Inert gases

The inert gases, listed in the table, constitute group 18 of the periodic table of the elements. They are now better known as the noble gases, since stable compounds of xenon have been prepared. The noble gases are all monatomic.

All these gases occur to some extent in the Earth's atmosphere, but the concentrations of all but argon are exceedingly low. Argon is plentiful, constituting almost 1% of the air.

All the gases are colorless, odorless, and tasteless. They are all slightly soluble in water, the solubility increasing with increasing molecular weight. They can be liquefied at low temperatures, the boiling point being proportional to the atomic weight. All but helium can be solidified by reducing the temperature sufficiently, and helium can be solidified at temperatures of less than 2°F above absolute zero (0–1 K) by the application of an external pressure of 25 atm (2.5 megapascals) or more. See also Argon; Helium; Krypton; Neon; Radon; Xenon.