Rutherford’s Protons and Prout’s Protyles – and bonus nuclear information

by NICHOLAS MEE on NOVEMBER 27, 2020

John Dalton (1766-1844) grew up in the English Lake District and spent most of his working life in Manchester. In 1808 he published A New System of Chemical Philosophy where he argued that each chemical element is composed of a distinct type of atom. Dalton listed the elements in a table (shown on the right), assigning a chemical symbol to each and giving its atomic mass as a multiple of that of hydrogen. His figures were, however, rather inaccurate.

Dalton believed, quite correctly, that compounds consist of combinations of atoms and that these combinations are rearranged in chemical reactions. He illustrated his ideas with small wooden balls of varying sizes, similar to the much later ball and stick models used to represent molecules.

Prout’s Hypothesis

William Prout (1785-1850)

William Prout (1785-1850) was a contemporary of Dalton’s. He was a medical doctor and accomplished chemist who studied organic materials and was the first to classify foodstuffs as fats, carbohydrates and proteins.

Prout published two papers in 1815 and 1816 where he observed that the atomic mass of every element so far measured was a multiple of the atomic mass of hydrogen. He suggested that this might be because there was just one fundamental unit or protyle, as he called it, from which all the elements were formed. This idea became known as Prout’s hypothesis. It implied that an atom of hydrogen was formed of a single protyle, whereas each atom of the heavier elements was composed of several protyles that were somehow bound together. This was a great idea. Unfortunately, the experimental basis on which it was constructed seemed to be flawed.

Jöns Jacob Berzelius

Berzelius

The Swedish chemist Jöns Jacob Berzelius (1779–1848) was one of the founders of modern chemistry and made wide-ranging contributions throughout the subject. One of his achievements was to replace the old alchemical symbols with the much more flexible modern chemical notation. He also conclusively demonstrated that elements combine in fixed proportions thereby lending support to Dalton’s atomic hypothesis. But Berzelius’s measurements were far more accurate than Dalton’s. And when he published a table of atomic masses in his Textbook of Chemistry in 1826 he showed that they could not be strict integer multiples of the atomic mass of hydrogen. Berzelius’s results discredited Prout’s hypothesis and it fell out of favour with chemists. 

Lord Rayleigh

But Prout’s hypothesis was never completely forgotten. The simplicity of the idea appealed especially to physicists. In 1904 when Lord Rayleigh accepted the Nobel Prize for Physics he recalled that Prout’s hypothesis had inspired his research.

The subject of the densities of gases has engaged a large part of my attention for over 20 years. In 1882 in an address to the British Association I suggested that the time had come for a re-determination of these densities, being interested in the question of Prout’s law. At that time the best results were those of Regnault, according to whom the density of oxygen was 15.96 times that of hydrogen. The deviation of this number from the integer 16 seemed not to be outside the limits of experimental error.

Rayleigh attempted to resolve the issue by investigating the atomic mass of nitrogen, and this led to the discovery of the noble gas argon and Rayleigh’s Nobel Prize, as discussed here: William Ramsay’s Noble Quest.

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