Which is a correct Lewis structure for carbonic acid, H2 C03
? A. B. C.
The Correct Answer and Explanation is:
Correct Answer: C.
Lewis Structure of Carbonic Acid (H₂CO₃)
Carbonic acid is a weak diprotic acid with the molecular formula H₂CO₃, composed of two hydrogen atoms, one carbon atom, and three oxygen atoms. The correct Lewis structure is essential to understand its molecular geometry, bonding, and acid-base behavior.
Correct Lewis Structure Explanation:
The correct structure (Option C) shows:
- A central carbon atom double-bonded to one oxygen atom (C=O).
- Two hydroxyl groups (-OH) each bonded to the central carbon through a single bond (C–O–H).
- Each oxygen atom in the hydroxyl groups has two lone pairs of electrons.
- The carbon atom has four bonds total (satisfying the octet rule): one double bond and two single bonds.
- Each hydrogen is bonded to an oxygen atom, not directly to carbon.
The Lewis structure of carbonic acid can be written as:
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H–O–C–O–H
In this structure:
- The carbon atom forms one double bond with an oxygen and two single bonds with the hydroxyl oxygens.
- Each oxygen in the –OH group has two lone pairs.
- The double-bonded oxygen has two lone pairs as well.
- All atoms obey the octet rule (except hydrogen, which needs only two electrons).
Why This Is Correct:
This structure minimizes formal charges and satisfies the octet rule for all atoms involved:
- Carbon has 4 bonds (no lone pairs).
- Each oxygen has 2 bonds and 2 lone pairs, resulting in a formal charge of zero.
- Hydrogens each form one bond and are stable.
Incorrect structures often misplace the hydrogen atoms (e.g., bonding H directly to carbon), violate the octet rule, or create unbalanced formal charges.
Thus, Option C is the correct Lewis structure for carbonic acid.
