# How do isotopes of an element differ? In what ways are they the same?

Let's take the element hydrogen as an exemplar, which so far as I know is the most abundant element in the universe. Most hydrogen nuclei are ""^(1)H; their nuclei each contain just the one proton, just the one fundamental, massive, positively-charged nuclear particle - we would call this nucleus the $\text{protium isotope}$.
A few hydrogen atoms, are substituted by an extra massive nuclear particle, the neutron, which we would represent as ""^2H, $\text{the deuterium isotope}$. Note that the element is still hydrogen, because $Z$, $\text{the atomic number}$, is still $1$. And a smaller few hydrogen nuclei are substituted by a nuclide with 2 neutrons, i.e. ""^3H, $\text{the tritium isotope}$.
And thus all isotopes of the same element necessarily contain $\text{THE SAME NUMBER OF PROTONS,}$ but $\text{DIFFERENT NUMBERS OF NEUTRONS}$. Most elements have several different isotopes, and their weighted average gives rise to the quoted atomic mass. Capisce?