or inert gas
Any of the six chemical elements that make up the rightmost group of the periodic table as usually arranged: helium , neon , argon , krypton , xenon , and radon .
All are colourless, odourless, and nonflammable and occur in tiny amounts in the atmosphere (though helium is the most plentiful element in the universe). Their stable electronic configuration s, with no unpaired electron s to share, make them extremely unreactive
hence "noble" (i.e., aloof) or inert
though the three heaviest, with outer electrons held less firmly, can form compounds (mainly with fluorine ). These gases absorb and give off electromagnetic radiation in a much less complex way than other substances, a property exploited in their use in fluorescent lighting devices and discharge lamps. They glow with a characteristic colour when confined in a transparent container at low pressure with an electric current passing through it. Their very low boiling and melting points make them useful as refrigerants for low-temperature research (see cryogenic s).