Question

For thiocyanate ion ($SC{N}^{-}$), three resonating structures are possible with the electron-dot method as shown below:

The decreasing order of $\%$ contribution to the resonance hybrid is:

• A. $a>b>c$
• B. $a>c>b$
• C. $c>b>a$
• D. Cannot be predicted

Answer 1

The correct option is A $a>b>c$

Out of all the possible resonating structures, a more stable structure will contribute more to the resonance hybrid and the stability of any resonating structure can be discussed on the basis of its formal charge. Formal charges help in the selection of the most stable structure from all the possible structures.
Formal charge on any atom is calculated as:

Formal charge $\left(FC\right)$ = $V-L-\frac{B}{2}$
where,
V = Total number of valence electrons in an atom.
L = Total number of non-bonding (lone pair) electrons in an atom.
B = Total number of bonding (shared) electrons in that particular atom.

In structure (a),
Formal charges on $S,C\&N$ are $-1,0,0$

In structure (b), Formal charges on $S,C\&N$ are $0,0,-1$

Formal charges on

In structure (c).
Formal charges on $S,C\&N$ are $1,0,-2$

Out of the three, structure (c) is the most unstable because it has more formal charges when compared to the other two structures. Hence, it is considered to be less contributing. Also, even though N is more electronegative than S, a negative charge on N is more unstable because it has a smaller size than S.
Therefore most contributing resonance structure is (a) , where the negative charge is present at the $S$ end.
$\therefore$ correct order is : $a>b>c$