A way of representing a chemical compound using symbols for the atoms present. Subscripts are used for the numbers of atoms. The molecular formula simply gives the types and numbers of atoms present. For example, the molecular formula of ethanoic acid is C2H4O2. The empirical formula gives the atoms in their simplest ratio; for ethanoic acid it is CH2O. The structural formula gives an indication of the way the atoms are arranged. Commonly, this is done by dividing the formula into groups; ethanoic acid can be written CH3.CO.OH (or more usually simply CH3COOH). Structural formulae can also show the arrangement of atoms or groups in space.