a. Draw resonance contributors for the following species, showing all the lone pairs:
b. For each species, indicate the most stable resonance contributor.
3. NO₂^-

Verified step by step guidance

1. Identify the structure of the NO2- ion. It consists of a nitrogen atom bonded to two oxygen atoms, with one negative charge distributed across the molecule. Nitrogen has a formal charge of -1, and the molecule has a total of 18 valence electrons (5 from nitrogen, 6 from each oxygen, and 1 extra electron for the negative charge).
2. Distribute the valence electrons to satisfy the octet rule for each atom. Start by placing single bonds between nitrogen and each oxygen atom, then distribute the remaining electrons as lone pairs on the oxygen atoms and nitrogen.
3. Draw the resonance contributors by moving electrons. In one resonance structure, one oxygen atom forms a double bond with nitrogen, while the other oxygen atom retains a single bond and carries a negative formal charge. In the second resonance structure, the double bond shifts to the other oxygen atom, and the first oxygen atom now carries the negative formal charge.
4. Indicate the lone pairs on all atoms in each resonance structure. Each oxygen atom should have two lone pairs when it is double-bonded to nitrogen and three lone pairs when it is single-bonded and carries the negative charge. Nitrogen will have one lone pair in both resonance structures.
5. Determine the most stable resonance contributor. The most stable structure is the one where the negative charge is on the more electronegative atom (oxygen). Both resonance structures are equivalent in stability because the negative charge is delocalized equally between the two oxygen atoms, making the resonance hybrid the most accurate representation of the molecule.

Key Concepts

Resonance Structures

Resonance structures are different Lewis structures for the same molecule that depict the same arrangement of atoms but differ in the distribution of electrons. These structures help illustrate the delocalization of electrons within a molecule, which can stabilize the overall structure. In the case of NO2-, resonance contributors can be drawn to show how the negative charge and lone pairs can be distributed across the nitrogen and oxygen atoms.

Lone Pairs

Lone pairs are pairs of valence electrons that are not involved in bonding and are localized on a specific atom. In resonance structures, it is crucial to accurately represent lone pairs, as they can influence the molecule's reactivity and stability. For NO2-, the placement of lone pairs on the nitrogen and oxygen atoms will affect the resonance contributors and the overall charge distribution.

Stability of Resonance Contributors

The stability of resonance contributors is determined by factors such as the octet rule, formal charges, and the electronegativity of atoms. More stable resonance structures typically have a complete octet for all atoms, minimized formal charges, and negative charges on more electronegative atoms. Identifying the most stable resonance contributor for NO2- involves evaluating these factors to determine which structure best represents the molecule's electron distribution.