<P> 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 . The melting and boiling points for a given noble gas are close together, differing by less than 10 ° C (18 ° F); that is, they are liquids over only a small temperature range . </P> <P> Neon, argon, krypton, and xenon are obtained from air in an air separation unit using the methods of liquefaction of gases and fractional distillation . Helium is sourced from natural gas fields which 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 (since those compounds give off alpha particles). Noble gases have several important applications in industries such as lighting, welding, and space exploration . A helium - oxygen breathing gas is often used by deep - sea divers at depths of seawater over 55 m (180 ft) to keep the diver from experiencing oxygen toxemia, the lethal effect of high - pressure oxygen, nitrogen narcosis, the distracting narcotic effect of the nitrogen in air beyond this partial - pressure threshold, and carbon dioxide poisoning (hypercapnia), the panic - inducing effect of excessive carbon dioxide in the bloodstream . After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and balloons . </P> <P> Noble gas is translated from the German noun Edelgas, first used in 1898 by Hugo Erdmann 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 . 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 . </P> <P> Pierre Janssen and Joseph Norman Lockyer discovered a new element on August 18, 1868 while looking at the chromosphere of the Sun, and named it helium after the Greek word for the Sun, ἥλιος (hḗlios). No chemical analysis was possible at the time, but helium was later found to be a noble gas . 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 . 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 . Along with Scottish scientist William Ramsay at University College, London, Lord Rayleigh theorized that the nitrogen extracted from air was mixed with another gas, leading to an experiment that successfully isolated a new element, argon, from the Greek word ἀργός (argós, "idle" or "lazy"). With this discovery, they realized an entire class of gases was missing from the periodic table . During his search for argon, Ramsay also managed to isolate helium for the first time while heating cleveite, a mineral . In 1902, having accepted the evidence for the elements helium and argon, Dmitri Mendeleev included these noble gases as group 0 in his arrangement of the elements, which would later become the periodic table . </P>

Where does the name noble gases come from