Here's why:

* Atomic Number: The number of protons defines an element. All atoms of the same element have the same number of protons.

* Mass Number: The mass number is the sum of protons and neutrons.

* Isotopes: Atoms of the same element with different numbers of neutrons are called isotopes. They have the same atomic number but different mass numbers.

Example: Carbon-12 and Carbon-14 are both isotopes of carbon. They both have 6 protons, but Carbon-12 has 6 neutrons while Carbon-14 has 8 neutrons.