Let's say I want to denote argon-4040. An element XX is written as ""_Z^YXYZX, where ZZ is the element's proton number, and YY is the mass number.
So here, argon would be written as ArAr. We know that argon-4040 has a mass of 40 \ "amu", as its isotope suggests, and its atomic number is 18.
Therefore, we would write it as ""_18^40Ar. However, we can also simplify it into ""^40Ar, as we know that an element is purely based on its proton number, and so we don't have to write it, although it is still useful sometimes, such as in the case of writing nuclear equations.
