Concept: Lewis structures represent the valence electrons of atoms within a molecule or ion, showing how they are arranged into bonding pairs and lone pairs. The octet rule (or duet rule for hydrogen) is a guiding principle, where atoms tend to achieve a stable electron configuration with eight valence electrons (or two for hydrogen).

Why (A) is correct:

1. Calculate total valence electrons: Nitrogen (Group 15) has 5 valence electrons. Each Oxygen (Group 16) has 6 valence electrons. The ion has a -1 charge, so we add 1 electron.

Total valence electrons = $5+(3\times 6)+1=5+18+1=24$ electrons.

2. Draw skeletal structure: Nitrogen is the central atom as it is less electronegative than oxygen.

O — N — O
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O

3. Distribute electrons: Form single bonds between the central nitrogen and the three oxygen atoms. This uses $3\times 2=6$ electrons.

Remaining electrons = $24-6=18$ electrons.

4. Complete octets of terminal atoms: Distribute the remaining 18 electrons to the oxygen atoms to complete their octets. Each oxygen needs 6 more electrons (3 lone pairs).

$3\times 6=18$ electrons used.

At this point, all oxygen atoms have an octet (2 from bond + 6 from lone pairs). However, the central nitrogen atom only has 6 electrons (3 single bonds). To satisfy the octet rule for nitrogen, one of the lone pairs from an oxygen atom must be converted into a double bond with nitrogen.

5. Form double bond and check octets: If one N=O double bond is formed, and two N-O single bonds, the nitrogen atom will have 8 electrons (4 bonds, 0 lone pairs). The oxygen involved in the double bond will have 2 lone pairs, and the two oxygen atoms involved in single bonds will each have 3 lone pairs.

In the resonance structures of the nitrate ion, the nitrogen atom always forms one double bond and two single bonds with the oxygen atoms. Therefore, around the central nitrogen atom, there are 4 bond pairs (one double bond counts as one bond pair for VSEPR, but two bonding pairs for electron counting, however, for the question asking about 'bond pairs' in the Lewis structure, it refers to the number of bonds formed by the central atom) and 0 lone pairs.

More precisely, in the Lewis structure, nitrogen forms 4 bonds (one double bond and two single bonds). A double bond consists of 2 bonding pairs of electrons, and a single bond consists of 1 bonding pair of electrons. So, the nitrogen atom is surrounded by 4 electron pairs that are involved in bonding (2 from the double bond, 1 from each single bond). There are no lone pairs on the nitrogen atom.

Therefore, the number of bond pairs around the nitrogen atom is 4, and the number of lone pairs is 0.

Option Analysis:

• A) 4,0: This is correct. Nitrogen forms one double bond and two single bonds, totaling 4 bond pairs, and has no lone pairs.
• B) 2,0: This would imply only two bonds, which does not satisfy nitrogen's octet.
• C) 3,1: This would imply three bonds and one lone pair, which would give nitrogen 8 electrons, but it's not the correct bonding arrangement for nitrate.
• D) 3,2: This would imply three bonds and two lone pairs, giving nitrogen 10 electrons, violating the octet rule for nitrogen.

Correct Answer: (A)