Why Does Fusion Only Occur for Elements in the Periodic Table Up to Iron?

Atoms and atomic nuclei can undergo various transformations, and such chemical and nuclear reactions are essential to all life. Nuclear fusion involves the production of heavier atomic nuclei from lighter ones and is the process responsible for the Sun's energy output. Fusion reactions generally release energy only for elements lighter than iron.

Atomic Structure

The atom is composed of protons, neutrons and electrons. Protons and neutrons are known as nucleons since they compose the nucleus. Electrons are found outside of the nucleus, orbiting around it. Neutrons have no charge, but protons are positively charged and, therefore, so is the nucleus. An electron has a negative charge that exactly counterbalances the charge of one proton. If an atom contains more electrons than protons, it becomes a negative ion; if it contains more protons than electrons, it is then a positive ion.

Nuclear Fusion

In fusion, the nuclei of two or more atoms actually combine to become a single, new, heavier nucleus. Because the nucleus of an atom is positively charged, when two or more nuclei are brought together they repel each other strongly. Consequently, it takes very large amounts of energy to overcome the repulsion and bring about fusion. When sufficient amounts of energy are supplied and the nuclei are close enough, the nuclear force actually becomes strong enough to attract the nuclei together and cause their fusion.

Fusion of Lighter Elements

Even though it takes a lot of energy to overcome the repulsive forces between nuclei to bring them together, once fusion takes place there can actually be a release of energy that is greater than the energy that was originally required to overcome nuclear repulsion. This means that the overall fusion process releases energy, or is exothermic. This is only the case, however, for elements up to iron. In the case of elements heavier than iron, the overall fusion process actually requires an input of energy, which is why fusion generally occurs only for lighter elements.

The Sun

Fusion reactions in the Sun are the reason for its energy output. In the core of the star, the element hydrogen is converted into the element helium. The actual fusion reaction requires four hydrogen nuclei and produces one helium nucleus. In each such reaction, a very small amount of matter is converted into a small amount of energy because the single helium nucleus is slightly lighter than the four starting hydrogen nuclei. Innumerable such reactions take place in the Sun each second and are collectively responsible for its enormous output of energy.