Question

How is the mass number of an isotope expressed in the name of an atom?

Answer 1

The mass number, the number of nuclear particles, is usually expressed as a superscript to the right of the elemental symbol.

Explanation:

If we take the hydrogen atom as an exemplar, most hydrogen atoms contain the one proton in their nuclei, i.e. the `"protium"` isotope, and are represented as `""^1H` (of course most of the time we assume that it is protium).

A few hydrogen atoms contain the 1 proton and a neutron in their nuclei: this is the `"deuterium"` isotope, `""^2H`, or sometimes just `D`. And a smaller few hydrogen atoms contain 2 neutrons to give the tritium isotope, `""^3H`.

As I have written before, `""^2H_2O`, `D_2O`, heavy water, is quite cheap as deuterated solvents go. It is an excellent reagent to label organic compounds with deuterium nuclei if you can find an active hydrogen (one that will exchange) on the original compound. Because deuterium has spectroscopic properties different to protium (for a start its mass!) many experiments can be performed.