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.