All isotopes of a particular element have the same atomic number. How then do the isotopes of a particular element differ?
The number of neutrons vary.
Atomic Number: The number of protons in an atom, NOT the number of neutrons
Isotope: An element with a varying number of neutrons
Remember, we CANNOT change the number of protons without changing the element.
However, by adding or subtracting neutrons, we can change an element's properties.
This site goes into the different isotopes of H.