Here's why:

* Elements are defined by the number of protons in their nucleus. This is called the atomic number.

* Isotopes of the same element have the same number of protons (same atomic number) but different numbers of neutrons. This difference in neutrons changes the atomic mass of the isotope.

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. This means Carbon-14 is heavier than Carbon-12.

Let me know if you have any other questions!