CBSE Class 9 Answered

In chemistry, ions can be a single charged atom, or they can be a group of atoms that act as an ion. These groups of atoms are called as polyatomic ions. In polyatomic ions each ion carry a specific charge, which is determined by their numbers of valence electrons.

Step 1

Determine the oxidation number of each atom in the polyatomic ion. For example, consider the hydroxide ion, which has an oxygen atom and a hydrogen atom. The oxidation number of oxygen is -2, and the oxidation number of hydrogen is +1.

Step 2

Add together the oxidation numbers of all the atoms in the polyatomic ion.  In the above example, -2 +1 = -1. This is the charge on the hydroxide ion which is a polyatomic ion.

Step 3

Write this charge as a superscript to the right of the polyatomic ion's formula. For a single charge, write - or + instead of 1- or 1+. In the example, the hydroxide ion is expressed as OH¯.