Here's why:

* Atoms of the same element have the same number of protons. This number is called the atomic number and defines the element.

* Isotopes are atoms of the same element that have different numbers of neutrons. This difference in neutrons changes the atom's mass.

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 are different isotopes due to the different number of neutrons.