Hydrogen and helium are the two most ancient, abundant and primitive elements in the universe. As can be seen from Figure 1, hydrogen consists of a single proton and a single electron, while helium contains two electrons surrounding a nucleus of four elementary particles—two protons and two neutrons. Now, it’s possible to add neutrons to either atom without changing the element. For example, if we added a neutron to hydrogen, it would be almost twice as heavy (since protons and neutrons have about the same mass), but it would still be hydrogen. We would call it deuterium, which is the special name for this isotope of hydrogen, but it would be hydrogen nevertheless. Add a neutron to helium and you get a heavy isotope of helium. But if you added a proton to helium, you wouldn’t have helium anymore—you’d have an isotope of lithium.

This brings up an important fundamental concept: an element is defined by the number of protons in its atoms. Not electrons. Not neutrons. The number of protons in the atom is the atomic number, and the atomic number identifies the element. Any atom with an atomic number of 2 is an atom of helium, no matter how heavy, no matter how many electrons or neutrons it has.