The release of energy occurs when nuclei that are less stable react to form nuclei that are more stable. Three processes that can occur in the nucleus to form more stable nuclei are nuclear fusion, nuclear fission, and radioactive decay. First, let’s focus on nuclear fusion.
Nuclear fusion is a process in which nuclei with a light mass combine to form a nucleus with a heavier mass. This occurs because the heavier nuclei are more stable. Fusion occurs at very high temperatures such as those that exist in stars like our sun. In the stars, hydrogen (atomic number = 1) nuclei combine to produce a helium (atomic number = 2) nucleus. This produces the energy that powers our sun. Fusion cannot occur at normal temperatures because it takes tremendous amounts of energy to bring protons close enough for the strong force to overcome the electrostatic repulsion between them. The extremely high temperatures necessary to sustain these reactions are very difficult to sustain here on Earth.
About 4 x 1026 Joules of energy are released every second from the fusion of hydrogen into helium. Unfortunately, we cannot currently use this process on Earth because of the high heat and pressure needed to start the reaction.
Fusion gives off a large amount of energy but cannot be used to generate energy on Earth because it takes very high temperatures to get it started. However, the fusion of hydrogen is what powers stars like our sun. View this slideshow to learn how the uranium used in power plants here on Earth was originally produced from fusion reactions that occurs in stars.
During most of a star’s life, the reaction fusing different isotopes of hydrogen (atomic number = 1) into helium (atomic number = 2) is occurring in the core. There are also reactions occurring that are producing the elements lithium (atomic number = 3) and beryllium (atomic number = 4).
The nuclear equation for the primary reaction in which hydrogen-2 and hydrogen-3 combine to form helium and a neutron is shown here.
\(_{1}^{2}{\text{H} +_{1}^{3}{\text{H} \rightarrow_{2}^{4}{\text{He} +_{0}^{1}\text{n}}}}\)
When the core of the star runs out of hydrogen, the star begins to die. Then it expands into a red giant. At this phase, fusion reactions occur that combine helium (atomic number = 2) atoms to form carbon (atomic number = 6) atoms.
The nuclear equation for this reaction is
\(_{2}^{4}{\text{He} +_{2}^{4}{\text{He} +_{2}^{4}{\text{He} \rightarrow_{6}^{12}{\text{C}\ + \ \gamma}}}}\).
If the star is more massive, it will form elements with atomic numbers between oxygen (atomic number = 8) and iron (atomic number = 26).
The following equation represents the nuclear reaction for the formation of iron.
\(_{13}^{28}\text{Si} +_{13}^{28}{\text{Si} \rightarrow_{26}^{56}\text{Fe}}\)
When the core of a star is made up mostly of iron, the core will collapse. When a very massive star reaches this point, it will explode and become a supernova. The supernova releases a large amount of energy that is used to produce elements heavier than iron, like silver (atomic number = 47), gold (atomic number = 79), and uranium (atomic number = 92). When the supernova explodes, all the elements are expelled out into space.
All naturally occurring elements with an atomic number greater than lithium (atomic number = 3) were formed by nuclear fusion in stars. In this way the uranium, which is used in nuclear power plants, was originally produced in supernova explosions of the stars.
Let's Practice
Complete this ordering activity to see how well you understand how fusion is responsible for the formation of elements heavier than hydrogen. Place the steps in order and keep trying until your order is correct.
Stars convert hydrogen to helium.
Stars fuse helium atoms to make elements like carbon.
Stars form elements like oxygen through iron in their core.
In a super nova, heavier elements such as uranium form.
Reflection
Describe how nuclear fusion can contribute to nuclear power, even though fusion reactions cannot happen here on Earth.
