Chemical elements
    Physical Properties

History of Neon discovery

It has already been stated that after argon and helium had been discovered, there was some controversy as to their position in the periodic classification of the elements, and that the discoverers ultimately came to the conclusion that these gases must be placed by themselves in a new group. Helium then occupied the same horizontal position in the new group as lithium in Group I., while argon came next to potassium. A gap was thus left for an element which would occupy a position in Group 0 between argon and helium, and would have an atomic weight two or three units less than that of sodium.

In the hope of discovering this gas Ramsay and Travers made a careful examination of 18 litres of argon, obtained from air in the usual manner. The gas was liquefied in a Dewar tube cooled in liquid air boiling under reduced pressure, and 25 c.c. of clear liquid were obtained; the temperature was then allowed to rise gradually and the gas that boiled off was collected in fractions.

The first fractions had a density of 14.7 (approx.), which approached that anticipated for the element between helium and argon; moreover, the spectrum of this gas was new and contained in particular a yellow line (λ 5852.65) distinct from those of sodium, helium, and krypton. The colour of the light from the vacuum tube varied from fiery red to brilliant orange as the pressure fell, and the gas was absorbed by the aluminium electrodes.

When the first fraction was again cooled with liquid air it was found that a large proportion did not liquefy; this gas had a density of 9.65, but was mixed with a certain proportion of helium and argon. These impurities were removed by fractionally distilling a liquefied mixture of the original gas with a suitable proportion of oxygen; the middle fraction was freed from oxygen by passage over red-hot copper and then had a density of 10.1.

The new gas was characterised by a spectrum distinct from that of helium or argon (vide infra), and was named neon (from the Greek viov, new).

Toward the end of the fractionation of the original supply of liquid argon, a solid substance was obtained which distilled very slowly and could therefore be obtained in a state of considerable purity. This substance was at first supposed to be another new element, and received the name " metargon ";

but it was later found by Ramsay and Travers themselves that the peculiar spectrum which led to its " discovery " was probably due to argon containing a small amount of some volatile carbon compound. Neon was not obtained again in the pure state until 1910 by Watson, and for this reason less is known of the properties of neon than of the other gases.

Sir J. J. Thomson, by his positive-ray method, has shown that atmospheric neon contains two kinds of atoms; one of mass 20, the ordinary neon atom, the other of mass 22 which could not be attributed to any known element. To this latter substance the name raetaneon has been given. Attempts have been made, without success, to separate neon and metaneon by repeated fractionation over cooled charcoal, but by a series of fractional diffusions Aston has obtained a gas having a density greater than that of ordinary neon. On the other hand, Leduc, who has had considerable experience in fractionating atmospheric neon from the same source as Aston's, considers it possible that the increase in density is due to the concentration of a trace of nitrogen in the end fractions.

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