All isotopes of a particular element have the same atomic number. How then do the isotopes of a particular element differ?

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.

For an example: https://tutor.hix.ai

This site goes into the different isotopes of H.

A given element's isotopes have different mass numbers because different nuclei have different numbers of neutrons; while all isotopes of the same element have the same number of protons (and therefore the same atomic number), different neutron counts can cause differences in mass numbers.

Isotopes of a particular element differ in their mass numbers, which is the sum of the number of protons and neutrons in the nucleus of an atom. Different isotopes of an element have different numbers of neutrons while maintaining the same number of protons (which determines the atomic number of the element).