Introduction group VIIIA

Group VIIIA includes following elements - the noble gases: Helium - [He], Neon - [Ne], Argon - [Ar], Krypton - [Kr], Xenon - [Xe], Radon - [Rn].

In 1785 H. Cavendish in his classic work on the composition of air noted that, after repeatedly sparking a sample of air with an excess of O2, there was a small residue of gas which he was unable to remove by chemical means and which he estimated with astonishing accuracy to be “not more than 1/120-th part of the whole”. He could not further characterize this component of air, and its identification as argon had to wait for more than a century. But first came the discovery of helium, which is unique in being the only element discovered extraterrestrially before being found on earth. During the solar eclipse of 18 August 1868, a new yellow line was observed close to the sodium D lines in the spectrum of the sun’s chromosphere. This led J. N. Lockyer and E. Frankland to suggest the existence of a new element which, appropriately, they named helium (Greek - the sun). The same line was observed by L. Palmieri in 1881 in the spectrum of volcanic gas from Mount Vesuvius, and the terrestrial existence of helium was finally confirmed by W. Ramsay in the course of his intensive study of atmospheric gases which led to the recognition of a new group in the periodic table. This work was initiated by the physicist, Lord Rayleigh, and was recognized in 1904 by the award of the Nobel Prizes for Chemistry and Physics to Ramsay and Rayleigh respectively.

In order to test Prout’s hypothesis (that the atomic weights of all elements are multiples of that of hydrogen) Rayleigh made accurate measurements of the densities of common gases and found, to his surprise, that the density of nitrogen obtained from air by the removal of O2, CO2 and H2O was consistently about 0.5% higher than that of nitrogen obtained chemically from ammonia. Ramsay then treated “atmospheric nitrogen” with heated magnesium (3Mg + N2 → Mg3N2), and was left with a small amount of a much denser, monatomic gast which, in a joint paper, was identified as a new element which was named argon (Greek - idle or lazy) because of its inert nature. Unfortunately there was no space for a new and unreactive, gaseous, element in the periodic table, which led to Ramsay’s audacious suggestion that a whole new group might be accommodated. By 1898 Ramsay and M. W. Travers had isolated three further new elements by the low-temperature distillation of liquid air (which had only recently become available) and characterized them by spectroscopic analysis: krypton (Greek - hidden, concealed), neon (Greek - new) and xenon (Greek - strange).

In 1895 Ramsay also identified helium as the gas previously found occluded in uranium minerals and mistakenly reported as nitrogen. Five years later he and Travers isolated helium from samples of atmospheric neon.

Element 86, the final member of the group, is a short-lived, radioactive element, formerly known as radium-emanation or niton or, depending on which radioactive series it originates in (i.e. which isotope) as radon, thoron, or actinon. It was first isolated and studied in 1902 by E. Rutherford and F. Soddy and is now universally known as radon (from radium and the termination-on adopted for the noble gases; Latin - radius, ray).

Once the existence of the new group had been established it was apparent that it not only fitted into the periodic table but actually improved it by providing a bridge between the strongly electronegative halogens and strongly electropositive alkali metals. The elements became known as “inert gases” comprising Group 0, though A. von Antropoff suggested that a maximum valency of eight might be attainable and designated them as Group VIIIB. They have also been described as the “rare gases” but, since the lighter members are by no means rare and the heavier ones are not entirely inert, “noble” gases seems a more appropriate name and has come into general use during the past three decades as has their designation as Group 18 of the periodic table.

The apparent inertness of the noble gases gave them a key position in the electronic theories of valency as developed by G. N. Lewis and W. Kossel and the attainment of a “stable octet” was regarded as a prime criterion for bond formation between atoms. Their monatomic, non-polar nature makes them the most nearly “perfect” gases known, and has led to continuous interest in their physical properties.