Most of these “heavy elements” are created by atomic nuclei capturing neutrons. However, looking at the periodic table, we see that that leaves two-thirds of the elements’ formation still unaccounted for.
The heavier elements, up to iron, were forged later, in the hearts of stars and in supernovae. A: The lightest elements in the universe - hydrogen, helium, and a little lithium - were born shortly after the Big Bang.