Question

1. which of these is a correct lewis structure for carbonate ion (co₃²⁻)?

Explanation:

Step1: Recall Lewis Structure Rules

For \( \text{CO}_3^{2-} \), C (4 valence e⁻) + 3×O (6×3 = 18) + 2 (charge) = 24 valence e⁻. C is central, needs octet (or expanded, but C usually octet). O can have single/double bonds.

Step2: Analyze Each Option

• First Option: Two double bonds (\( \text{O}=\text{C}=\text{O} \)) and one single? Wait, structure has \( \text{O}=\text{C}=\text{O} \) and \( :\text{O}: \). Count e⁻: Each double bond (4 e⁻ shared), single? Wait, no, the third O has 6 lone pairs? Wait, no, let's count total valence e⁻. C (4) + 3O (18) + 2 (charge) = 24. First option: \( \text{O}=\text{C}=\text{O} \) (two double bonds, each O has 2 lone pairs? Wait, no, the structure shown has two double bonds and one O with 3 lone pairs? Wait, no, let's check the third option.
• Third Option: Structure is \( [:\overset{..}{\text{O}}-\text{C}=\overset{..}{\text{O}} \atop \quad \quad :\overset{..}{\text{O}}:]^{2-} \). Let's count bonds and lone pairs. C has 4 bonds (1 single, 1 double, 2 single? Wait, no: one single bond (2 e⁻), one double bond (4 e⁻), and one single bond? Wait, C is bonded to three O: one single (\( \text{O}-\text{C} \), 2 e⁻), one double (\( \text{C}=\text{O} \), 4 e⁻), and one single (\( \text{C}-\text{O} \))? Wait, no, the third O is bonded with a single bond? Wait, no, the structure is \( :\text{O}^- - \text{C} = \text{O} \) (with lone pairs) and \( :\text{O}: \) (with lone pairs). Wait, let's calculate formal charges. For \( \text{CO}_3^{2-} \), resonance structures have one double bond and two single bonds (with formal charges on O). The correct Lewis structure should have C with octet (4 bonds: 1 double, 2 single, or 3 single? No, C needs 4 bonds. Wait, the third option: C is bonded to one O (single, 2 e⁻), one O (double, 4 e⁻), and one O (single, 2 e⁻) – total 8 e⁻ (octet). Each O: the double-bonded O has 2 lone pairs (4 e⁻), single-bonded O⁻ has 3 lone pairs (6 e⁻), and the other single-bonded O? Wait, no, the third option has one double bond and two single bonds, with the single-bonded O's having 3 lone pairs (formal charge -1 each) and the double-bonded O having 2 lone pairs (formal charge 0). Total formal charge: 2×(-1) + 0 + C (0) = -2, which matches the charge. Also, total valence e⁻: Double bond (4) + two single bonds (2 each) + lone pairs: \( :\text{O}^- \) (6 lone e⁻) + \( \text{O}=\text{C} \) (O has 4 lone e⁻) + \( :\text{O}: \) (6 lone e⁻)? Wait, no, let's count all electrons:
• Double bond: \( \text{C}=\text{O} \): 4 e⁻ shared.
• Two single bonds: \( \text{C}-\text{O} \) (each 2 e⁻ shared).
• Lone pairs: \( :\text{O}^- \) (6 e⁻), \( \text{O}=\text{C} \) (O has 4 e⁻ lone), \( :\text{O}: \) (6 e⁻). Wait, total shared e⁻: 4 + 2 + 2 = 8. Lone e⁻: 6 + 4 + 6 = 16. Total: 8 + 16 = 24, which matches (4 + 18 + 2 = 24).
• Other options: Second option has all single bonds? C would have 3 bonds (incomplete octet, 6 e⁻), which is wrong. Fourth option: all single bonds? C has 3 bonds (incomplete octet). First option: two double bonds, C would have 4 bonds (but O count? No, first option has two double bonds, which would give C 4 bonds, but the third O has 3 lone pairs? Wait, no, the first option's structure has \( \text{O}=\text{C}=\text{O} \) and \( :\text{O}: \) with 3 lone pairs? Wait, no, the third option is the only one with C having octet (4 bonds: 1 double, 2 single) and correct formal charges (two O⁻ with -1, one O with 0, total -2).

Answer:

The third option (the one with \( [:\overset{..}{\text{O}}-\text{C}=\overset{..}{\text{O}} \atop \quad \quad :\overset{..}{\text{O}}:]^{2-} \))