Question

Valency of carbon in $C{{O}_{3}}^{2-}$ ion is:
(A) 2
(B) 3
(C) 4
(D) 5

Answer 1

Hint: Valency is basically the combining capacity of an element. The compound will form covalent bonds equal to its valency or equal to its valence electrons (when equal).

Complete Solution :
Let us know some basic steps before answering the illustration;
Valency-
It is the capacity of an atom to combine with other atoms to form a stable compound.
Process to find valency of any atom in a compound-
The overall valency would be stated on the compound. If it is negative, we can call it as anion; if it is positive, we can call it as cation; if it is zero, we can call it as a neutral compound. We will understand this by considering an example.
For example: we have XYZ (a neutral compound),
Y has a charge of +1, Z has a charge of -2 and we need to analyse the valency of X. This can be done as;
$\begin{align}
  & X+1+\left( -2 \right)=0 \\
 & X-1=0 \\
 & X=+1 \\
\end{align}$
Hence, the valency of X is 1.

Illustration-
We need to find the valency of carbon in $C{{O}_{3}}^{2-}$, so let us follow the above procedure considering the valency of carbon as x.
The overall charge of $C{{O}_{3}}^{2-}$ is -2 and we know that the charge on oxygen atom is -2 thus,
$\begin{align}
  & C{{O}_{3}}=-2 \\
 & x+\left[ 3\times \left( -2 \right) \right]=-2 \\
 & x-6=-2 \\
 & x=+4 \\
\end{align}$
Thus, the valency of carbon in $C{{O}_{3}}^{2-}$ is 4.
So, the correct answer is “Option C”.

Note: Alternatively, by stating the electronic configuration of carbon atom we can find its valency in $C{{O}_{3}}^{2-}$. Also, option (D) can never be the answer as 5 cannot be the valency of any atom; though valence electrons in atoms can be 5.