The stable electronic configuration of the noble gases probably restricted the attempts to prepare chemical compounds of the elements. Every attempt to prepare inert gas compounds of the usual types by treatment with oxidizing or reducing agents has met with failure. But inspite of unfavourable conditions, some compounds formation has been reported. The only compounds known before 1962 were the hydrates and clathrates. In such compounds, the noble gas atoms (guest) are caged in cavities in the crystal lattice of other compounds (host) whose molecules are joined together by hydrogen bond e.g. quinol (1,4-dihydroxybenzene). Quinol forms clathrates with argon, krypton and xenon when crystallized in the presence of these gases from benzene or water under pressure of 10-40 atmospheres. X-ray studies show that three quinol molecules form a somewhat spherical cage of about 4? diameter. Hence the ratio of quinol to entrapped atom or molecule is usually 3:1.



The argon-quinol clathrate crystals are stable and can be kept for years. When the clathrate are dissolved, the hydrogen bonded quinol molecules separate out and thus the inert gas escapes. It must be noted that due to their small sizes the helium and neon can escape from the cavities and hence they do not form clathrate compounds. Similarly, when water is allowed to freeze in the presence of argon, krypton or xenon under pressure, the atoms of the noble gases get trapped in the crystal structure of ice yielding clathrates of composition approximating to 5.75 molecules of water for every 1 molecule of noble gas.

The noble gas hydrates are crystalline compounds which are stable at very low temperatures. At high temperatures, the noble gas escapes out. Again like quinol clathrates, hydrate clathrates are not formed by helium and neon. It is because of the fact that the helium and neon atoms are too small and too volatile to be retained in cavities and thus these gases do not form hydrates.

The essential conditions for clathrate formation are that the crystal lattice of the host should be such as to have cavities of approximate size and the guest molecules or atoms should be approximate size to fit in tightly into these cavities. It is important to note that there is no definite chemical bond between the noble gas (guest molecules) and quinol (host molecules) molecules, and they are linked only by means of weak intermolecular forces. Hence clathrates should be regarded as compounds. They are stable at ordinary temperatures and decompose to individual compounds on melting or dissolution. Since helium and neon do not form clathrate compounds with hydroquinone while the argon, krypton and xenon form such compounds, the former two inert gases can be separated from the latter three.