Question

Carbon has a valency $2$in $CO$ and $4$in $C{{O}_{2}}$and $C{{H}_{4}}$. What is its valency in acetylene $\left( {{C}_{2}}{{H}_{2}} \right)$.
A. $1$
B. $2$
C. $3$
D. $4$

Answer 1

Hint:Valency: The combining capacity of atom is known as the valency of that atom. Or in other simple words it means the number of bonds than an atom can form in a compound. Valency of an element in an organic compound is generally fixed and does not change.

Complete step-by-step answer:Let us understand firstly the actual concept of valency or to be more precise, what is the significance of valency.
We know that noble gases are the most stable elements in the periodic table as they have eight electrons in the outermost shell. Rest other elements in the periodic table do not have eight valence electrons in their outermost shell. So, they will either lose, gain or share the electrons in order to attend stability. Always remember, every element, atom, molecule, compound always tends to be in a more stable state.
Valency is basically defined by this gain or loss of electrons from an atom or element. If we want to calculate the valency then we need to actually see the electrons lost, gained or shared and the bonds formed by the element whose valency we want to calculate.
For example: In $CO$ carbon has a valency of $2$. This is because carbon shares its two electrons with oxygen and oxygen in turn share its two electrons with carbon. As a result, both carbon and oxygen complete their octet and the compound carbon monoxide formed is stable.
Now in question we are asked the valency of carbon in $\left( {{C}_{2}}{{H}_{2}} \right)$. We know that the electronic configuration of carbon is $1{{s}^{2}},2{{s}^{2}},2{{p}^{2}}$. There are only four electrons in the valence shell. So, to complete the octet either it will gain four electrons, or lose four electrons or can even share its electrons.
In $\left( {{C}_{2}}{{H}_{2}} \right)$carbon prefers sharing its electrons. Since, it needs four electrons so it shares its four electrons. two electrons with one hydrogen each and the remaining two with the carbon. In turn carbon and hydrogen also share four electrons in total with carbon and thus carbon completes its octet.
Since, four electrons are being shared to complete the octet. Therefore, the valency of carbon in $\left( {{C}_{2}}{{H}_{2}} \right)$is $4$.

Hence, the correct option is Option D.

Note:Valency of an element in an organic compound is generally fixed. It should be noted that there is a difference between valency and oxidation number. The oxidation number is just a hypothetical charge on an atom in a molecule and it is only the measure of its apparent capacity to either lose or gain electrons in that species. On the other hand, valency has no charge and is generally fixed for an element.