Draw the Lewis dot structure for carbonic acid (H2CO3). Do not forget to draw all lone pairs.
The Correct Answer and Explanation is:
Lewis Structure for Carbonic Acid (H₂CO₃)
Step 1: Count the total number of valence electrons.
- Hydrogen (H): 1 electron × 2 = 2 electrons
- Carbon (C): 4 electrons
- Oxygen (O): 6 electrons × 3 = 18 electrons
Total = 2 + 4 + 18 = 24 valence electrons
Step 2: Determine the central atom.
Carbon is the central atom because it forms the most bonds and can connect with multiple oxygen atoms.
Step 3: Arrange the atoms.
Attach the three oxygen atoms to the carbon atom. Two of the oxygen atoms will each have a hydrogen atom bonded to them, forming hydroxyl groups (–OH). The third oxygen will be double bonded to the carbon.
Step 4: Place bonds.
- Form single bonds between carbon and two oxygen atoms (that are each connected to hydrogen).
- Form one double bond between carbon and the third oxygen.
- Add hydrogen atoms to the singly bonded oxygen atoms.
Step 5: Distribute remaining electrons as lone pairs.
Each oxygen atom should have a complete octet (8 electrons), counting bonding pairs. Hydrogen atoms only need 2 electrons.
Final Lewis Structure:
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H–O–C–O–H
- The double-bonded oxygen has two lone pairs.
- Each singly bonded oxygen (–OH) has two lone pairs.
- Carbon has 4 bonds total (2 single, 1 double).
- Hydrogen atoms have one bond each (no lone pairs).
Explanation
Carbonic acid (H₂CO₃) is an important weak acid in biological systems and carbon cycling. To draw its Lewis structure correctly, we start by calculating the total number of valence electrons, which adds up to 24. Carbon, being the central atom, is bonded to three oxygen atoms. Two of these oxygen atoms are further bonded to hydrogen atoms, forming hydroxyl groups (–OH), while the third oxygen is double bonded to the carbon.
In the Lewis structure, we show each bond as a shared pair of electrons and ensure that each atom (except hydrogen) has an octet. Hydrogen atoms follow the duet rule and are each attached to an oxygen atom. Carbon forms four bonds in total, which satisfies its tetravalency. Each oxygen atom in the structure either forms two bonds and has two lone pairs (like in the –OH groups) or forms a double bond and has two lone pairs (as with the carbonyl oxygen).
The placement of lone pairs is essential. Each oxygen atom must have enough electrons to complete its octet when bonding electrons are counted. That means each oxygen ends up with two lone pairs in this structure. The hydrogen atoms do not carry lone pairs since they only need two electrons to complete their outer shell.
This structure of carbonic acid reflects its resonance stability and role in buffering systems in organisms. It also exists in equilibrium with water, carbon dioxide, and bicarbonate in aqueous solutions. Drawing the Lewis structure accurately helps visualize electron distribution, molecular geometry, and reactivity.
