The noble gases (historically called also inert gas, but this term is not strictly accurate because several of them do take part in chemical reactions) 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 and made up of atoms with complete electron shells. They are the most stable due to having the maximum number of valence electrons their outer shell can hold. Therefore, they rarely react with other elements since they are already stable.
Other characteristics of the noble gases are that they all conduct electricity, fluoresce, and are used in many conditions when a stable element is needed to maintain a safe and constant environment. They constitute the group 18 of the periodic table according to the current IUPAC nomenclature. Noble gases are all monoatomic gases, not easily liquefied, typically non-reactive, present in the atmosphere in various percentages. 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.