The subscript represents the number of atoms of an element in a compound. The number is found to the lower right of the element's symbol.
The subscript of one is never written because it is considered to be understood. In a compound, the subscript appears after the end parentheses. For instance, in Ca(NO3)2, the two is the subscript. It represents that two nitrates are present in the compound calcium nitrate.
Subscripts cannot be changed in a compound's chemical makeup. If the subscript is changed, it is no longer the chemical it originally represented. When dealing with polymers, the subscript can sometimes be a letter.