Here's why:

* Atomic Number: The atomic number represents the number of protons in an atom's nucleus.

* Protons Determine Identity: The number of protons defines what element an atom is. For example, all carbon atoms have 6 protons, all oxygen atoms have 8 protons, and so on.

* Isotopes: Isotopes are atoms of the same element (same atomic number) but with different numbers of neutrons. This means they have the same number of protons but different mass numbers (total number of protons and neutrons).

Example:

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

* Both have 6 protons (atomic number 6).

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

Let me know if you'd like to know more about isotopes!