A chemical formula is an easy way of expressing information about the atoms that constitute a particular chemical compoun, and how the relationship between those atoms changes in chemical reactions. For molecular compounds it is also known as the molecular formula, and identifies each constituent element by its chemical symbol and indicates the number of atoms of each element found in each discrete molecule of that compound. If a molecule contains more than one atom of a particular element, this quantity is indicated using a subscript after the chemical symbol (although 19th-century books often used superscripts). For ionic compounds and other non-molecular substances, the subscripts indicate the ratio of elements in the empirical formula.