Question

In ${\text{SnCl}}_{\text{3}}^{\text{ - }}$ ion, calculate the formal charge on
(A) ${\text{Sn}}$
(B) $+ 1$
(C) $- 1$
(D) $0$

Answer 1

To answer this question, you must recall the formula for calculating formal charges on atoms in a compound. The formal charges on the respective atoms of a compound help us to determine the most stable structure possible for the compound.

Formula used: ${\text{FC}} = {\text{V}} - {\text{N}} - {\text{B/2}}$
Where, ${\text{FC}}$ represents the formal charge on the atom under observation
${\text{V}}$ represents the valence electrons present on the atom in its free state.
${\text{N}}$ represents the number of non- bonding electrons present on the atom
And, ${\text{B}}$ represents the number of bonds formed by the atom in the species thus, the term ${\text{B/2}}$ represents the number of valence electrons of the atom involved in bond formation.

Complete step by step solution
To find the formal charge we must first determine the central atom in the given species. In ${\text{SnCl}}_{\text{3}}^{\text{ - }}$ ion, the central atom is ${\text{Sn}}$. Next we calculate the valence electrons present on the atom. The number of valence electrons in tin are 4.
In the given species, tin carries a lone pair of electrons and forms three bonds with chlorine atoms.
So using the formula, ${\text{FC}} = {\text{V}} - {\text{N}} - {\text{B/2}}$, we can calculate the formal charge on ${\text{Sn}}$as,
${\text{FC}} = 4 - \left[ {2 + \dfrac{1}{2}\left( 6 \right)} \right]$
$\Rightarrow {\text{FC}} = 4 - 5 = - 1$
The correct answer is B.

Note
Formal charge is a theoretical concept which is used when studying the properties of a species at the atomic level. It is a fraudulent charge associated with each atom of the species. The formal charge does not indicate a real charge separation as the real charge is spread uniformly on the entire species. It is useful for the determination of the most stable structure among various possible resonance structures of a compound.