electrons. Thus, for any of the resonance structures, we have the following arrangement in the plane of the ion:

Notice that we have accounted for a total of 18 electrons—6 in the three N—O σ bonds, and 12 as nonbonding pairs on the O atoms. The remaining six electrons will reside in the π system of the ion.

The four p orbitals—one on each of the four atoms—are used to build the π system. For any one of the three resonance structures shown, we might imagine a single localized N—O π bond formed by the overlap of the p orbital on N and a p orbital on one of the O atoms. The remaining two O atoms have nonbonding pairs in their p orbitals. Thus, for each of the resonance structures, we have the situation shown in Figure 9.27. Because each resonance structure contributes equally to the observed structure of NO$_3^-$, however, we represent the p bonding as delocalized over the three N—O bonds, as shown in the figure. We see that the NO$_3^-$ ion has a six-electron π system delocalized among the four atoms in the ion.