When the number of neutrons in an atom is changed, you form an isotope of that element.

Here's why:

* Atomic Number: The number of protons defines the element. Changing the number of protons would change the element entirely.

* Isotopes: Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons. This changes the atom's mass but not its chemical properties.

Example:

* Carbon-12 has 6 protons and 6 neutrons.

* Carbon-14 has 6 protons and 8 neutrons.

Both are carbon, but they are different isotopes.

Important Note: Changing the number of neutrons can sometimes lead to unstable isotopes called radioisotopes, which undergo radioactive decay.