The universe began with 9 out of 10 atoms being hydrogen, and it still is that ratio even 13.8 billion years later. Hydrogen is the lightest atom because its nucleus has the fewest particles, just one proton. Helium is the second most abundant element in the universe and the second lightest, containing four particles…

Actually, four hydrogen combine to form one helium-4 isotope. The nuclei (protons) of two of the hydrogen atoms convert to the two neutrons found in helium by emitting positrons (positive electrons) and a neutrino.