# Question #e5346

Sep 29, 2016

First you need $Z$, $\text{the atomic number}$.

#### Explanation:

And $\text{Mass number"-"Atomic number}$ $=$ $\text{Number of neutrons}$.

Both neutrons and nuclear protons are the massive particles of the atom, and both contribute to the mass of an isotope. You need both to make the determination of mass.