Isotopes are atoms of the same element that have the same number of protons (and therefore the same atomic number), but different numbers of neutrons. This difference in neutrons leads to a different atomic mass.

Here's a breakdown:

* Atomic Number: Determines the element. It's the number of protons in the nucleus.

* Atomic Mass: The total number of protons and neutrons in the nucleus.

For example:

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

* They both have 6 protons (atomic number 6), making them both carbon.

* Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons.

* This gives Carbon-12 an atomic mass of 12 and Carbon-14 an atomic mass of 14.