chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, the whole number rule states that the masses of the isotopes are

whole number An integer is the number zero ( 0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number ( −1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative ...

multiples of the mass of the

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

atom. The rule is a modified version of Prout's hypothesis proposed in 1815, to the effect that

atomic weight Relative atomic mass (symbol: ''A''; sometimes abbreviated RAM or r.a.m.), also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a giv ...

s are multiples of the weight of the hydrogen atom. It is also known as the Aston whole number rule after Francis W. Aston who was awarded the Nobel Prize in Chemistry in 1922 "for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule".

Law of definite proportions

The

law of definite proportions In chemistry, the law of definite proportions, sometimes called Proust's law or the law of constant composition, states that a given chemical compound contains its constituent elements in a fixed ratio (by mass) and does not depend on its source ...

was formulated by

Joseph Proust Joseph Louis Proust (26 September 1754 – 5 July 1826) was a French people, French chemist. He was best known for his discovery of the law of definite proportions in 1797, stating that chemical compounds always combine in constant proportions. ...

around 1800 and states that all samples of a chemical compound will have the same elemental composition by mass. The

atomic theory Atomic theory is the scientific theory that matter is composed of particles called atoms. The definition of the word "atom" has changed over the years in response to scientific discoveries. Initially, it referred to a hypothetical concept of ...

John Dalton John Dalton (; 5 or 6 September 1766 – 27 July 1844) was an English chemist, physicist and meteorologist. He introduced the atomic theory into chemistry. He also researched Color blindness, colour blindness; as a result, the umbrella term ...

expanded this concept and explained matter as consisting of discrete

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s with one kind of atom for each element combined in fixed proportions to form compounds.

Prout's hypothesis

In 1815,

William Prout William Prout Fellow of the Royal Society, FRS (; 15 January 1785 – 9 April 1850) was an English chemist, physician, and natural theologian. He is remembered today mainly for what is called Prout's hypothesis. Biography Prout was born in H ...

reported on his observation that the

atomic mass Atomic mass ( or ) is the mass of a single atom. The atomic mass mostly comes from the combined mass of the protons and neutrons in the nucleus, with minor contributions from the electrons and nuclear binding energy. The atomic mass of atoms, ...

es of the elements were whole multiples of the atomic mass of

. He then hypothesized that the hydrogen atom was the fundamental object and that the other elements were a combination of different numbers of hydrogen atoms.

Aston's discovery of isotopes

In 1920, Francis W. Aston demonstrated through the use of a

mass spectrometer Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...

that apparent deviations from Prout's hypothesis are predominantly due to the existence of

isotopes Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), but ...

. For example, Aston discovered that neon has two isotopes with masses very close to 20 and 22 as per the whole number rule, and proposed that the non-integer value 20.2 for the atomic weight of neon is due to the fact that natural neon is a mixture of about 90% neon-20 and 10% neon-22). A secondary cause of deviations is the

binding energy In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...

or mass defect of the individual isotopes.

Discovery of the neutron

During the 1920s, it was thought that the atomic nucleus was made of protons and electrons, which would account for the disparity between the

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of its atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (''n''p) or the number of pro ...

of an atom and its

.Friedlander G., Kennedy J.W. and Miller J.M. (1964) ''Nuclear and Radiochemistry'' (2nd edition), Wiley, pp. 22–23 and 38–39 In 1932,

James Chadwick Sir James Chadwick (20 October 1891 – 24 July 1974) was an English nuclear physicist who received the Nobel Prize in Physics in 1935 for his discovery of the neutron. In 1941, he wrote the final draft of the MAUD Report, which inspired t ...

discovered an uncharged particle of approximately the mass as the proton, which he called the

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

. The fact that the atomic nucleus is composed of protons and neutrons was rapidly accepted and Chadwick was awarded the

Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...

in 1935 for his discovery. The modern form of the whole number rule is that the

of a given elemental

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

is approximately the

mass number The mass number (symbol ''A'', from the German word: ''Atomgewicht'', "atomic weight"), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is appro ...

(number of protons plus neutrons) times a dalton (approximate mass of a proton, neutron, or hydrogen-1 atom). This rule predicts the

nuclide Nuclides (or nucleides, from nucleus, also known as nuclear species) are a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by the A ...

s and isotopes with an error of at most 1%, with most of the error explained by the mass deficit caused by

nuclear binding energy Nuclear binding energy in experimental physics is the minimum energy that is required to disassemble the nucleus of an atom into its constituent protons and neutrons, known collectively as nucleons. The binding energy for stable nuclei is alwa ...

References

External links