Atomic Weights of Argon, History

As argon forms no definite compounds its atomic weight cannot be determined in the usual manner; indeed, the usual chemical conception of the atom as the least part of an element which exists in the molecules of its compounds cannot be held in reference to argon or any other of the inert gases. The molecular weight is determined accurately from the density; and there is much evidence for the belief that the molecule is monatomic and that, therefore, the atomic weight is identical with the molecular weight.

Argon shows a close resemblance to the monatomic vapour of mercury in possessing a very low dielectric cohesion, and the value obtained for the thermal conductivity of the gas points to simplicity of molecular structure. What is usually regarded as the most conclusive evidence in this respect is obtained by the determination of the ratio of the specific heats at constant pressure and constant volume: the value 1*66, obtained for argon, is in agreement with that calculated from theoretical considerations and found in the case of other gases which are almost certainly monatomic. J. J. Thomson's method of analysis by positive rays points definitely to the same conclusion.

The work of Eotvos, Ramsay, and Shields has shown that for non- associating liquids the value of the rate of change of the molecular surface energy with the temperature is approximately constant (C.G.S. - 2.12 units). The surface tension of liquid argon has been determined by observation of its rise in a capillary tube, and from these results the molecular weight of argon in the liquid state is calculated to be 45.9, a figure in sufficiently good agreement with the accepted value 39.9 (O = 16).

Lastly, there is the evidence of the general relationship of argon to the other inert gases and to the elements in the first and seventh groups of the periodic system. With one exception these relationships are in agreement with the position in the system occupied by argon, if it is given the atomic weight 39-9. The exception is, of course, the anomaly that exists in the fact that argon, if it is to occupy a place among the elements of Group 0, must precede potassium, the atomic weight of which is 39.1. This disturbing fact must not, however, be permitted to obscure our vision of the greater number of considerations which confirm the accepted atomic weight.