Compounds composed of metals and nonmetals contain charged species. The charged species are known as ions. Ions may consist of a single charged atom or a group of atoms that have a net charge on them. An ion can be negatively or positively charged. A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’.

Take, for example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+ ) and negatively charged chloride ions (Cl). A group of atoms carrying a charge is known as a polyatomic ion

Formula Ions Name. NaCl = Na+ + Cl , sodium chloride. K2S = 2 K+ + S2- , potassium sulfide. MgO = Mg2+ + O2- magnesium oxide. CaI2 = Ca2++ 2 I , calcium iodide. Al2O3 = 2Al3++ 3O2- aluminum oxide.

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