Helium-4 (or )'s a non-radioactive and light isotope of helium. It's by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus's the same as an alpha particle, consisting of two protons and two neutrons. The total spin of the nucleus's an integer so it's a boson. Alpha decay's a common decay mode for many radioactive isotopes. In fact, alpha decay of heavy elements's the source of most naturally occurring helium-4 o… (More on Helium-4)