Covalent Bond
Easychemistry
The formal charge on the nitrogen atom in the nitrate ion (NO₃⁻) is:
Select the correct option:
Solution
Incorrect! Answer:
+1
- Draw the Lewis Structure of NO₃⁻: Nitrogen is the central atom bonded to three oxygen atoms. One N=O double bond and two N–O single bonds (with negative formal charges on the singly-bonded oxygens) is one resonance structure. Total valence electrons = 5+3(6)+1=24.
- Apply the Formal Charge Formula: FC=V−N−2B, where V = valence electrons of free atom, N = non-bonding electrons, B = bonding electrons.
- Calculate for Nitrogen (in the resonance structure with one double bond and two single bonds):
- V = 5 (nitrogen has 5 valence electrons)
- N = 0 (nitrogen has no lone pairs in this structure)
- B = 8 (four bonds: one double + two single = 2 + 2 + 2 + 2 shared electrons)
- FC=5−0−28=5−0−4=+1.
- Verify Charge Balance: In each resonance structure, the doubly-bonded O has FC = 0, each singly-bonded O has FC = −1. Total: (+1)+0+(−1)+(−1)=−1, which matches the overall charge of NO₃⁻.
- Conclusion: The formal charge on nitrogen in NO₃⁻ is +1, despite nitrogen being bonded to more electronegative oxygen atoms. Formal charge is a bookkeeping tool and does not represent actual electron density.
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About This Question
- Subject
- chemistry
- Chapter
- chemical bonding and molecular structure
- Topic
- covalent bond
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
+1
- Draw the Lewis Structure of NO₃⁻: Nitrogen is the central atom bonded to three oxygen atoms. One N=O double bond and two N–O single bonds (with negative formal charges on the singly-bonded oxygens) is one resonance structure. Total valence electrons = 5+3(6)+1=24.
- Apply the Formal Charge Formula: FC=V−N−2B, where V = valence electrons of free atom, N = non-bonding electrons, B = bonding electrons.
- Calculate for Nitrogen (in the resonance structure with one double bond and two single bonds):
- V = 5 (nitrogen has 5 valence electrons)
- N = 0 (nitrogen has no lone pairs in this structure)
- B = 8 (four bonds: one double + two single = 2 + 2 + 2 + 2 shared electrons)
- FC=5−0−28=5−0−4=+1.
- Verify Charge Balance: In each resonance structure, the doubly-bonded O has FC = 0, each singly-bonded O has FC = −1. Total: (+1)+0+(−1)+(−1)=−1, which matches the overall charge of NO₃⁻.
- Conclusion: The formal charge on nitrogen in NO₃⁻ is +1, despite nitrogen being bonded to more electronegative oxygen atoms. Formal charge is a bookkeeping tool and does not represent actual electron density.
This easy difficulty chemistry question is from the chapter chemical bonding and molecular structure, covering the topic of covalent bond. It appeared in the 2025 exam.
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