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

Mar 9, 2017

#### Answer:

The sum of protons and neutrons is the mass number.

#### Explanation:

And this sum gives the identity of the isotope.

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?