The number that indicates the number of atoms of each element in a molecule of a substance is called a subscript. Subscripts are written as small numbers to the right of the element's symbol in a chemical formula.
For example, in the chemical formula for water (H₂O), the subscript "2" indicates that there are two hydrogen (H) atoms in each molecule of water. Similarly, the absence of a subscript for oxygen (O) implies that there is only one oxygen atom in each molecule of water.
Subscripts are essential for representing the correct ratio of atoms in a molecule and are used to express the chemical composition of compounds.