Prout's hypothesis was an early 19th century attempt to explain the existence of the various
chemical elementA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s through a hypothesis regarding the internal structure of the
atomThe atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
. In 1815 and 1816, the
EnglishEngland is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
chemist
William ProutWilliam Prout FRS was an English chemist, physician, and natural theologian. He is remembered today mainly for what is called Prout's hypothesis.-Biography:...
published two papers in which he observed that the
atomic weightAtomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
s that had been measured for the elements known at that time appeared to be whole multiples of the atomic weight of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. 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. Prout's hypothesis was an influence on
Ernest RutherfordErnest Rutherford, 1st Baron Rutherford of Nelson OM, FRS was a New Zealand-born British chemist and physicist who became known as the father of nuclear physics...
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
protonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s.
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
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s and the
neutronThe neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
. Prout's hypothesis was then found to be correct for
atomic massThe atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
es of individual isotopes, to an accuracy not less than 99%.
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 BerzeliusJöns Jacob Berzelius was a Swedish chemist. He worked out the modern technique of chemical formula notation, and is together with John Dalton, Antoine Lavoisier, and Robert Boyle considered a father of modern chemistry...
in 1828 or
Edward TurnerEdward Turner was a British motorcycle designer. He was born in Camberwell in the London Borough of Southwark, on the day King Edward VII was proclaimed King....
in 1832, disproved the hypothesis. In particular the atomic weight of
chlorineChlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, which is 35.45 times that of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, could not at the time be explained in terms of Prout's hypothesis. Some came up with the
ad hocAd hoc is a Latin phrase meaning "for this". It generally signifies a solution designed for a specific problem or task, non-generalizable, and not intended to be able to be adapted to other purposes. Compare A priori....
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 weightAtomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
s was later understood to be the result natural occurrence of multiple
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s 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. All the individual isotopes (
nuclideA nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
s) 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 energyBinding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
in atomic nuclei, when they are formed.
Although all elements are the product of
nuclear fusionNuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
of hydrogen into higher elements, it is now understood that atoms consist of both
protonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s (hydrogen nuclei) and
neutronThe neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s. The modern version of Prout's rule is that the
atomic massThe atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
of a nucleus of proton number
P and
neutron numberThe neutron number, symbol N, is the number of neutrons in a nuclide.Atomic number plus neutron number equals mass number: Z+N=A....
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 numberThe mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...
M =
P +
N times an
atomic mass unitThe unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
(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-56Iron-56 is the most common isotope of iron. About 91.754% of all iron is iron-56.Of all isotopes, iron-56 has the lowest mass per nucleon. With 8.8 MeV binding energy per nucleon, iron-56 is one of the most tightly bound nuclei....
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 nucleosynthesisStellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...
).
Literary Allusion
In his 1891 story,
The Doings of Raffles HawThe Doings of Raffles Haw is a novel by Scottish author Sir Arthur Conan Doyle.-Plot outline:A mysterious millionaire Raffles Haw comes to reside in Tamfield in Staffordshire. Even before he arrives, people start gossiping about him...
, its author,
Arthur Conan DoyleSir Arthur Ignatius Conan Doyle DL was a Scottish physician and writer, most noted for his stories about the detective Sherlock Holmes, generally considered a milestone in the field of crime fiction, and for the adventures of Professor Challenger...
, talks about turning elements into other elements of decreasing
atomic numberIn chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
, 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.
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