# In the nuclear symbol for deuterium, ""_1^2H, what is the atomic number and the mass number?

And since it is an hydrogen isotope we can IMMEDIATELY say that ${Z}_{\text{atomic number}} = 1$. Why? Because for elemental hydrogen $Z = 1$ BY SPECIFICATION. And since $Z$ represents the number of massive, CHARGED nuclear particles, we say that ALL hydrogen nuclei contain one proton, i.e. one massive particle, with a formal POSITIVE charge.
And 99.98% of all hydrogen nuclei in this universe is the ""^1H isotope. But the nucleus could also contain massive particles of ZERO charge, and these are so-called neutrons... The deuterium isotope, ""^2H, approx. 0.02% abundant, contains the one (and defining proton), and ALSO a neutron, a massive particle of ZERO charge. And a trace percentage of hydrogen atoms contain TWO NEUTRONS, $\text{the tritium isotope}$, the which we would represent as ""^3H. Note that we do not have to specify the atomic number $Z$, given that the atomic number, here $H$, ALREADY specifies that $Z = 1$.
The mass number is simply the number of $\text{MASSIVE NUCULAR PARTICLES}$, for the $\text{protium, deuterium, and tritium isotopes}$, the mass numbers are $1 , 2 , 3$ respectively... With me?