(called also noble gases, rare gases), chemical elements forming the principal subgroup of the eighth group of Mendeleev’s periodic system: helium, He (atomic number 2); neon, Ne (10); argon, Ar (18); krypton, Kr (36); xenon, Xe (54); and radon, Rn (86). Of all the inert gases, only Rn has no stable isotopes and is a radioactive chemical element.

The name “inert gases” reflects the chemical inertness of the elements of this subgroup, which is explained by the presence in the atoms of inert gases of a stable outer electron shell, in which He has two electrons while the other inert gases each have eight. The removal of electrons from this shell requires great energy expenditures in accordance with the high ionization potentials of the atoms of inert gases (see Table 1).

Because of the chemical inertness, inert gases could not be detected for a long time and were discovered only in the second half of the 19th century. A spectroscopic study of solar protuberances conducted by the Frenchman P. J. C. Jansen and the Englishman J. N. Lockyer in 1868 led to the discovery of the first inert gas—helium. The other inert gases were discovered between 1892 and 1908.

Inert gases are always present in the air in free form. Under normal conditions, 1 m³ of air contains about 9.4 liters of inert gases, primarily argon (see Table 1). Apart from the air, inert gases are present in solution in water and in some minerals and rocks. Helium is found in subterranean gases and gases liberated by mineral sources. The other stable inert gases can be obtained from air on separation. Radioactive preparations of uranium, radium, and other elements serve as a source of radon. After use, stable inert gases return to the atmosphere once again, and therefore their reserves (except for light He, which is gradually dispersing from the atmosphere into outer space) do not decrease.

The molecules of inert gases are monatomic. None of the inert gases has color, odor, or taste; they are colorless in the solid and liquid states. The presence of the filled outer shell is responsible not only for the high chemical inertness of the inert gases but also for the difficulty of producing them in the liquid and solid states (see Table 1). (For information on other physical properties of inert gases see the articles on the individual elements.)

Attempts to produce chemical compounds of inert gases were long unsuccesful. The Canadian scientist N. Bartlett, who in 1962 reported the synthesis of a compound of Xe and PtF₆, succeeded in putting an end to the concepts of the absolute chemical inactivity of inert gases. In the years since then, a large number of compounds of Kr, Xe, and Rn, in which inert gases have oxidation levels +1, +2, +4, +6, and +8, have been produced. Here it is significant that no fundamentally new concepts of the nature of the chemical bond were required to explain the structure of these compounds and that the bond in compounds of inert gases can be described well, for example, by the method of molecular orbitals. Because of the rapid radioactive decay of Rn, its compounds have been produced in negligibly small quantifies, and their compositions have been roughly established. Compounds of Xe are much more stable than compounds of Kr, and as yet it has not proved possible to produce stable compounds of Ar and the lighter inert gases. Fluorine takes part in most reactions of inert gases: some substances are produced by acting on inert gases with fluorine or agents that contain fluorine (such as SbF₅ or PtF₆, while others form on decomposition of fluorides of inert gases. There are indications that reactions between Xe and Kr on the one hand and chlorine on the other may take place. Oxides (Xe0₃ and XeO₄) and oxyhalides of inert gases also have been obtained.

In addition to the compounds indicated above, inert gases form inclusion compounds at low temperatures. Thus, all inert gases except He produce crystal hydrates of the type Xe·6H₂O on interaction with water, while heavy inert gases produce compounds of the type Xe·3C₆H₅OH on interaction with phenol.

The industrial uses of inert gases are based on their low chemical activity or on specific physical properties. (Examples of the uses of inert gases may be found in the articles on the individual elements.)