Noble gas

Lore & Background

The noble gases are colorless, odorless, and monatomic under standard conditions, with the first six members—helium, neon, argon, krypton, xenon, and radon—exhibiting very low chemical reactivity and cryogenic boiling points. Their defining characteristic is a full outer shell of valence electrons, which gives them little tendency to participate in chemical reactions; only a few hundred noble gas compounds are known. The intermolecular forces between their atoms are the very weak London dispersion forces, resulting in all their boiling points being cryogenic. Their inertness makes them useful where chemical reactions are unwanted: argon serves as a shielding gas in welding and a filler in incandescent bulbs, helium provides buoyancy in blimps and balloons, and helium and neon are used as refrigerants due to their low boiling points. Industrial quantities of all noble gases except radon are obtained by liquefying and fractionally distilling air; helium is also a byproduct of natural gas mining, while radon is isolated from the radioactive decay of dissolved radium, thorium, or uranium compounds. The seventh member, oganesson, is an unstable synthetic element with uncertain chemistry, predicted to be a solid under standard conditions and reactive enough not to qualify functionally as “noble.” The term “noble gas” was first used in 1900 by Hugo Erdmann, drawing an analogy to noble metals. Their discovery began in 1868 when helium was found on the Sun, followed by argon in 1895, and then krypton, neon, and xenon in 1898; radon was identified in 1898 but not recognized as a noble gas until 1904.

Reader's Guide

The discovery of the noble gases was unique in the history of chemistry, as an entirely new group of elements with no previously known representative. Their defining characteristic is a full outer shell of valence electrons, which gives them a very low tendency to react chemically; only a few hundred compounds are known. Under standard conditions, the first six are odorless, colorless, monatomic gases with cryogenic boiling points, held together only by weak London dispersion forces. This inertness makes them valuable where chemical reactions are unwanted: argon serves as a shielding gas in welding and a filler in incandescent bulbs, helium provides buoyancy for blimps and balloons, and both helium and neon are used as refrigerants. Industrial quantities of all except radon are obtained from air via liquefaction and fractional distillation; helium is also a byproduct of natural gas mining. Radon is isolated from the radioactive decay of radium, thorium, or uranium compounds. Oganesson, the seventh member, is an unstable synthetic element with uncertain chemistry; only five very short-lived atoms have been synthesized, and it is predicted to be a solid under standard conditions and reactive enough not to qualify functionally as a noble gas. The discovery of this group—starting with helium on the Sun in 1868, then argon, krypton, neon, xenon, and radon—helped develop atomic structure theory, leading to the octet rule and the understanding that for all noble gases except helium, the outermost shell contains eight electrons.

Did You Know?

• Argon forms about 0.94% by volume of Earth's atmosphere due to decay of radioactive potassium-40.
• Helium is the only element that cannot be solidified by cooling at atmospheric pressure; a pressure of 25 standard atmospheres at 0.95 K is required.