Question

How do we know the valency of a polyatomic ion? Such as hydroxide and many others.

Answer 1

• Ions can be a single charged atom or a group of atoms that act as an ion. When a group of atoms combine to form an ion then they are called polyatomic ions.
• As valency is the combining capacity of the atom or an ion. So, the valency of the polyatomic ions is determined by the resultant charge after the combination of valencies/oxidation numbers of the atoms present in them.

Examples:

1. Hydroxide ion$\left({\mathrm{OH}}^{-}\right)$, has an oxygen atom$\left(\mathrm{O}\right)$ and a hydrogen atom$\left(\mathrm{H}\right)$. The oxidation number of oxygen is -2, and the oxidation number of hydrogen is +1. Adding the oxidation numbers of all the atoms in the polyatomic ion$\left[-2+1=-1\right]$ so the resultant charge comes up to be -1. Hence, the hydroxide ion is expressed as ${\mathrm{OH}}^{-}$.
2. Ammonium ion$\left({\mathrm{NH}}_4^{+}\right)$, has a nitrogen atom$\left(\mathrm{N}\right)$ and four hydrogen atoms$\left(\mathrm{H}\right)$. The oxidation number of Nitrogen is -3, and the oxidation number of hydrogen is +1. Adding the oxidation numbers of all the atoms in the polyatomic ion$\left[-3+(4\times (+1\left)\right)=+1\right]$ the resultant charge comes up to be +1. So, the hydroxide ion is expressed as ${\mathrm{NH}}_4^{+}$.