Answer: Isotopes
Explanation: Every atom/element is defined by the number of protons. So if it has 1 proton it's hydrogen no matter what, if it has 2 protons it's helium no matter what and so on. So if you know the number of protons, you know the element.
You can change the neutrons and you will get isotopes. Atoms that have different numbers of neutrons are called isotopes of each other. However, the element is the same but one is going to be a little bit heavier or lighter than another.
