When nonequivalent resonance structures exist, the most likely resonance structure is the one with formal charges closest to zero. A general rule of chemical stability is that the localization of positive or negative charges within a molecule is destabilizing. Consider the following two possible inequivalent resonance structures for BCl3 shown here with calculated formal charges.
The resonance structure on the right is less likely than the one on the left because it has localized formal charges of +1 and -1. In addition, chlorine (χ = 3.0) is more electronegative than boron (χ = 2.0), so a resonance structure that has a positive formal charge on the more electronegative element (and a negative formal charge on the less electronegative element) is probably not stable.
The three nonequivalent resonance structures of CO2 are shown here with calculated formal charges.
The central structure is the most likely resonance structure for CO2 because it has formal charges of zero on each atom, whereas both of the other resonance structures have formal charges of +1 and -1 on the O atoms.
The cyanate ion, OCN-, has three nonequivalent resonance structures.
None of the three resonance structures have formal charges of zero on all atoms. The first resonance structure is not very likely because of the large formal charges (+1 and -2). The other two resonance structures, however, each have small formal charges. In this case, electronegativity can help determine the best resonance structure. The electronegativity of oxygen (3.5) is greater than that of nitrogen (3.0). Therefore, oxygen is more likely to carry a negative formal charge in a Lewis structure, and according to formal charges, the resonance structure on the right is most likely.
Three inequivalent resonance structures for the chlorate ion are shown here. Assign formal charges to all atoms in the resonance structures and identify the more likely resonance structure.
You are asked to use formal charge to identify the most likely resonance structure of those given for a polyatomic ion.
You are given three valid resonance structures.
Structure B is the most likely resonance structure. The formal charges in B are close to zero, and the highly electronegative oxygen atom carries a -1 formal charge. Structure A is unlikely because of the large positive formal charge on chlorine. Structure C is unlikely because the least electronegative element in the ion (chlorine) has a negative formal charge while the most electronegative element in the ion (oxygen) has a formal charge of zero.