Here's why:

* Atomic number: The atomic number defines the element. It represents the number of protons in the nucleus of an atom. Since protons determine what element something is, changing the number of protons would change the element itself.

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

Example:

* Carbon-12 and Carbon-14 are isotopes of carbon. They both have 6 protons (atomic number 6) but differ in the number of neutrons (6 for Carbon-12 and 8 for Carbon-14).

To find the atomic number of an element:

* Periodic table: Look up the element on the periodic table. The atomic number is usually located above the element's symbol.

Let me know if you have any other questions!