Which is the correct Lewis structure for the carbonate anion, CO32-? Note, carbon has four valence elctrons and oxygen has six valence electrons.
The Correct Answer and Explanation is :
The correct Lewis structure for the carbonate anion (CO₃²⁻) involves one carbon atom bonded to three oxygen atoms, with one double bond and two single bonds, and each of the single-bonded oxygens holding a negative charge.
Here’s how we determine it:
- Valence Electrons:
- Carbon (C) has 4 valence electrons.
- Oxygen (O) has 6 valence electrons.
- The carbonate anion has an overall charge of -2, which means we add 2 extra electrons to the total. For the CO₃²⁻ ion:
[
(1 \, \text{C} \times 4 \, \text{electrons}) + (3 \, \text{O} \times 6 \, \text{electrons}) + 2 \, \text{electrons} = 24 \, \text{total valence electrons}.
]
- Forming Bonds:
Carbon, the central atom, forms three bonds with the three oxygen atoms. To satisfy the octet rule for each atom, we distribute the 24 electrons. - Single and Double Bonds:
Since carbon can only form 4 bonds, and oxygen prefers to form 2 bonds, we place one double bond between carbon and one of the oxygens. The other two oxygens are connected by single bonds. This leaves two lone pairs of electrons on each oxygen atom involved in single bonds, while the double-bonded oxygen has two lone pairs. - Formal Charges:
The oxygen atoms involved in single bonds will carry a negative charge, giving us the 2- overall charge of the ion. The formal charge on carbon is zero. Hence, the structure maintains charge balance. - Resonance Structures:
The actual structure of CO₃²⁻ is a resonance hybrid of three equivalent forms, where the double bond between carbon and oxygen shifts among the three oxygen atoms. This resonance delocalizes the negative charge over the oxygen atoms.
Thus, the carbonate ion’s correct Lewis structure is a resonance hybrid where each oxygen shares the negative charge, and each bond between carbon and oxygen is a partial double bond.