A century later, in 1895, Lord Rayleigh discovered that samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. Before them, in 1784, the English chemist and physicist Henry Cavendish had discovered that air contains a small proportion of a substance less reactive than nitrogen. No chemical analysis was possible at the time, but helium was later found to be a noble gas. Pierre Janssen and Joseph Norman Lockyer had discovered a new element on 18 August 1868 while looking at the chromosphere of the Sun, and named it helium after the Greek word for the Sun, ἥλιος ( hḗlios). Helium was first detected in the Sun due to its characteristic spectral lines. Rare gases is another term that was used, 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. The noble gases have also been referred to as inert gases, but this label is deprecated as many noble gas compounds are now known. The name makes an analogy to the term ' noble metals', which also have low reactivity. Noble gas is translated from the German noun Edelgas, first used in 1898 by Hugo Erdmann to indicate their extremely low level of reactivity. After the risks caused by the flammability of hydrogen became apparent in the Hindenburg disaster, it was replaced with helium in blimps and balloons. A helium-oxygen breathing gas is often used by deep-sea divers at depths of seawater over 55 m (180 ft). Noble gases have several important applications in industries such as lighting, welding, and space exploration. Helium is sourced from natural gas fields that have high concentrations of helium in the natural gas, using cryogenic gas separation techniques, and radon is usually isolated from the radioactive decay of dissolved radium, thorium, or uranium compounds. Neon, argon, krypton, and xenon are obtained from air in an air separation unit using the methods of liquefaction of gases and fractional distillation. The melting and boiling points for a given noble gas are close together, differing by less than 10 ☌ (18 ☏) that is, they are liquids over only a small temperature range. The properties of the noble gases can be well explained by modern theories of atomic structure: Their outer shell of valence electrons is considered to be 'full', giving them little tendency to participate in chemical reactions, and it has been possible to prepare only a few hundred noble gas compounds. For example, argon is used in incandescent lamps to prevent the hot tungsten filament from oxidizing also, helium is used in breathing gas by deep-sea divers to prevent oxygen, nitrogen and carbon dioxide toxicity.

The inertness of noble gases makes them very suitable in applications where reactions are not wanted. Noble gases are typically highly unreactive except when under particular extreme conditions. Because of the extremely short 0.7 ms half-life of its only known isotope, its chemistry has not yet been investigated.įor the first six periods of the periodic table, the noble gases are exactly the members of group 18. Although IUPAC has used the term 'noble gas' interchangeably with 'group 18' and thus included oganesson, it may not be significantly chemically noble and is predicted to break the trend and be reactive due to relativistic effects. Oganesson (Og) is a synthetically produced highly radioactive element. The six naturally occurring noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). The noble gases (historically also the inert gases sometimes referred to as aerogens ) make up a class of chemical elements with similar properties under standard conditions, they are all odorless, colorless, monatomic gases with very low chemical reactivity.

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