Which of the following is a reasonable resonance structure for the nitrate ion (NO_3^-)?

• A. A structure with one double bond between N and O, and two single bonds to the other O atoms, each of which carries a negative charge.
• B. A structure with two double bonds between N and two O atoms, and one single bond to the third O atom.
• C. A structure with three single bonds between N and each O atom, and the nitrogen atom carrying a negative charge.
• D. A structure with one triple bond between N and one O atom, and two single bonds to the other O atoms.

Answer: (A)

Verified step by step guidance

1. Recall that the nitrate ion (NO_3^-) has a total of 24 valence electrons: 5 from nitrogen, 6 from each oxygen (3 oxygens × 6 = 18), plus 1 extra electron due to the negative charge (5 + 18 + 1 = 24).
2. Draw a skeletal structure with nitrogen as the central atom bonded to three oxygen atoms. Initially, connect each oxygen to nitrogen with a single bond.
3. Distribute the remaining electrons to satisfy the octet rule for each atom, starting with the outer atoms (oxygen). Remember that each single bond counts as 2 electrons.
4. To minimize formal charges and create a stable resonance structure, form one double bond between nitrogen and one oxygen atom, while the other two oxygens remain single bonded and carry a negative charge each. This arrangement balances the formal charges and satisfies the octet rule.
5. Evaluate the other options by calculating formal charges and checking octet completion: structures with two double bonds or a triple bond often result in unfavorable formal charges or incomplete octets, and the structure with nitrogen carrying a negative charge is less stable due to nitrogen's electronegativity.