When the number of protons in an atom is different, it's called a different element.

Here's why:

* Protons define an element: The number of protons in an atom's nucleus is its atomic number. This number uniquely identifies an element. For example, all carbon atoms have 6 protons, while all oxygen atoms have 8 protons.

* Isotopes: If you're talking about atoms of the *same* element with different numbers of neutrons, those are called isotopes. For example, carbon-12 and carbon-14 are both isotopes of carbon, but they have different numbers of neutrons (6 and 8, respectively).

Let me know if you have any other questions!