Question

What is the formal charge on carbon atom in $\text{CO}_3^{2-}$ ion ?

• A. $-2$
• B. $-4$
• C. $+4$
• D. zero

Hint:

A highly reliable shortcut: If carbon forms exactly four chemical bonds (e.g., four singles, two doubles, or one triple and one single) and possesses no lone pairs, its formal charge will absolutely always be zero!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We need to calculate the formal charge residing specifically on the central carbon atom within the carbonate polyatomic ion ($\text{CO}_3^{2-}$).

Step 2: Key Formula or Approach:
The formal charge (FC) on any atom in a Lewis structure is determined by the formula:
$$\text{FC} = V - N - \frac{B}{2}$$
Where $V$ is the number of valence electrons of the free neutral atom, $N$ is the number of non-bonding (lone pair) electrons, and $B$ is the total number of shared bonding electrons.

Step 3: Detailed Explanation:
First, consider the correct Lewis structure for the carbonate ion ($\text{CO}_3^{2-}$).
The central carbon atom is bonded to three oxygen atoms: it forms one double bond and two single bonds.
Carbon belongs to Group 14, meaning it naturally possesses 4 valence electrons ($V = 4$).
In this structure, the carbon atom utilizes all of its valence electrons for bonding, leaving it with no lone pair electrons ($N = 0$).
The carbon atom forms 4 bonds total ($1\text{ double} + 2\text{ single}$), meaning it shares 8 bonding electrons ($B = 8$).
Substituting these values into the formal charge formula:
$$\text{FC} = 4 - 0 - \frac{8}{2}$$
$$\text{FC} = 4 - 0 - 4$$
$$\text{FC} = 0$$

Step 4: Final Answer:
The formal charge on the carbon atom is strictly zero, making option (D) correct.