Helium-4 (also written He) is an isotope of helium. It is the most common isotope of helium in the universe. Out of all the helium on the Earth, 99.99986% is helium-4. The nucleus (the center) of a helium-4 atom is also known as the alpha particle. Helium-4 is made of 2 protons (the part of an atomic nucleus with a positive charge), 2 neutrons (the part of an atomic nucleus with no charge) and 2 electrons (the part of an atom that goes around the nucleus with a negative charge).