The format ion, HCO2–, is a simple carboxylate anion that exhibits resonance. To draw the Lewis structures and show the formal charges, we need to start with the basic valence structures of the molecule.
1. **First Resonance Structure**: In the first structure, the negative charge is located on one of the oxygen atoms. The Lewis structure can be represented as:
O
||
H -- C -- O-
In this structure, the carbon atom is single bonded to one oxygen (O) and double bonded to another oxygen. The formal charge can be calculated as follows:
- For Carbon (C): 4 (valence electrons) – 0 (non-bonding electrons) – 4/2 (bonding electrons) = 0
- For Oxygen (with double bond): 6 (valence electrons) – 4 (non-bonding electrons) – 4/2 (bonding electrons) = 0
- For Oxygen (with single bond): 6 (valence electrons) – 6 (non-bonding electrons) – 2/2 (bonding electrons) = -1
So the formal charges for this resonance structure are: C = 0, O = 0 (double-bonded), and O = -1 (single-bonded).
2. **Second Resonance Structure**: In the second structure, we can move the double bond between carbon and oxygen over to the other oxygen. The new structure looks like this:
O- || H -- C -- O
Here, the formal charges are recalculated:
- For Carbon (C): 0 (as previously calculated)
- For Oxygen (with double bond): 0 (as previously calculated)
- For Oxygen (now with single bond): -1 (as previously calculated)
So in the second resonance structure, the formal charges remain the same: C = 0, O = 0 (single-bonded) and O = -1 (double-bonded).
Therefore, the format ion HCO2– has two resonance forms, and in both forms, the total charge is conserved, illustrating the stability of the molecule through resonance.