inert gas or noble gas, any of the elements in Group 18 of the periodic table. In order of increasing atomic number they are: helium, neon, argon, krypton, xenon, and radon. They are colorless, odorless, tasteless gases and were once believed to be entirely inert, i.e., forming no chemical compounds; however, some compounds of these elements have been produced, i.e., fluorides of krypton, xenon, and radon. The low chemical activity of the inert gases is due to the fact that their outermost, or valence, electron shell is complete, containing two electrons in the case of helium and eight in the remaining cases. The inert gases are sometimes called the rare gases, although argon is not rare (it makes up about 1% of the atmosphere) and helium is commercially extracted from natural gas and the atmosphere.

or inert gas

Any of the seven chemical elements that make up the rightmost group of the periodic table as usually arranged: helium, neon, argon, krypton, xenon, radon, and element 118. All are colourless, odourless, and nonflammable and, except for element 118, occur in tiny amounts in the atmosphere (though helium is the most plentiful element in the universe after hydrogen). Their stable electronic configurations, with no unpaired electrons to share, make them extremely unreactive—hence “noble” (i.e., aloof) or inert—though krypton, xenon, and radon, 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 cryogenics).

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