Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.

Let's break it down:

* Same element: This means they have the same atomic number, which is defined by the number of protons.

* Same number of protons: As mentioned, this is what determines the element.

* Different number of neutrons: This is the key difference between isotopes. The number of neutrons affects the atom's mass, but not its chemical properties.

Example:

Carbon-12 and Carbon-14 are isotopes of carbon. They both have 6 protons (making them carbon), but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons.