Noble Gases

Noble Gases, also inert gases, group of six gaseous chemical elements constituting the group 18 (or VIIIa) of the periodic table (see Periodic Law). They are, in order of increasing atomic weight, helium, neon, argon, krypton, xenon, and radon.

For many years chemists believed that these gases, because their outermost shells were completely filled with electrons, were inert—that is, that they would not enter into chemical combinations with other elements or compounds. This is now known not to be true, at least for the four heaviest inert gases—argon, krypton, xenon, and radon. In 1962, Neil Bartlett, a British chemist working in Canada, succeeded in making the first complex xenon compound. His work was confirmed by scientists at Argonne National Laboratory in Illinois, who made the first simple compound of xenon and fluorine (xenon tetrafluoride) and later made radon and krypton compounds. Although krypton compounds were made with considerable difficulty, both xenon and radon reacted readily with fluorine, and additional reactions to produce other compounds of xenon and radon could be accomplished. Researchers at the University of Helsinki created the first argon compound, argon fluorohydride (HArF), in 2000.

The forces between the outermost electrons of these three elements and their nuclei are diluted by distance and the interference of other electrons. The energy gained in creating a xenon or radon fluoride is greater than the energy required for promotion of the reaction, and the compounds are chemically stable, although xenon fluorides and oxides are powerful oxidizing agents. The usefulness of radon compounds is limited because radon itself is radioactive and has a half-life of 3.82 days. The energy gain is also greater in the case of krypton, but only slightly so. Compounds of helium, neon, or argon, the electrons of which are more closely bound to their nuclei, are unlikely to be created.

Liquefied noble gases under pressure, particularly xenon, are employed as solvents in infrared spectroscopy. They are useful for this because they are transparent to infrared radiation and therefore do not obscure the spectra of the dissolved substances.