The number placed in front of an element’s symbol in a chemical formula is called a coefficient. Coefficients indicate the number of molecules or moles of that substance in a chemical equation.
In contrast, subscripts are used to indicate the number of atoms of an element in a molecule (e.g., in H₂O, the '2' is a subscript showing there are two hydrogen atoms). Atomic symbol and chemical symbol generally refer to the one or two-letter representation of an element (e.g., H for hydrogen, O for oxygen) and are not associated with numbers in the same way as coefficients or subscripts.