In a molecular formula, a subscript is written immediately after the letter denoting an element and refers to the amount of atoms that element has. A molecular formula, or empirical formula, is noted as a series of letters and numbers that represent a compound, or a molecule. Each letter in a formula symbolizes one element from the periodic table of elements.
For example, carbon monoxide is written out as CO, whereas carbon dioxide is written out as CO2. The number two is the subscript, denoting there are two atoms of oxygen in the compound. If an element's symbol does not have a subscript, such as in CO, then it is assumed that there is only one atom of each element.