Let's say I want to denote argon-#40#. An element #X# is written as #""_Z^YX#, where #Z# is the element's proton number, and #Y# is the mass number.
So here, argon would be written as #Ar#. We know that argon-#40# 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.
