how do i type atomic numbers and mass numbers in Word

[Jay Freedman]

Use these shortcuts to change the format to subscript or superscript:

Ctrl and the = key for subscript.
Ctrl and the + key (that is, Ctrl and Shift and =) for superscript.

 

[Klaus Linke]

And if you need both, you could either try an eq field:
{ eq \a(235,92)}U
(format the numbers in a smaller font size)
or the Equation Editor (second row, third item on the Equation Editor toolbar).

Regards,
Klaus