Isotope symbols ....
Ideally the atom number is written as a subscript to the left of the symbol and the mass number is written as a superscript to the left of the symbol. See the symbols written at the bottom. But that is difficult with with the usual word processing software, so special programs have to be used to do that (the symbols at the bottom were written in Microsoft Equation Editor 3.0). Therefore, there are several work-arounds which are used as seen directly below.
This is the notation that you will see in the Handbook of Chemistry and Physics.
A variation of this can be done on one line, and is found in emails and forum's like this where it is difficult to do subscripts and superscripts. It is the ZEA notation where Z is the atomic number, E is the symbol of the element and A is the mass number.
2He4 ... and .... 16S34
Since the atomic number and the element symbol are intrinsically tied together, the atomic number is sometimes omitted and the symbols written as:
He-4 ... and ... S-34
It takes special software to write the atomic number and mass number as subscripts and superscripts on the left side of the symbol. This is why there are other work-arounds, particularly the one used by the CRC Handbook.