Here's why:

* Isotopes are atoms of the same element that have the same number of protons (defining the element) but different numbers of neutrons.

* Protons determine the atomic number of an element and its charge. Uncharged elements have an equal number of protons and electrons.

* Neutrons contribute to the atomic mass of an element. Different numbers of neutrons lead to different atomic masses, creating the various isotopes of an element.

For example:

* Carbon-12 has 6 protons and 6 neutrons.

* Carbon-14 has 6 protons and 8 neutrons.

Both are carbon atoms because they have 6 protons, but they have different numbers of neutrons, making them isotopes with different atomic masses.