Noble Gases

The noble gases are a group of chemical elements in the eighteenth column of the periodic table. They consist of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). That column also contains oganesson (Og), a superheavy element created in laboratories, whose properties remain incompletely characterized.

The defining characteristic is their extremely low reactivity. The reason is that the outermost electron shell is complete—three electron pairs for helium and eight for the others. This means they do not readily donate or accept electrons, and so exist in a stable, single-atom state under ordinary conditions.

Physical Properties

All are colorless, odorless gases with no taste. Their boiling and melting points are very low, and increase as atomic mass increases: helium has the lowest boiling point of any substance, making it useful for extreme cooling (cryogenics). Krypton and radon have the highest boiling points among the group.

Chemistry and Compounds

For a long time they were thought to form no compounds at all, hence the name 'noble gases'. However, in the mid-twentieth century xenon compounds were synthesized containing fluorine and oxygen, such as xenon hexafluoroplatinate and xenon tetrafluoride. Krypton and radon form a small number of compounds. Helium and neon have not been shown to form stable compounds under ordinary conditions.

Occurrence

Argon is the most abundant in the Earth's atmosphere, making up roughly one percent. Neon, helium, krypton, and xenon are much rarer and are generally obtained by fractional distillation of liquid air. Helium is also found in natural gas wells and is released by radioactive decay of elements in the Earth's crust. Radon is a radioactive product of uranium decay in the soil, and can accumulate in basements.

Applications

Neon is used in colored sign tubes. Argon is used to fill light bulbs and welding spaces to protect metal from oxygen. Helium fills balloons and aircraft, and cools the high-temperature magnets used in medical imaging equipment. Xenon is used in high-intensity lamps and certain anesthetic drugs.