At the centre of every atom lies the nucleus. This is the core of the atom and constitutes more than 99% of the atom's mass. It consists of neutrons and protons that are held together by a strong nuclear force. Heavier atoms consist of heavier nuclei, that is, there are more protons and neutrons. These tiny components are responsible for some of the most energetic (and destructive) reactions that occur on Earth. Since humans have learned that nuclei can be split apart or fused, we have been curious to understand the energies involved in these reactions. In this article, we will discuss the splitting and combining of nuclei, also known as fission and fusion respectively.
The meaning of fission and fusion
Smaller atoms can sometimes combine to form heavier atoms when their nuclei collide and merge. This process is known as nuclear fusion, during which, energy is released. Heavier nuclei can also be split in a process called nuclear fission, which produces smaller nuclei and some energy.
Nuclear fission reactions
Some heavy atoms are unstable and they maintain stability by emitting radioactive particles such as alpha and beta particles. This process occurs spontaneously and randomly. Large nuclei can also become more stable by splitting into two smaller nuclei. This process is known as nuclear fission and is not random. It requires a neutron to collide with the heavy nucleus for the splitting to occur. After the neutron collides with the nucleus, it causes the nucleus to become unstable. The nucleus then splits into two smaller nuclei, that are similar in size, and releases two or three neutrons in the process along with large amounts of energy in the form of gamma rays. The smaller nuclei, known as the fission products, are usually also unstable and may release alpha or beta particles to attain stability. Part of the energy that is released is in the form of the kinetic energy of the fission products. The example below shows how fission occurs.
The figure below is an example of the fission of uranium-235 into barium-139 and krypton-95.
A uranium-235 nucleus is struck by a neutron and splits into barium-139 and krypton-95. Two neutrons are released in the process along with 200 MeV of energy, Wikimedia Commons CC BY-SA 3.0
A neutron is fired into a stable uranium-235 nucleus, making it momentarily unstable. It then splits into barium-139 and krypton-95 which are both smaller nuclei. Two neutrons are released in the process andof energy (this is equivalent to). The barium and krypton nuclei may undergo alpha and beta decay and form even smaller nuclei. The fission products (barium, krypton and nuclei) all have some kinetic energy after the fission occurs.
Nuclear fission diagram
Moving neutrons are responsible for fission to occur and the fission products include neutrons with kinetic energy. These neutrons that are produced may then be used to cause further nuclear fissions. This process is called a nuclear chain reaction. The example above showed uranium-235 undergoing fission to form barium-139 and krypton-95 and two neutrons. These two neutrons can cause two further uranium nuclei to undergo fission producing four neutrons which may cause four fissions to occur. The number of fissions will therefore grow exponentially with time. This is called an uncontrolled chain reaction and can release an enormous amount of energy in a short amount of time. It is this principle that was used to build the atomic bomb. Just a small amount of uranium undergoing fission has the potential to destroy an entire city.
If we could control the number of product neutrons used to initiate further fissions then we could control the total amount of energy that is released. If only one product neutron is used to cause the fission of one nucleus, then the chain reaction is controlled. This is how nuclear fission reactors are created using uranium. The fission diagram below shows the uncontrolled chain reaction that would occur from the fission of uranium-236.
Uranium-236 undergoes fission to produce barium-144 and krypton-89. The two neutrons produced are then used to cause the fission of two further uranium-236 nuclei. The process continues in an uncontrolled chain reaction, Wikimedia Commons CC BY-SA 4.0
Nuclear fission example
We must first recall that the symbol for any neutral atomis written in the following way:
where the atomic numberrepresents the number of protons in the nucleus of this atom and the mass numberrepresents the number of protons and neutrons in the nucleus. Now, if we consider the previous example of the splitting of uranium-235 into barium-139 and krypton-95, we can write an equation for the reaction to represent the balance of the reactants and products. This is called a nuclear equation and the equation for the example described looks as follows. The mass and atomic numbers on both sides of a nuclear equation must balance for the nuclear reaction to occur. Note that this equation contains a rightward-facing arrow rather than the equality symbol.
The reactants are on the left side of this arrow and the products are on the right. The atomic numbers and mass numbers on either side of this equation balance, which means that the fission can indeed occur in this manner. The example below shows how another nuclear equation can be written.
can absorb a neutron and undergo nuclear fission, producing, and three neutrons. Write the nuclear equation for this fission reaction.
The nuclear equation must contain all the reactants and products and we must ensure that the mass numbers and atomic numbers balance on either side of the equation. We can write the equation as
which is the correct, balanced equation for this fission reaction. Let's check that it is a balanced equation by comparing atomic numberson the left-hand side (LHS) and right-hand side (RHS) of the equation.
The atomic numbers are both 92 so they indeed balance. Let's check the mass numbers next:
The mass numbers also balance, which is a consequence of the conservation of mass.
Nuclear fusion reactions
If two light nuclei merge, they can form one heavier nucleus and release energy in the process. This is known as nuclear fusion. The interaction requires a significant amount of energy to occur. Nuclear fusion occurs in stars, like the sun, and is the main source of their energy. The temperatures and pressures on stars are great enough to fuse two hydrogen atoms (two protons each) into a helium nucleus (two protons and two neutrons). In the process, two protons are converted into neutrons. The mass of the helium nucleus is less than the mass of the hydrogen atoms that fused to form it, so the remainder of the mass is released in the form of energy after the fusion reaction. Nuclear fusion creates no radioactive products and this makes it a good candidate as a clean energy source for the future.
Nuclear fusion diagram
The fusion diagram below represents the fusion between two isotopes of hydrogen; deuteriumand tritium. Helium is produced by this fusion, along with a neutron with kinetic energy and a significant amount of energy in the form of heat.
A diagram showing the fusion of two isotopes of hydrogen; deuterium and tritium that fuse to form helium. A neutron is released in the process along with a significant amount of energy, Wikimedia Commons CC BY-SA 3.0
Nuclear fusion example
Let us consider the figure above and attempt to write and balance the nuclear equation for the reaction.
For the fusion of deuterium and tritium above we can write a nuclear equation as follows:
Let us first check that the atomic numbers balance on either side of this equation.
The atomic numbers are both equal to 2, so we can now check the mass numbers.
The mass number is 5 on either side of this equation and so there is a balance. This fusion reaction can indeed occur.
Differences and similarities between fission and fusion
The following table contains some of the differences and similarities between nuclear fission and nuclear fusion.
Differences between fission and fusion | Similarities between fission and fusion |
Fission is the splitting of nuclei and fusion is the merging of nuclei. | Fission and fusion are both nuclear processes that involve the interactions of atomic nuclei. |
Fission typically involves large nuclei whereas fusion involves smaller nuclei. | Fission and fusion reactions both release energy. |
Fission has radioactive products and fusion does not. | Fission and fusion can both be used as sources of power. |
Fission and Fusion - Key takeaways
- Nuclear fission occurs when large, unstable nuclei split into two smaller nuclei (fission products).
- Two or three neutrons are also produced by nuclear fission along with a significant amount of energy.
- The process is initiated when a moving neutron collides with the nucleus, making it unstable.
- The fission products may also be unstable and decay by releasing alpha or beta particles.
- The fission products gain kinetic energy after the reaction.
- The released neutrons may initiate further fissions in a process called a chain reaction.
- Uncontrolled chain reactions initiate fission reactions at an exponential rate.
- Controlled chain reactions occur when a fission reaction initiates only one other reaction.
- Nuclear power stations use controlled fission reactions to generate electricity.
- Boron control rods are used in nuclear fission reactors to control the rate of fission reactions.
- Nuclear equations can be used to describe fission reactions.
- Nuclear fusion occurs when light nuclei fuse to become a single, heavier nucleus.
- Fusion requires a significant amount of energy in order to be initiated.
- Fusion occurs in stars when hydrogen is converted into helium due to high temperatures and pressures.
- Fusion is a candidate for clean energy production.
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