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 suceeded 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, which he named protons.
The discrepancy between Prout's hypothesis and the known variation of 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. Prout's hypothesis was then found to be correct for atomic masses of individual isotopes, to an accuracy not less than 99%.
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[edit] Influence
Prout's hypothesis remained influential in chemistry throughout the 1820s. However, more careful measurements of the atomic weights, such as those compiled by Jöns Jakob Berzelius in 1828 or Edward Turner in 1832, disproved the hypothesis. In particular the atomic weight of chlorine, which is 35.45 times that of hydrogen, could not at the time be explained in terms of Prout's hypothesis. Some came up with the ad hoc claim that the basic unit was one-half of a hydrogen atom, but further discrepancies surfaced. This resulted in the hypothesis that one-quarter of a hydrogen atom was the common unit. Although they turned out to be wrong, these conjectures catalyzed further measurement of atomic weights, a great benefit to chemistry.
The discrepancy in the atomic weights was later understood to be the result natural occurrence of multiple isotopes of the same element. For example, in 1925 the problematic chlorine was found to be composed of the isotopes Cl-35 and Cl-37, in proportions such that the average weight of natural chlorine was about 35.45 times that of hydrogen.[4] All the individual isotopes (nuclides) were eventually found to have masses very close to equal to an even number of hydrogen atoms, to an accuracy always better than 99% (that is, the inaccuracy was always less than 1%). This is a near miss to Prout's law being correct. Nevertheless, the rule was not found to predict isotope masses better than this for all isotopes, due mostly to mass-defects resulting from release of binding energy in atomic nuclei, when they are formed.
Although all elements are the product of nuclear fusion of hydrogen into higher elements, it is now understood that atoms consist of both protons (hydrogen nuclei) and neutrons. The modern version of Prout's rule is that the atomic mass of a nucleus of proton number P and neutron number N is equal to sum of the masses of its constituent protons and neutrons, minus the mass of the nuclear binding energy, the mass defect. This is roughly, but not exactly, its mass number M = P + N times an atomic mass unit (u), plus or minus binding energy discrepancy – atomic mass unit being the modern approximation for "mass of a proton, neutron, or hydrogen atom". For example iron-56 atoms (which have among the highest binding-energies) weigh only about 99.1% as much as 56 hydrogen atoms. The missing 0.9% of mass represents the energy lost when the nucleus of iron was made from hydrogen inside a star. (See stellar nucleosynthesis).
[edit] Literary Allusion
In his 1891 story, The Doings of Raffles Haw, its author, Arthur Conan Doyle, talks about turning elements into other elements of decreasing atomic number, until a gray matter is reached. In his 1959 novel "Life and Fate", Vassily Grossman's principal character, the physicist Viktor Shtrum, reflects on Prout's hypothesis about hydrogen being the origin of other elements (and the felicitous fact that Prout's incorrect data led to an essentially correct conclusion), as he worries about his inability to formulate his own thesis.
[edit] See also
[edit] References
- ^ William Prout (1815). On the relation between the specific gravities of bodies in their gaseous state and the weights of their atoms. Annals of Philosophy, 6: 321–330. Online reprint
- ^ William Prout (1816). Correction of a mistake in the essay on the relation between the specific gravities of bodies in their gaseous state and the weights of their atoms. Annals of Philosophy, 7: 111–13. Online reprint
- ^ Lederman, Leon (1993). The God Particle.
- ^ Harkins WD (1925). "The Separation of Chlorine into Isotopes (Isotopic Elements) and the Whole Number Rule for Atomic Weights". Proc. Natl. Acad. Sci. U.S.A. 11 (10): 624–8. Bibcode 1925PNAS...11..624H. doi:10.1073/pnas.11.10.624. PMC 1086175. PMID 16587053. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1086175.
[edit] Further reading
- Gladstone, Samuel (1947). "William Prout (1785-1850)". Journal of Chemical Education 24 (10): 478–481. doi:10.1021/ed024p478.
- Benfey, O. Theodore (1952). "Prout's Hypothesis". Journal of Chemical Education 29 (2): 78–81. doi:10.1021/ed029p78.
- Siegfried, Robert (1956). "The Chemical Basis for Prout's Hypothesis". Journal of Chemical Education 33 (6): 263–266. doi:10.1021/ed033p263.
