Nuclear reactions change the nuclei of atoms so that atoms change from one element into another. The two basic types of nuclear reactions are nuclear fusion and nuclear fission reactions.
Sometimes people have trouble remembering which is fusion and which is fission. Remember that the word fuse means join together, so fusion reactions are lighter atoms joining together into heavier atoms. A fissure is a split, so fission reactions involve splitting heavy atoms into lighter elements.
Nuclear Fusion
The simplest nuclear fusion reaction is hydrogen to helium. Four hydrogen atoms fuse to make one helium atom and some byproducts, including energy. Like most nuclear reactions, the reaction does not occur in one step. In several steps, four hydrogen atoms fuse into helium.
This reaction releases energy because the helium atom produced has very slightly less mass than the four hydrogen atoms that started the reaction. In all energy producing nuclear reactions, the total mass of the products after the reaction is less than the total mass before the reaction. The lost mass is converted into energy.
Hydrogen into helium is the most common nuclear fusion reaction. These reactions provide the energy for our Sun and other stars. When the hydrogen runs out, elements as heavy as iron are also made by nuclear fusion reactions in massive stars.
Fusing atoms of elements heavier than iron requires energy rather than releases energy. So fusion reactions making atoms heavier than iron only occur only when there is a huge abundance of energy, such as when a massive star explodes as a supernova.
Iron is the boundary between fusion and fission
Nuclear Fission
In nuclear fission reactions atoms of heavy elements, such as uranium or plutonium, split to form atoms of lighter elements. Like fusion reactions, nuclear fission reactions occur via a series of steps. Elements lighter than iron do not normally undergo fission reactions because they would require rather than release energy.
When atoms heavier than iron undergo fission reactions, energy is released because the total mass of the final products is less than the starting mass. The extra mass is converted into energy.
Examples of Fission and Fusion Reactions
Nuclear power plants produce energy by nuclear fission reactions. Their fuel is radioactive elements like uranium. The end products of these reactions are also often radioactive. With proper precautions, nuclear power can be safe, but many people fear the radioactive fuels and end products.
Many scientists are working to produce controlled nuclear fusion reactions, but have not yet been successful. If they do succeed the fuel and end products, hydrogen and helium, will not be dangerously radioactive.
Nuclear fusion reactions require very high temperatures, such as found in the core of the Sun. A few scientists have claimed to have produced cold fusion reactions where they fused hydrogen into helium at approximately room temperature. If they were possible, such reactions would immensely help the world's energy problems. However other scientists have been unable to duplicate the work and the cold fusion claims have been discredited.
Nuclear weapons can use either nuclear fission or fusion. Atomic bombs use nuclear fission of uranium or plutonium atoms to produce their blast. More powerful hydrogen bombs produce their energy by fusing hydrogen into helium
Nuclear fusion and fission reactions play an important role in both solar and nuclear energy production. They also power the most dangerous weapons the world has known.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
