Hi,
you can definitely define custom commands for often used isotopes.
It’s quite simple:
#import "@preview/physica:0.9.4": isotope
#let C13 = isotope("C", a:"13")
#C13
You could even use a show rule to replace normal text with the isotope notation:
#show "C13": isotope("C", a:"13")
This is an C13 isotope.
which results in:
I guess that’s the easiest way to accomplish isotope notation while writing your notes.