Question

How to calculate the valency of the polyatomic ions?

Answer 1

Hint: The valency of an element or ion refers to the measure of its combining capacity with other atoms while forming a chemical compound or molecule. In simple words it is the maximum number of bonds an atom or ion can form while forming a chemical compound.

Complete answer:
A polyatomic ion which is also known as a molecular ion, is a charged species that is composed of two or more atoms bonded to each other via a covalent bond to form a group and overall molecule contains a charge either positive or negative.
Now, to find the valency of polyatomic ions we must first add the number of valence electrons of each atom present in that ion and then add or subtract the number of electrons accordingly, as per charge on the ion given. If the polyatomic ion is positively charged that means ion is electron deficient, so the number of electrons corresponding to that charge gets subtracted whereas if the polyatomic ion is negatively charged that means ion is electron rich, so the number of electrons corresponding to that charge are added in the valency of the polyatomic ion. For better understanding, let us take an example:
Polyatomic ion: $O{H^ - }$
Valence electrons in oxygen atom $ = 6$
Valence electrons in hydrogen atom $ = 1$
The polyatomic ion is negatively charged and consists of a charge of $ - 1$ that means the molecule has one extra electron. Therefore, the valency of the polyatomic ion $ = 6 + 1 + 1 \Rightarrow 8$.

Note:
Remember that the valency of an atom and oxidation state are two different terms as oxidation state is a hypothetical charge of an atom in a molecule or ion which represents the actual number of electrons shared by that atom whereas valency is the maximum number of electrons an atom can share.