Question 8.9: Three possible Lewis structures for the thiocyanate ion, NCS...
Three possible Lewis structures for the thiocyanate ion, NCS^{-}, are
(a) Determine the formal charges in each structure.
(b) Based on the formal charges, which Lewis structure is the dominant one?
(a) Neutral N, C, and S atoms have five, four, and six valence electrons, respectively. We can determine the formal charges in the three structures by using the rules we just discussed:
As they must, the formal charges in all three structures sum to 1-, the overall charge of the ion.
(b) The dominant Lewis structure generally produces formal charges of the smallest magnitude (guideline 1).
That eliminates the left structure as the dominant one. Further, as discussed in Section 8.4, N is more electronegative than C or S. Therefore, we expect any negative formal charge to reside on the N atom (guideline 2). For these two reasons, the middle Lewis structure is the dominant one for NCS^{-}.