Draw the Lewis structure of the following compound K2CO3 and select how many covalent double bonds it has and how many non-bonding electrons C.
The correct answer and explanation is:
Correct Answer:
- Number of covalent double bonds: 1
- Number of non-bonding electrons on carbon (C): 0
Lewis Structure and Explanation (Approx. 300 words)
Potassium carbonate (K₂CO₃) is an ionic compound composed of two potassium ions (K⁺) and a carbonate ion (CO₃²⁻). To draw the Lewis structure and analyze the bonding, focus on the carbonate ion (CO₃²⁻), since potassium is a metal that donates electrons and does not form covalent bonds in this compound.
Step 1: Count total valence electrons
- Carbon (C): 4 valence electrons
- Oxygen (O): 6 valence electrons × 3 = 18
- Total = 4 + 18 = 22
- Add 2 more electrons for the 2⁻ charge: 22 + 2 = 24 valence electrons
Step 2: Build the skeletal structure
- Place carbon in the center and bond it to the three oxygen atoms.
Step 3: Distribute electrons
- Initially assign single bonds between carbon and each oxygen (3 bonds = 6 electrons).
- Place lone pairs on each oxygen to complete octets.
- You’ll find that you exceed the number of valence electrons, so a double bond is needed to satisfy the octet rule without adding extra electrons.
Step 4: Final structure
- One oxygen will form a double bond with carbon.
- The other two oxygens will each have a single bond and carry a negative charge.
- All atoms (including carbon) now obey the octet rule.
Covalent double bonds and non-bonding electrons on carbon
- The carbon atom forms three sigma bonds (two single, one double) and one pi bond (in the double bond).
- Carbon has no lone pairs (non-bonding electrons) because it uses all 4 valence electrons in bonding.
- One double bond is present in the carbonate ion.
Resonance
There are three resonance structures of CO₃²⁻ where the double bond rotates among the three oxygen atoms, but each still has only one double bond at a time.
Summary:
- Double bonds: 1
- Non-bonding electrons on C: 0