Formation of Elements

Elements are identified by the nuclei of the atoms of which they are made. For example, an atom having six protons in its nucleus is carbon, and one having 26 protons is iron. There are over 80 naturally occurring elements, with uranium (92 protons) being the heaviest (heavier nuclei have been produced in reactors on Earth). Nuclei also contain certain neutrons, usually in numbers greater than the number of protons.

Heavy elements can be formed from light ones by nuclear fusion reactions; these are nuclear reactions in which atomic nuclei merge together. The simplest reactions involve hydrogen, whose nucleus consists only of a single proton, but other fusion reactions, involving mergers of heavier nuclei, are also possible. When the universe formed in an initial state of very high temperature and density called the big bang, the first elements to exist were the simplest ones: hydrogen, helium (two protons), and little else. But we and the earth are made of much heavier elements, so a major question for scientists is how these heavier elements were created.

During the formation of the universe in the so-called big bang, only the lightest elements were formed: hydrogen, helium, lithium, and beryllium. Hydrogen and helium dominated; the lithium and beryllium were only made in trace quantities. The other 88 elements found in nature were created in nuclear reactions in the stars and in huge stellar explosions known as supernovas. Stars like the Sun and planets like Earth containing elements other than hydrogen and helium could only form after the first generation of massive stars exploded as supernovas, and scattered the atoms of heavy elements throughout the galaxy to be recycled.