The noble gases (historically also the inert gases; sometimes referred to as aerogens[1]) are the naturally occurring members of group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Under standard conditions, these chemical elements are odorless, colorless, monatomic gases with very low chemical reactivity and cryogenic boiling points.
The inertness of noble gases is due to their electron configuration: Their outer shell of valence electrons is "full", giving them little tendency to participate in chemical reactions. Only a few hundred noble gas compounds are known to exist. For the same reason, noble gas atoms are small and the only intermolecular force between them is the very weak London dispersion force, so their boiling points are all cryogenic, below 120 K (−153 °C; −244 °F).
The inertness of noble gases makes them useful whenever chemical reactions are not wanted. For example, argon is used as a shielding gas in welding, and as a filler gas in incandescent light bulbs. After the risks caused by the flammability of hydrogen became apparent in the Hindenburg disaster, it was replaced with helium in blimps and balloons. Helium and neon are also used as refrigerants due to their low boiling points. Industrial quantities of the noble gases, except for radon, are obtained by separating them from air using the methods of liquefaction of gases and fractional distillation. Helium is also a by-product of the mining of natural gas. Radon is usually isolated from the radioactive decay of dissolved radium, thorium, or uranium compounds.
The seventh member of group 18 is oganesson (Og), an unstable synthetic element, whose chemistry is still uncertain because only five atoms have ever been synthesized (as of 2020[2]). IUPAC uses the term "noble gas" interchangeably with "group 18" and thus includes oganesson;[3] however, due to relativistic effects, oganesson is predicted to be a solid under standard conditions, and reactive enough not to qualify as "noble".[2] In the rest of this article, the term "noble gas" should be understood not to include oganesson unless it is specifically mentioned.[update]
Noble gas is translated from the German noun Edelgas, first used in 1898 by Hugo Erdmann[4] to indicate their extremely low level of reactivity. The name makes an analogy to the term "noble metals", which also have low reactivity. The noble gases have also been referred to as inert gases, but this label is deprecated as many noble gas compounds are now known.[5] Rare gases is another term that was used,[6] but this is also inaccurate because argon forms a fairly considerable part (0.94% by volume, 1.3% by mass) of the Earth's atmosphere due to decay of radioactive potassium-40.[7]