# What is the "mass number" of an atom?

$\text{The mass number of an atom.............}$
The mass number of an atom is its $\text{atomic number,}$ $Z$, PLUS its number of neutrons.....i.e. it is the number of massive nuclear particles. The mass number is specific to a particular isotope. Can you tell me the mass numbers of the following hydrogen isotopes: ""^1H, ""^2H, and ""^3H?