Atomic Weights of Helium, History

As helium does not form any definite compound, the equivalent referred to hydrogen or oxygen cannot be determined - indeed, it cannot be said to have an equivalent. We are thus reduced to other and less satisfactory methods for the determination of its atomic weight.

Assuming the truth of Avogadro's hypothesis, it follows from determinations of the density that the molecular weight of helium is 4.002 (O₂ = 32), and as the ratio of the specific heats is 1.66, it is probable that the atomic weight is identical with the molecular weight.

This conclusion is based (a) on reasoning from the assumptions of the kinetic theory of gases, from which it appears that the highest possible ratio of the specific heats - which one would expect to find in cases of the greatest molecular simplicity - is about 1.66, and (b) on the fact that the vapour of mercury, known on other grounds to be monatomic (e.g. from the vapour density), has the ratio of the specific heats, Cp/Cv = 1.66.

Confirmatory evidence of molecular simplicity is to be found in the nature of the positive rays in helium, the Zeeman effect in helium, and in the thermal conductivity of the gas.

The view that the atomic weight is approximately 4 receives further support from the way which helium then falls into place with the other elements of its group in the periodic classification of the elements.

Detection. - The simplest method for the detection of helium consists in introducing the gas into a vacuum tube connected with a side tube containing charcoal cooled in liquid air; all heavier gases are thus absorbed. In this way it was found possible to detect the helium and neon in 50 c.c. of air. This method has also been used for the detection of helium in minerals.