Question

What does the superscript of an isotope notation mean?

Answer 1

This represents the NUMBER of nuclear particles....

Explanation:

Let's take a simple example, i.e. for hydrogen, the MOST abundant element in this universe. Most hydrogen nuclei contain the ONE nuclear particle, i.e. one proton....and we would represent this ISOTOPE as `""^1H`, `"protium"`, to reflect its mass.

A smaller percentage of hydrogen atom necessarily contain the DEFINING proton, and ALSO a neutron...to give the `"deuterium isotope"`....i.e. `""^2H`..here in the nucleus there are one proton (necessarily) and ALSO ONE neutron.

And an even smaller percentage of hydrogen atoms, contain the `"tritium isotope"`, i.e. `""^3H`. How many neutrons does this isotope contain?

And so the superscripted number is the number of `"nucular particles"`, protons, and neutrons. The atomic symbol defines `Z`, the atomic number. To a first approximation (which does not work so well for hydrogen), the chemistry of the isotopes are identical.