A chemical formula is a way to represent a substance using symbols for its constituent elements. Each element is denoted by its chemical symbol, such as H for hydrogen and O for oxygen. The formula also indicates the number of atoms of each element present in the compound, using subscripts. For example, H2O represents water, which contains two hydrogen atoms and one oxygen atom. Chemical formulas can be classified into two main types: empirical and molecular. An empirical formula shows the simplest ratio of elements in a compound, while a molecular formula provides the actual number of atoms of each element. For instance, the empirical formula for C6H12O6 (glucose) is CH2O.