Physical Properties

Physical Properties

All members of Group 18 exist as single, uncombined atoms (monoatomic gases) rather than as diatomic or polyatomic molecules — a direct consequence of their filled-shell inertness, since there is no drive to pair up with a neighbouring atom. They are uniformly colourless, odourless and tasteless, and only sparingly soluble in water.

Their melting and boiling points are all exceptionally low. This follows from the nature of the forces holding the atoms together in the liquid or solid state: since a noble gas atom cannot form ordinary chemical bonds, the only attraction between neighbouring atoms is the weak, temporary attraction known as dispersion (van der Waals) forces. Such forces are easily overcome by thermal energy, so very little heat is needed to melt or boil these elements. …