Final answer:
Structure (a) with formal charges of C: 0, N: 0, and O: -1, is the preferred Lewis structure for the cyanate ion due to lower formal charges and the negative charge on the more electronegative oxygen.
Step-by-step explanation:
To determine the most stable Lewis structure for the cyanate ion (CNO-), we should assign formal charges to each atom in the given structures and select the one with the lowest formal charges consistent with the overall charge of the ion. The formal charge is calculated using the formula:
Formal charge = [Valence electrons] - [Non-bonding electrons] - 1/2[Shared electrons]
For structure (a), [:C=N=O:], the formal charges are as follows:
- Carbon (C): (4 - 2 - 1/2*6) = 0
- Nitrogen (N): (5 - 2 - 1/2*6) = 0
- Oxygen (O with two dots): (6 - 6 - 1/2*2) = -1
For structure (b), [:N=C-O:], the formal charges are:
- Nitrogen (N): (5 - 0 - 1/2*8) = +1
- Carbon (C): (4 - 4 - 1/2*4) = 0
- Oxygen (O): (6 - 4 - 1/2*4) = 0
Comparing the formal charges, structure (a) has no positive charges and one negative charge that is placed on the more electronegative oxygen, which is consistent with Guideline 4 and results in a neutral charge for carbon and nitrogen. Therefore, structure (a) is the preferred Lewis structure for the cyanate ion.
The formal charge distributions in structures similar to the thiocyanate ion reinforce the choice of the most stable structure with the lowest number of nonzero formal charges.