Prout’s Hypothesis.
§ 77. It was observed by a chemist of the name of Prout, that, the atomic weight of hydrogen being taken as the unit, the atomic weights of nearly all the elements approximated to whole numbers; and in 1815 he suggested as the reason for this regularity, that all the elements consist solely of hydrogen. Prout’s Hypothesis received on the whole a very favourable reception; it harmonised Dalton’s Theory with the grand concept of the unity of matter—all matter was hydrogen in essence; and Thomas Thomson undertook a research to demonstrate its truth. On the other hand, however, the eminent Swedish chemist, Berzelius, who had carried out many atomic weight determinations, criticised both Prout’s Hypothesis and Thomson’s research (which latter, it is true, was worthless) in most severe terms; for the hypothesis amounted to this—that the decimals in the atomic weights obtained experimentally by Berzelius, after so much labour, were to be regarded as so many errors. In 1844, Marignac suggested half the hydrogen atom as the unit, for the element chlorine, with an atomic weight of 35·5, would not fit in with Prout’s Hypothesis as originally formulated; and later, Dumas suggested one-quarter. With this theoretical division of the hydrogen-atom, the hypothesis lost its simplicity and charm, and was doomed to downfall. Recent and most accurate atomic weight determinations show clearly that the atomic weights are not exactly whole numbers, but that, nevertheless, the majority of them (if expressed in terms of O = 16 as the unit) do approximate very closely to such. The Hon. R. J. Strutt has recently calculated that the probability of this occurring, in the case of certain of the commoner elements, by mere chance is exceedingly small (about 1 in 1,000),[93] and several attempts to explain this remarkable fact have been put forward. Modern scientific speculations concerning the constitution of atoms tend towards a modified form of Prout’s hypothesis, or to the view that the atoms of other elements are, in a manner, polymerides of hydrogen and helium atoms. As has been pointed out, it is possible, according to modern views, for elements of different atomic weight to have identical chemical properties, since these latter depend only upon the number of free electrons in the atom and not at all upon the massive central nucleus. By a method somewhat similar to that used for determining the mass of kathode particles (see [§ 79]), but applied to positively charged particles, Sir Joseph Thomson and Dr. F. W. Aston discovered that the element neon was a mixture of two isotopic elements in unequal proportions, one having an atomic mass of 20, the other (present only to a slight extent) having an atomic mass of 22. Dr. Aston has perfected this method of analysing mixtures of isotopes and determining their atomic masses.[94] The results are of great interest. The atomic weight of hydrogen, 1·008, is confirmed. The elements helium, carbon, nitrogen, oxygen, fluorine, phosphorus, sulphur, arsenic, iodine and sodium are found to be simple bodies with whole-number atomic weights. On the other hand, boron, neon, silicon, chlorine, bromine, krypton, xenon, mercury, lithium, potassium and rubidium are found to be mixtures. What is specially of interest is that the indicated atomic mass of each of the constituents is a whole number. Thus chlorine, whose atomic weight is 35·46, is found to be a mixture of two chemically-identical elements whose atomic weights are 35 and 37. Some of the elements, e.g., xenon, are mixtures of more than two isotopes.
[93] Hon. R. J. Strutt: “On the Tendency of the Atomic Weights to approximate to Whole Numbers,” Philosophical Magazine, [6], vol. i. (1901), pp. 311 et seq.
[94] F. W. Aston: “Mass-spectra and Atomic Weights,” Journal of the Chemical Society, vol. cix. (1921), pp. 677 et seq.
It is highly probable that what is true of the elements investigated by Dr. Aston is true of the remainder. It appears, therefore, that the irregularities presented by the atomic weights of the ordinary elements, which have so much puzzled men of science in the past, are due to the fact that these elements are, in many cases, mixtures. As concerns hydrogen, it is only reasonable to suppose that the close packing of electrically charged particles should give rise to a slight decrease in their total mass, so that the atomic weights of other elements referred to H = 1 should be slightly less than whole numbers, or, what is the same thing, that the atomic weight of hydrogen referred to O = 16 should be slightly more than unity.