<Tr> <Td_colspan="2"> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The noble gases (historically also the inert gases) make up a 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 (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). Oganesson (Og) 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 . </P> <P> 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 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 . </P>

What do helium argon and neon have in common