The following three Lewis structures can be drawn for N2...

The following three Lewis structures can be drawn for $$\begin{aligned} &\mathrm{N}_{2} \mathrm{O}: \\ &: \mathrm{N} \equiv \mathrm{N}-\ddot{O}: \longleftrightarrow: \ddot{\mathrm{N}}-\mathrm{N} \equiv \mathrm{O}: \longleftrightarrow: \dot{\mathrm{N}}=\mathrm{N}=\ddot{\mathrm{O}}: \end{aligned} $$ (a) Using formal charges, which of these three resonance forms is likely to be the most important? (b) The $\mathrm{N}-\mathrm{N}$ bond length in $\mathrm{N}_{2} \mathrm{O}$ is $1.12 \AA$, slightly longer than a typical $\mathrm{N} \equiv \mathrm{N}$ bond; and the $\mathrm{N}-\mathrm{O}$ bond length is $1.19 \AA$, slightly shorter than a typical $\mathrm{N}=\mathrm{O}$ bond. (See Table 8.5.) Rationalize these observations in terms of the resonance structures shown previously and your conclusion for (a).

The following three Lewis structures can be drawn for
$$\begin{aligned}
&\mathrm{N}_{2} \mathrm{O}: \\
&: \mathrm{N} \equiv \mathrm{N}-\ddot{O}: \longleftrightarrow: \ddot{\mathrm{N}}-\mathrm{N} \equiv \mathrm{O}: \longleftrightarrow: \dot{\mathrm{N}}=\mathrm{N}=\ddot{\mathrm{O}}:
\end{aligned}
$$
(a) Using formal charges, which of these three resonance forms is likely to be the most important? (b) The $\mathrm{N}-\mathrm{N}$ bond length in $\mathrm{N}_{2} \mathrm{O}$ is $1.12 \AA$, slightly longer than a typical $\mathrm{N} \equiv \mathrm{N}$ bond; and the $\mathrm{N}-\mathrm{O}$ bond length is $1.19 \AA$, slightly shorter than a typical $\mathrm{N}=\mathrm{O}$ bond. (See Table 8.5.) Rationalize these observations in terms of the resonance structures shown previously and your conclusion for (a).

Key Concepts

Lewis Structures

Lewis structures are schematic diagrams that depict the arrangement of electrons in a molecule, showing atomic connectivity and the distribution of valence electrons as dots and bonds. They provide the foundational representation used to analyze electron configuration, bonding patterns, and overall stability in molecules.

Formal Charges

Formal charges are calculated quantities used to determine the likely distribution of electrons within a Lewis structure. By assigning charges based on the difference between an atom’s valence electrons and the electrons it 'owns' in a particular structure, chemists can compare different resonance forms to identify the one that minimizes charges and allocates negative charges to more electronegative atoms, indicating greater stability.

Resonance Structures

Resonance structures are multiple valid Lewis representations of a molecule that differ only in the placement of electrons, not the arrangement of atoms. The true structure, known as the resonance hybrid, is a weighted average of these forms, accounting for the delocalization of electrons and showing properties like intermediate bond orders which can explain observed bond lengths.

Bond Order and Bond Length

Bond order refers to the number of bonds between two atoms, and it directly influences bond length and strength—with higher bond orders corresponding to shorter, stronger bonds. In the context of resonance, the bond order can be fractional, leading to bond lengths that differ slightly from the values expected based solely on typical single, double, or triple bonds.

Stability Criteria in Resonance Structures

When evaluating multiple resonance forms, the most stable structure is typically the one in which atoms have full octets, formal charges are minimized, and any negative formal charges reside on the more electronegative atoms. This rationale is used to predict which resonance form contributes most to the resonance hybrid and can help explain deviations in expected bond lengths based on pure Lewis structure bond orders.