Supercharge your learning!

Use adaptive quiz-based learning to study this topic faster and more effectively.

Nuclear reactions

Nuclear reactions are reactions, involving atomic nuclei, that have been induced by external stimuli (e.g. high temperatures, neutron bombardment, irradiation). They are therefore not spontaneous.

By contrast, nuclear decay (in which a nucleus splits into two or more nuclei) is spontaneous and random.

The difference between a nuclear reaction and nuclear decay is only in the initiation of the action.

An alpha particle can be emitted both through stimulation (in a nuclear reaction) or spontaneously (in alpha decay).

Nuclear reactions can be divided into two broad categories:

  • nuclear fusion (merging two small atoms)
  • nuclear fission (splitting one big atom)

The number of protons and neutrons (and thus of nucleons) and the total energy-mass are conserved in both nuclear reactions and nuclear decay. The rest mass of the individual particles is not conserved.

The conservation of the number of protons and neutrons is related to the nature of the strong force that preserves the intrinsic properties of a nucleon.

Nuclear fusion is the creation of one larger nucleus from two or more smaller nuclei.

For reactants with small proton numbers $$\ce{Z}$$, this process releases energy. The released energy can take the form of the masses of the outgoing particles and kinetic energy (of the large nuclei, free neutrons, alpha and beta particles) as well as electromagnetic radiation (photons).

The final products of nuclear fusion after a complete reaction are generally more stable than the reactant nuclei. The intermediate products are typically very unstable.

As long as the atomic mass is below 56 ($$\ce{A}\lt 56$$), larger nuclei generally have higher binding energies per nucleon than smaller nuclei.

The fusion of hydrogen isotopes such as that of deuterium $$\ce{^{2}_{1}H}$$ and tritium $$\ce{^{3}_{1}H}$$ is the most widespread nuclear fusion reaction in the universe. Both of them fuse into a helium nucleus $$\ce{^{4}_{2}He}$$ and a free neutron $$\ce{^{1}_{0}n}$$: $$$\ce{^{2}_{1}H + ^{3}_{1}H \rightarrow ^{4}_{2}He + ^{1}_{0}n}$$$This reaction is the source of energy for the sun (and many stars) and requires very high temperatures and pressures due to the electrostatic repulsion between the positively charged nuclides.

Harnessing the energy of nuclear fusion to generate electricity has thus far had limited success. Using current technology, more energy is required to maintain the conditions for a fusion reaction than the energy released by the reaction itself.

Fusion on hydrogen isotopes.
Fusion on hydrogen isotopes.

Nuclear fission is the reaction of a large nucleus with external particles, causing the nucleus to break up into two or more smaller nuclei.

This process releases energy in the form of the masses of the new particles created and in the form of kinetic energy (of the smaller nuclei, free neutrons, alpha and beta particles) as well as electromagnetic radiation (photons).

Nuclear fission can take place spontaneously (i.e. in nuclear decay) in natural or man-made radioactive isotopes. But it can also be induced by bombarding a nucleus with a neutron.

The final products (i.e. the smaller nuclei) which form after nuclear fission are generally more stable than the parent nuclei. The intermediate products are typically very unstable.

This is because the parent nuclei are typically large (i.e. $$A\gt56$$). The binding energy per nucleon of the products is therefore higher than that of the parent nuclei.

An example of a nuclear fission reaction is the fission of a uranium isotope into krypton and barium isotopes and free neutrons (commonly occurring in nuclear reactors):$$$\ce{^{1}_{0}n + ^{235}_{92}U\rightarrow^{92}_{36}Kr + ^{141}_{56}Ba + 3\text{ }^{1}_{0}n}$$$

Nuclear fission is used in nuclear power stations to generate electricity, as well as in nuclear bombs. These reactions are self-sustaining as the reactants generate more catalysts (free neutrons) that trigger more reactions.

Unlike nuclear fusion, nuclear fission does not require any energy input as there is no electromagnetic repulsive force to overcome.

Fission of a uranium isotope.
Fission of a uranium isotope.