Question

In thiocyanate ion, what is the formal charge on the sulfur atom?

Answer 1

Hint: Thiocyanate ion consists of a Sulphur atom, carbon atom and nitrogen atom with a negative charge on nitrogen atom. The formal charge on the Sulphur atom can be calculated from the valence electrons of Sulphur, number of bonds formed from the Sulphur and number of non-bonded electrons in Sulphur.

Complete answer:
Every atom has the electrons which are core and valence electrons. Only valence electrons involved in the bond formation. All the valence electrons do not involve a bond, some of them exist as non-bonded electrons called a lone pair of electrons.
Thiocyanate ion consists of a Sulphur atom, carbon atom and nitrogen atom. The total electrons present on the three atoms are \[6 + 4 + 5 + 1 = 16\] electrons. Sulphur has \[6\], carbon has \[4\], and nitrogen has \[5\] valence electrons and one negative charge as it is an ion. These \[16\] electrons are involved in the bond formation between these atoms in thiocyanate ion.
The Sulphur and carbon have double bond, the carbon and nitrogen have double bond. Thus, \[8\] electrons were utilized. Remaining \[8\] electrons exist as lone pairs on sulphur and nitrogen atoms.
The formal charge on sulphur atom will be (valence electrons) \[ - \] (Number of bonds \[ + \] Number of non-bonded electrons) is given as:
\[6 - \left( {2 + 4} \right) = 0\]
Thus, the formal charge on sulphur atom is \[0\].

Note:
The formal charge can also be calculated by substituting the number of bonded electrons divided by two, in place of the number of bonds as two electrons make a single bond. As thiocyanate ion is an ion the negative charge is associated with nitrogen due to more electronegativity.