Formula is a representation of one simple chemical unit (or a molecule) of the compound or element with the help of the symbols of the elements with their required numbers written as subscripts. For example, the formula of sodium chloride is NaCl. The formula of ozone molecule is O~3~, a sulfur molecule is S~8~, and so on. While NaCl is a compound, O~3~ and S~8~ are elements. ### Types of Chemical Formula: (a) Empirical Formula (b) Molecular Formula #### Empirical Formula: An *empirical formula* for a compound represents the simplest whole-number ratio between +ve and –ve ions (ionic compound) or between atoms of elements (covalent compounds). ** a) Empirical Formula for Ionic Compounds: For Ionic compounds, the formula we use is always empirical formulae while for covalent compounds(to be discussed later), the formulae are molecular formulae. ** This is because, an ionic compound is a network solid in which +ve and –ve ions are