Which of the following Lewis structures for the nitrate ion (NO_3^-) has the lowest formal charge on each atom?

• A. A structure where the nitrogen atom has a double bond to one oxygen and single bonds to two other oxygens, with each singly bonded oxygen carrying a negative charge.
• B. A structure where the nitrogen atom has a double bond to each oxygen, and the nitrogen carries a positive charge.
• C. A structure where the nitrogen atom has three single bonds to three oxygens, and the nitrogen carries a negative charge.
• D. A structure where the nitrogen atom has a triple bond to one oxygen and single bonds to two other oxygens, with no charges on any atom.

Answer: (A)

Verified step by step guidance

1. Recall that the formal charge (FC) on an atom in a Lewis structure is calculated using the formula: \(\text{FC} = V - (N + \frac{B}{2})\), where \(V\) is the number of valence electrons in the free atom, \(N\) is the number of nonbonding (lone pair) electrons on the atom in the structure, and \(B\) is the number of bonding electrons shared in bonds with other atoms.
2. Determine the total number of valence electrons for the nitrate ion (NO\(_3^-\)). Nitrogen has 5 valence electrons, each oxygen has 6, and there is an extra electron due to the negative charge, so total valence electrons = \$5 + 3 \times 6 + 1$.
3. For each proposed Lewis structure, assign electrons to atoms as lone pairs and bonding pairs, then calculate the formal charge on each atom using the formula from step 1.
4. Compare the formal charges on all atoms in each structure. The most stable Lewis structure typically has formal charges closest to zero and places negative charges on the more electronegative atoms (oxygen in this case).
5. Identify the structure where nitrogen has a double bond to one oxygen and single bonds to two other oxygens, with each singly bonded oxygen carrying a negative charge, as this arrangement minimizes formal charges and aligns with known resonance structures of the nitrate ion.