Notation of mass number of an element

[Paradox101]

In a lot of books I have seen the mass number of an element being written on left superscript of its chemical symbol,while my professor and other teachers write it in right superscript.However, it is written at the bottom of the symbol in the periodic table.Which is/are correct, or are all correct but different ways to represent the same thing?

 

[Borek]

It should be - $^{137}_{\phantom{0}55}\mathrm{Cs}$ - that is, mass number on the top, Z (nucleus charge) on the bottom. Generally speaking Z is not necessary when the element symbol is used, but it still makes following the nuclear reaction equations easier.

 

[Paradox101]

Thanks