Easy Lewis structure Based on formal charges, choose the best Lewis structure for COF
(a) :F\text{-}C\text{-}O\text{-}F: (b) :F\text{-}C\text{=}O\text{-}F: (c) :F\text{-}C\text{=}O \text{ }: (d) :F\text{-}C\text{-}F: \text{ }: \text{ }O: (a) (b)
The Correct Answer and Explanation is:
The correct answer is (c).
Explanation:
To determine the best Lewis structure for phosgene, COF₂, the primary considerations are satisfying the octet rule for all atoms and minimizing the formal charges on each atom. The ideal structure will have formal charges as close to zero as possible.
1. Calculate Total Valence Electrons:
First, one must find the total number of valence electrons available for bonding in the molecule.
- Carbon (C) is in Group 14, providing 4 valence electrons.
- Oxygen (O) is in Group 16, providing 6 valence electrons.
- Fluorine (F) is in Group 17, and with two atoms, it provides 2 × 7 = 14 valence electrons.
- Total valence electrons = 4 + 6 + 14 = 24 electrons.
2. Analyze the Proposed Structures:
A valid Lewis structure must account for all 24 valence electrons.
- Structure (a) and (d): These structures are immediately incorrect because they depict 26 valence electrons, not the 24 available for COF₂. Structure (a) shows 18 lone pair electrons and 8 bonding electrons (4 bonds), totaling 26. Structure (d) shows 20 lone pair electrons and 6 bonding electrons (3 bonds), also totaling 26.
- Structure (b): This structure shows 24 valence electrons (16 lone pair electrons and 8 bonding electrons). However, its formal charges are not minimized.
- Formal Charge (FC) = (Valence e⁻) – (Lone Pair e⁻) – (Bonds).
- FC on Carbon: 4 – 2 – 3 = -1
- FC on Oxygen: 6 – 2 – 3 = +1
- FC on each Fluorine: 7 – 6 – 1 = 0
This structure has a +1 charge on the highly electronegative oxygen atom, which is a very unfavorable arrangement.
- Structure (c): This structure correctly shows 24 valence electrons (16 lone pair electrons and 8 bonding electrons). It places the least electronegative atom, carbon, in the center. All atoms satisfy the octet rule. Let’s calculate the formal charges:
- FC on Carbon: 4 – 0 – 4 = 0
- FC on Oxygen: 6 – 4 – 2 = 0
- FC on each Fluorine: 7 – 6 – 1 = 0
Conclusion:
Structure (c) is the best Lewis structure because it correctly uses all 24 valence electrons, satisfies the octet rule for every atom, and results in a formal charge of zero for all atoms in the molecule. This represents the most stable and accurate electron distribution for COF₂.
