Physical Properties of Radon

Radon is a gas. This can be said with certainty, as it can be manipulated as a gas by means of a Topler pump, can be seen to flow in the manner of a gas through the glass tubes of the apparatus, obeys Boyle's law, and can be liquefied and solidified by cooling. It is colourless by transmitted light.

The density of the gas has been determined by direct weighing on a microbalance sensitive to 2×10^-6 milligram, with a weight of radon of the order of 1/1400 mgm.; - the mean of several experiments gave the value 111.5 (H = 1). This figure is in general agreement with the results of the effusion experiments of Debierne and Perkins, which gave the values 110 and 117.5 respectively.

As radium (atomic weight = 226.0) is transformed into radon with the loss of one a-particle, and as the α-particle is believed, on excellent grounds, to be identical with the atom of helium, it is to be expected that the atomic weight of radon would be 226.0 – 4.0 = 222.0. This figure agrees well with the molecular weight calculated from the density determinations quoted above; the one adopted by the International Commission is 222.4.

Radon is soluble in water: its absorption coefficient is approximately 0.5 at 0°, 0.3 at 15°, 0.27 at 18°, 0.23 at 20°, and 015 at 40°, and thus has a large temperature coefficient. Above 75° C. the value of the absorption coefficient becomes very nearly constant and approximately equal to 0.111. It is much more soluble in organic liquids, except glycerine, as the following values of the absorption coefficient at 15° C. indicate: -

Ethyl alcohol – 7.2; Amyl alcohol – 9.3; Petroleum – 9.5; Aniline – 4; Toluene – 11.7; Carbon disulphide – 24; Glycerine – 0.21;

It is less soluble in salt solution than in pure water, and Henry's Law is followed exactly in every case.

When subjected to an electrical discharge, radon gives a distinct spectrum, which is the same whether the discharge is direct or oscillatory, and appears to be of the same character as the spectra of the other inert gases.13 There is, however, some evidence that the relative intensity of the lines may vary with the pressure. The radon spectrum disappears after the discharge has been passed for three to five minutes, giving place first to the primary, and later to the secondary spectrum of hydrogen. The source of this hydrogen is not definitely known, but it is very probably occluded in the electrodes. The radon appears to be driven to the walls of the vacuum tube, and can be partially liberated again by prolonged heating.

The first measurements were made visually by Ramsay and Collie, who observed that a very strong and persistent line, λ = 5595, coincided with a strong line seen by Pickering in the spectrum of lightning. Since then numerous attempts have been made to map the spectrum by visual and photographic methods. Their results are summarised and supplemented with new data in a paper by Watson, which should be consulted for further information on this point.

The minute amount of emanation usually obtained is found to condense and volatilise sharply between -152° and -154°, but with larger amounts it has been found possible to determine its vapour pressure at various temperatures.

Its critical temperature is 104.5°, and its critical pressure is 62.5 atmospheres. It boils at -62° and freezes at -71°, and its vapour pressure at the latter temperature is 500 mm. When examined under the microscope, the liquid is seen to be colourless and transparent; its density is about 5. The solid is opaque, but its colour cannot be seen on account of its intrinsic luminosity.

Radon is absorbed by cocoanut charcoal at the ordinary temperature, and is again evolved at higher temperatures, and this fact may be utilised in its purification.