1 Answer
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Formal charges ....
Formal charges are assigned to individual atoms in a compound. Formal charges are not actual charges, they are hypothetical charges, assuming that all of the bonds are 100% covalent (Similarly, oxidation states are hypothetical charges assuming that all the bonds are 100% ionic. There are no 100% ionic bonds.) The sum of the formal charges must equal the total charge on the particle, zero for a compound, the ioniz charge for a polyatomic ion.
Formal charge = valence electrons - nonbonding electrons - 1/2 bonding electrons
FC = VE - NBE - 1/2BE
:C≡O: ....... FC of C = 4 - 2 - 3 = -1...... FC of O = 6 - 2 - 3 = +1
O=C=O .... FC of C = 4 - 0 - 4 = 0 ...... FC of O = 6 - 4 - 2 = 0
O−C=O (2-) .... plus two additional resonance structures
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.....O
FC of C = 4 - 0 - 4 = 0
FC of O in C=O = 6 - 4 -2 = 0
FC of O in C−O = 6 - 6 - 1 = -1 ..... Since there are two of these oxygen atoms, the total of the FCs equals the charge on the ion, -2.