 Noble gas, the noble gases historically also the inert gases make up the group of chemical elements with similar properties, under standard conditions, they are all odorless, colorless, monatomic gases with very low chemical reactivity. The six noble gases that occur naturally are helium, helium, neon, northeast are gone. They are krypton, k-R-Z non-X-E and the radioactive radon are them. These elements are all non-metals. Organic synod is variously predicted to be a noble gas as well or to break the trend due to relativistic effects. Its chemistry has not yet been investigated. For the first six periods of the periodic table, the noble gases are exactly the members of group 18. Noble gases are typically highly unreactive except when under particular extreme conditions. The inertness of noble gases makes them very suitable in applications where reactions are not wanted. For example, argon is used in incandescent light bulbs 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 hypercapnia toxicity. 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 degrees C 18 degrees F that is, they are liquids over only a small temperature range. 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. Carbon 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 sea water over 55 meters 180 feet 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 flatability of hydrogen became apparent, it was replaced with helium in blimps and balloons.