Final answer:
Subscripts in a molecular formula represent the number of each type of atom in a molecule, with no subscript indicating just one atom.
Step-by-step explanation:
Subscripts in a molecular formula indicate how many of each type of atom can be found within a molecule. For instance, the molecular formula for water is H2O, which shows that there are two hydrogen atoms and one oxygen atom in a water molecule. If only one atom of a specific type is present, there is no subscript used next to that chemical symbol.
A molecular formula provides an easy way to see the composition of a molecule but does not indicate the arrangement of atoms. For example, we write NH3 for ammonia to represent one nitrogen atom and three hydrogen atoms. Writing N3H, as opposed to NH3, would incorrectly suggest three nitrogen atoms and one hydrogen atom in the molecule, which is not the case for ammonia.