Here's the breakdown:

* Atomic Number: The number of protons in an atom's nucleus. This defines the element.

* Mass Number: The total number of protons and neutrons in an atom's nucleus.

Isotopes are atoms of the same element (same atomic number) that have different numbers of neutrons, leading to different mass numbers.

Example:

* Carbon-12 and Carbon-14 are isotopes of carbon.

* Both have 6 protons (atomic number = 6), defining them as carbon.

* Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons. This makes their mass numbers different (12 and 14, respectively).