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