RESONANCE STRUCTURES

Some molecules have a number of resonance structures that may be important and help contribute to their stability.

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Some molecules have equivalent resonance structures of equal importance that help contribute to their stability. Nitrate, is a good example.

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Thiocyanate, NCS–, provides an example in which the resonance structures are not equal. Structure A has low formal charges and the negative charge is on the most electronegative atom, however, a C=S double bond is not very strong. In structure B, although the negative charge is not on the most electronegative atom, short CN multiple bonds are very strong.  Structure C has large formal charges and a relatively weak C≡S triple bond. Thus, A and B are the most important resonance structures of NCS–.