# What atomic particles occupy the nucleus of an atom?

The nucleus is composed of protons, massive, positively charged, nuclear particles, and neutrons, massive neutrally charged nuclear particles. The number of protons, $Z$, is the atomic number and determines the identity of the element. $Z = 1 , H$, $Z = 2 , H e$, $Z = 3 , L i$..........
Different numbers of neutrons gives rise to the existence of isotopes: i.e. $\text{^1H,"protium}$, $\text{^2H,"deuterium}$, $\text{^3H,"tritium}$. Given what I have said, can you tell me the number of neutrons in each isotope? Why do they have different masses?