Prout's hypothesis

Prout's hypothesis was an early 19th-century attempt to explain the existence of the various chemical elements through a hypothesis regarding the internal structure of the atom. In 1815^[1] and 1816,^[2] the English chemist William Prout published two papers in which he observed that the atomic weights that had been measured for the elements known at that time appeared to be whole multiples of the atomic weight of hydrogen. He then hypothesized that the hydrogen atom was the only truly fundamental object, which he called protyle, and that the atoms of other elements were actually groupings of various numbers of hydrogen atoms.^[3]

Prout's hypothesis was an influence on Ernest Rutherford when he succeeded in "knocking" hydrogen nuclei out of nitrogen atoms with alpha particles in 1917, and thus concluded that perhaps the nuclei of all elements were made of such particles (the hydrogen nucleus), which in 1920 he suggested be named protons, from the suffix "-on" for particles, added to the stem of Prout's word "protyle".^[a] The assumption as discussed by Rutherford was of a nucleus consisting of Z + N = A protons plus N electrons somehow trapped within thereby reducing the positive charge to +Z as observed and vaguely explaining beta decay radioactivity. Such a nuclear constitution was known to be inconsistent with dynamics either classical or early quantum but seemed inevitable until the neutron hypothesis by Rutherford and discovery by English physicist James Chadwick.

The discrepancy between Prout's hypothesis and the known variation of some atomic weights to values far from integral multiples of hydrogen, was explained between 1913 and 1932 by the discovery of isotopes and the neutron. According to the whole number rule of Francis Aston, Prout's hypothesis is correct for atomic masses of individual isotopes, with an error of at most 1%.

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