Here's why:
* Atoms of the same element always have the same number of protons. This number is called the atomic number and defines the element. For example, all carbon atoms have 6 protons.
* The number of neutrons can vary. This leads to different isotopes of the same element. Isotopes have the same atomic number but different atomic masses. For example, carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons.
In summary, the number of neutrons is the only quantity that can vary among atoms of the same element, resulting in isotopes.
