Isotopes are atoms of the same element that have a different number of neutrons.

Here's why:

* Element: An element is defined by the number of protons in its nucleus. This is called the atomic number.

* Isotopes: Isotopes of the same element have the same number of protons (same atomic number) but different numbers of neutrons.

For 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.