# For a given atom, what is the mass number?

For the simplest (and most abundant element), hydrogen, MOST hydrogen nuclei are garden variety $\text{protium}$, ""^1H. A small proportion of hydrogen atoms are $\text{deuterium}$, ""^2H. An even smaller proportion of hydrogen atoms are $\text{tritium}$, ""^3H. What is the number of neutrons in each isotope?