The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or 12, which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: 12 6C. This is technically redundant, as each element is defined by its atomic number, so it is often omitted.

Answer this question: where is the atomic mass located on an element? Let's give stream of consciousness first:
This is the relevant information: The mass number is written either after the element name or as a superscript to the left of an element's symbol.
So, the final answer is either after the element name or as a superscript to the left of an element 's symbol.