Most processes existing in nature occur as a result of interactions between the atoms of elements and the transfer of electrons. Nuclear chemistry is focused on atomic nuclei. It examines their properties and the changes that occur as a result of their disintegration. Compared to other reactions, nuclear reactions lead to the formation of new chemical individuals. Besides, nuclear reactions entail the emission of extreme amounts of energy, which are even several hundred thousand times higher than in the case of a conventional chemical reaction.

Stability of atomic nuclei

Atoms are the basic, but not the smallest, units of matter. Each of them consists of an atomic nucleus and electrons circulating around it. An atomic nucleus is made of positive protons and neutral neutrons. Atoms with the same number of protons but a different number of neutrons are called isotopes. A vast majority of them are naturally stable, meaning they do not undergo chemical reactions, even after a long time. However, certain isotopes are unstable and easily desintegrate, these including beta plus, beta minus or alpha. For an isotope to be stable, the atomic nucleus should include an identical number of protons and neutrons. The heavier the nuclei (i.e. the higher their atomic number), the more frequent the difference between their quantities. As a result of that disproportion, the atomic nucleus is much less susceptible to radioactive disintegration. The heaviest naturally existing isotope that is stable is ¹⁰⁹Bi.

Both an excessive number of neutrons in the nucleus and their shortage in relation to the number of protons cause a nuclear reaction (or a series of reactions), which finally produces a stable atomic nucleus. Additionally, the stability of nuclei is affected by nuclear forces, which should be higher than the forces of electrostatic interactions (this is ensured by the appropriate number of neutrons in the nucleus relative to the number of protons) and the mass of the nucleus (the higher the mass, the less stable the nucleus).

The greatest focus of nuclear chemistry is put on those isotopes whose atomic nuclei are not stable and desintegrate easily. The chemical elements consisting of such isotopes are called radioactive.

Radioactivity of chemical elements

The radioactivity of chemical elements is a result of the disintegration of their unstable atomic nuclei. This is what we should know about it:

• We distinguish natural and artificial radioactivity of chemical elements. Natural radioactivity is where the substance emits radiation by itself. Artificial radioactivity is where a particular substance is not radioactive but emits rays when exposed to natural radiation coming from a radioactive substance.
• Natural reactions are classified into alpha, beta and gamma. Artificial reactions include nuclear reactions, nuclear fusions and nuclear fissions.
• The disintegration of atomic nuclei releases radiation that is called ionising or nuclear radiation. Radioactive isotopes may emit three types of rays: alpha, beta and gamma.
• Alpha particles are the nuclei of helium atoms. Electrons are beta particles (which are much lighter than alpha particles). The gamma radiation is not a particle, it belongs to the family of electromagnetic waves.
• Most nuclei of radioactive elements form new nuclei after disintegration. These are also unstable and undergo further disintegration. This is how the so-called radioactive series are formed.
• The radioactivity of a particular chemical material depends mainly on its quantity. It declines with time, as the quantity decreases. Other factors affecting the speed of nuclear reactions include temperature and pressure.

Nuclear and thermonuclear reactions

Many nuclear reactions of radioactive elements contained in the Earth’s crust occur naturally. However, certain reactions can be carried out in a chemical laboratory. The first such reaction conducted by a human took place in 1919. It was carried out by Ernest Rutherford. Today, the number of human-induced nuclear reactions is very large. The quantity of produced artificial isotopes considerably exceeds naturally occurring radioactive isotopes.

Nuclear reactions, which are amongst the issues analysed by nuclear chemistry, occur as a result of bombarding atomic nuclei with specific particles. It may be neutrons, protons, alpha particles or even carbon nuclei. The occurring nuclear reaction depends on the type of bombarding molecule and its energy. Nuclear reactions lead to the absorption (with the emission of one or two elementary particles) of the emitted bombarding molecule by the atomic nucleus or to the destruction of that nucleus. The former case occurs when the energy of the “bullet” is low (less than one hundred MeV). Nuclear spallation occurs at high energies of even several hundred MeV. Many simple nuclear reactions occur under the influence of alpha particles, which are emitted by natural radioactive elements. The fission of the atomic nucleus can be easily activated with the use of neutrons. Since their charge is neutral, they easily reach the nuclei as being insusceptible to electrostatic repulsion. An important characteristic of all nuclear reactions is that they go along with the absorption or emission of considerable amounts of energy.

Thermonuclear reactions take place in somewhat different conditions. They occur at very high temperatures ranging from 10⁷ to 10⁸ K (often only in such temperatures can they occur spontaneously). Temperatures that enable, for example, natural synthesis of helium from hydrogen, exist inside stars and cause them to emit large amounts of energy. As a result of thermonuclear reactions, the smallest nuclei (e.g. those of hydrogen or deuterium) combine to form larger nuclei. Scientists have carried out the thermonuclear synthesis of helium. This reaction produced extremely large amounts of energy. They were much larger than during the explosion of an uranium or plutonium bomb. However, the entire process was not controlled. It is estimated that the acquisition of energy in a fully controlled thermonuclear reaction will be possible once the technical difficulties are overcome.

What are the applications of the radiation of atomic nuclei?

• Imaging of tissues and organs;
• Treatment of neoplastic diseases;
• Medical diagnostics;
• Scientific research at research centres and in the industry;
• Electricity production in nuclear power plants;
• Sterilisation of medical equipment.