Whole number rule

File:Dalton's Element List.jpg

The law of definite proportions was formulated by Joseph Proust around 1800Proust, J.-L. (1799). Researches on copper, Ann. chim., 32:26-54. [http://web.lemoyne.edu/~giunta/proust.html Excerpt], in Henry M. Leicester and Herbert S. Klickstein, A Source Book in Chemistry, 1400-1900, Cambridge, MA: Harvard, 1952. Accessed 2008-05-08. and states that all samples of a chemical compound will have the same elemental composition by mass. The atomic theory of John Dalton expanded this concept and explained matter as consisting of discrete atoms with one kind of atom for each element combined in fixed proportions to form compounds.Dalton, J. (1808). A New System of Chemical Philosophy, volume 1, Manchester. [http://web.lemoyne.edu/~giunta/dalton.html Excerpt]. Accessed 2008-05-08.

In 1815, William Prout reported on his observation that the atomic weights of the elements were whole multiples of the atomic weight of hydrogen.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. [http://web.lemoyne.edu/~giunta/PROUT.HTML 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. [http://web.lemoyne.edu/~giunta/PROUT.HTML#prout2 Online reprint] 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.{{cite book|last=Lederman|author-link=Leon Lederman|first=Leon|title=The God Particle|url=https://archive.org/details/godparticle00leon|url-access=registration|year=1993|isbn=9780385312110 }}

In 1920, Francis W. Aston demonstrated through the use of a mass spectrometer that apparent deviations from Prout's hypothesis are predominantly due to the existence of isotopes.{{cite journal |title=The constitution of atmospheric neon |journal=Philosophical Magazine |year=1920 |first=Francis W. |last=Aston |volume=39 |issue=6 |pages=449–455 |url=https://zenodo.org/record/1430720 |doi=10.1080/14786440408636058}} 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 or mass defect of the individual isotopes.

{{Main|Discovery of the neutron}}

File:Groves and Chadwick 830308.jpg, director of the Manhattan Project.]]

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 of an atom and its atomic mass.{{cite journal|doi=10.1063/1.2995181|title=The idea of the neutrino|year=1978|last1=Brown|first1=Laurie M.|journal=Physics Today|volume=31|issue=9|pages=23–28|bibcode = 1978PhT....31i..23B }}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 discovered an uncharged particle of approximately the mass as the proton, which he called the neutron.{{cite journal |last=Chadwick |first=James |year=1932 |title=Possible Existence of a Neutron |journal=Nature |volume=129 |page=312 |doi=10.1038/129312a0 |bibcode = 1932Natur.129Q.312C |issue=3252|doi-access=free }} The fact that the atomic nucleus is composed of protons and neutrons was rapidly accepted and Chadwick was awarded the Nobel Prize in Physics in 1935 for his discovery.{{cite web |url=http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-bio.html |title=James Chadwick – Biography |publisher=The Nobel Foundation |access-date=21 April 2013}}

The modern form of the whole number rule is that the atomic mass of a given elemental isotope is approximately the mass number (number of protons plus neutrons) times an atomic mass unit (approximate mass of a proton, neutron, or hydrogen-1 atom). This rule predicts the atomic mass of nuclides and isotopes with an error of at most 1%, with most of the error explained by the mass deficit caused by nuclear binding energy.

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• {{cite journal |author=Harkins WD |author-link=William Draper Harkins |title=The Separation of Chlorine into Isotopes (Isotopic Elements) and the Whole Number Rule for Atomic Weights |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=11 |issue=10 |pages=624–8 |year=1925 |pmid=16587053 |doi=10.1073/pnas.11.10.624 |pmc=1086175 |bibcode = 1925PNAS...11..624H |doi-access=free }}

• [http://nobelprize.org/nobel_prizes/chemistry/laureates/1922/press.html 1922 Nobel Prize Presentation Speech]

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Category:Mass spectrometry

Category:Periodic table