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: Write the formula.
Formal charge is $\text{FC} = V - N - \dfrac{B}{2}$, where $V$ is valence electrons, $N$ is lone pair electrons, and $B$ is the shared bonding electrons.
Step 2: Draw the carbonate ion.
In $\text{CO}_3^{2-}$ the central carbon joins three oxygens, one by a double bond and two by single bonds.
Step 3: Find $V$.
Carbon is in group 14, so it has $4$ valence electrons. So $V = 4$.
Step 4: Find $N$.
Carbon uses all its electrons in bonding and keeps no lone pair, so $N = 0$.
Step 5: Find $B$ and compute.
Carbon makes four bonds in total (one double and two single), so it shares $8$ bonding electrons, giving $B = 8$. Now $\text{FC} = 4 - 0 - \dfrac{8}{2} = 4 - 4 = 0$.
Step 6: Final choice.
The formal charge on carbon is zero, which is option 4.
\[ \boxed{\text{zero}} \]