Group 18 or viii, commonly referred to as the noble gases, consists of six elements,helium,neon,argon,krypton,xenon, and radon. They are the most stable group of elements within the periodic table due to having a complete valence shell of electrons. This degree of stability gives a clear indication as to how reactive the elements are as a group.
The group's inability to enter into chemical combinations with other elements had led scientists for many years into naming the group the 'inert gases'. This has since been shown to be an incorrect analogy as in 1962 Neil Bartlett, a British chemist working in Canada, successfully made a compound of xenon and platinum (vi) fluoride (xenon hexafluoroplatinate). Today, over 100 compounds of group 18 elements have been made, with only helium still having no known compounds.
Atmospheric Abundance
Noble gases are monatomic gases and are far more abundant in the atmosphere than one might first imagine. For instance, Argon, symbol Ar, is more common in air than carbon dioxide, and at 9300 parts per million, it constitutes 0.93% of the earth's atmosphere, making it the third most prevalent gas around.
All other noble gases except for radon are present in the atmosphere in small amounts. Hence apart from radon, all other noble gases can be extracted from air by fractional distillation. Radon-222 is obtained by passing air through a solution of radium salt and collecting the air, contaminated with radon gas that was present in the solution
Trends In Physical Properties
The van der waals radius increases on descending the group - each atom has one shell of electrons more than the previous one.
Both the melting point and boiling point increases on descending the group - van der waals forces increase down the group.
Decreasing first ionisation energy on descending the group - the outer electrons are further from the positive nucleus, resulting in a weaker attraction between them.