An ion is a charged particle and can be negatively or positively charged.

A negatively charged ion is called an „anion‟ and the positively charged ion, a „cation‟. For example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl^–). Ions may consist of a single charged atom or a group of atoms that have a net charge on them. A group of atoms carrying a charge is known as a polyatomic ion e.g. Calcium oxide (Ca+2 O-2)