# What are two atoms of the same element with different mass numbers called?

You speak of $\text{isotopes......}$
All hydrogen nuclei contain the one proton, the one positively charged, massive nuclear particle. This is the $\text{protium isotope}$, ""^1H. A few hydrogen nuclei contain one proton PLUS one neutron, a massive, neutrally charged particle; this is the $\text{deuterium isotope}$, ""^2H. And a smaller few hydrogen nuclei contain 2 neutrons, to give the $\text{tritium isotope}$, ""^3H.