Editing History of atomic theory (section)

==Discovery of the proton==
{{Prose|section|date=November 2024}}
Back in 1815, [[William Prout]] observed that the atomic weights of the known elements were multiples of hydrogen's atomic weight, so he hypothesized that all atoms are agglomerations of hydrogen,  a particle which he dubbed "the protyle". [[Prout's hypothesis]] was put into doubt when some elements were found to deviate from this pattern—e.g. chlorine atoms on average weigh 35.45 [[dalton (unit)|daltons]]—but when [[isotopes]] were discovered in 1913, Prout's observation gained renewed attention.<ref name="Kragh-2010">{{Cite web |last=Kragh |first=Helge |date=October 2010 |title=Before Bohr: Theories of atomic structure 1850-1913. |url=https://css.au.dk/fileadmin/reposs/reposs-010.pdf |access-date=2024-11-29 |website=RePoSS: Research Publications on Science Studies, 10.}}</ref>

In 1898, J. J. Thomson found that the positive charge of a hydrogen ion was equal to the negative charge of a single electron.<ref>{{Cite journal |last=J. J. Thomson |date=1898 |title=On the Charge of Electricity carried by the Ions produced by Röntgen Rays |url=https://archive.org/details/londonedinburgh5461898lon/page/528/mode/2up |journal=The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science |series=5 |volume=46 |issue=283 |pages=528–545 |doi=10.1080/14786449808621229}}</ref>

In an April 1911 paper concerning his studies on [[alpha particle]] scattering, [[Ernest Rutherford]] estimated that the charge of an atomic nucleus, expressed as a multiplier of hydrogen's nuclear charge (''q''<sub>e</sub>), is roughly half the atom's [[atomic weight]].<ref>{{cite journal
 |author1=Ernest Rutherford
 |year=1911
 |title=The Scattering of α and β Particles by Matter and the Structure of the Atom
 |url=http://www.chemteam.info/Chem-History/Rutherford-1911/Rutherford-1911.html
 |journal=[[Philosophical Magazine]] |series=Series 6
 |volume=21 |issue= 125|pages=669–688
 |doi=10.1080/14786440508637080
 |ref=refRutherford1911
|url-access=subscription
 }}</ref>

In June 1911, Van den Broek noted that on the [[periodic table]], each successive chemical element increased in atomic weight on average by 2, which in turn suggested that each successive element's nuclear charge increased by 1 ''q''<sub>e</sub>.<ref>{{cite journal |author=Antonius van den Broek |date=23 June 1911 |title=The Number of Possible Elements and Mendeléeff's "Cubic" Periodic System |journal=Nature |volume=87 |issue=2177 |pages=78 |doi=10.1038/087078b0 |bibcode=1911Natur..87...78V |url=https://www.nature.com/articles/087078b0}}<br />"Hence, if this cubic periodic system should prove to be correct, then the number of possible elements is equal to the number of possible permanent charges of each sign per atom, or to each possible permanent charge (of both signs) per atom belongs a possible element."</ref> In 1913, van den Broek further proposed that the electric charge of an atom's nucleus, expressed as a multiplier of the [[elementary charge]], is equal to the element's sequential position on the periodic table. Rutherford defined this position as being the element's [[atomic number]].<ref>Eric Scerri (2020). ''The Periodic Table: Its Story and Its Significance'', p. 185</ref><ref>{{cite journal |author=Ernest Rutherford |date=March 1914 |title=The Structure of the Atom |journal=Philosophical Magazine |series=6 |volume=27 |pages=488–498 |url=https://www.chemteam.info/Chem-History/Rutherford-1914.html |quote=It is obvious from the consideration of the cases of hydrogen and helium, where hydrogen has one electron and helium two, that the number of electrons cannot be exactly half the atomic weight in all cases. This has led to an interesting suggestion by van den Broek that the number of units of charge on the nucleus, and consequently the number of external electrons, may be equal to the number of the elements when arranged in order of increasing atomic weight.}}</ref><ref>{{cite journal |author=Ernest Rutherford |date=11 Dec 1913 |title=The Structure of the Atom |journal=Nature |volume=92 |issue=423 |quote=The original suggestion of van der Broek that the charge on the nucleus is equal to the atomic number and not to half the atomic weight seems to me very promising.}}</ref>

In 1913, [[Henry Moseley]] measured the X-ray emissions of all the elements on the periodic table and found that the frequency of the X-ray emissions was a mathematical function of the element's atomic number and the charge of a hydrogen nucleus {{Crossreference|(see [[Moseley's law]])}}.{{citation needed|date=October 2024}}

In 1917 [[Ernest Rutherford|Rutherford]] bombarded [[nitrogen]] gas with [[alpha particle]]s and observed [[hydrogen]] ions being emitted from the gas. Rutherford concluded that the alpha particles struck the nuclei of the nitrogen atoms, causing hydrogen ions to split off.<ref>{{cite journal|author=Rutherford, Ernest|url=http://web.lemoyne.edu/~GIUNTA/rutherford.html |title=Collisions of alpha Particles with Light Atoms. IV. An Anomalous Effect in Nitrogen|journal=Philosophical Magazine|year=1919|volume=37|page=581|doi=10.1080/14786440608635919|issue=222|url-access=subscription}}</ref><ref>''The Development of the Theory of Atomic Structure'' (Rutherford 1936). Reprinted in ''Background to Modern Science: Ten Lectures at Cambridge arranged by the History of Science Committee 1936'':<br />"In 1919 I showed that when light atoms were bombarded by α-particles they could be broken up with the emission of a proton, or hydrogen nucleus. We therefore presumed that a proton must be one of the units of which the nuclei of other atoms were composed..."</ref>

These observations led Rutherford to conclude that the hydrogen nucleus was a singular particle with a positive charge equal to that of the electron's negative charge. The name "proton" was suggested by Rutherford at an informal meeting of fellow physicists in Cardiff in 1920.<ref>{{cite journal |author=Orme Masson |date=1921 |title=The Constitution of Atoms |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |volume=41 |issue=242 |pages=281–285 |doi=10.1080/14786442108636219 |url=https://archive.org/details/londonedinburg6411921lond/page/280/mode/2up}}<br />Footnote by Ernest Rutherford: 'At the time of writing this paper in Australia, Professor Orme Masson was not aware that the name "proton" had already been suggested as a suitable name for the unit of mass nearly 1, in terms of oxygen 16, that appears to enter into the nuclear structure of atoms. The question of a suitable name for this unit was discussed at an informal meeting of a number of members of Section A of the British Association [for the Advancement of Science] at Cardiff this year. The name "baron" suggested by Professor Masson was mentioned, but was considered unsuitable on account of the existing variety of meanings. Finally the name "proton" met with general approval, particularly as it suggests the original term "protyle" given by Prout in his well-known hypothesis that all atoms are built up of hydrogen. The need of a special name for the nuclear unit of mass 1 was drawn attention to by Sir Oliver Lodge at the Sectional meeting, and the writer then suggested the name "proton."'</ref>

The charge number of an atomic nucleus was found to be equal to the element's ordinal position on the periodic table. The nuclear charge number thus provided a simple and clear-cut way of distinguishing the [[chemical element]]s from each other, as opposed to [[Antoine Lavoisier|Lavoisier's]] classic definition of a chemical element being a substance that cannot be broken down into simpler substances by chemical reactions. The charge number or proton number was thereafter referred to as the [[atomic number]] of the element.  In 1923, the International Committee on Chemical Elements officially declared the atomic number to be the distinguishing quality of a chemical element.<ref>{{cite journal |author=Helge Kragh |date=2000 |title=Conceptual Changes in Chemistry: The Notion of a Chemical Element, ca. 1900-1925 |journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics |volume=31 |issue=4 |pages=435–450|doi=10.1016/S1355-2198(00)00025-3 |bibcode=2000SHPMP..31..435K }}</ref>

During the 1920s, some writers defined the atomic number as being the number of "excess protons" in a nucleus. Before the discovery of the [[neutron]], scientists believed that the atomic nucleus contained a number of "nuclear electrons" which cancelled out the positive charge of some of its protons. This explained why the atomic weights of most atoms were higher than their atomic numbers. Helium, for instance, was thought to have four protons and two nuclear electrons in the nucleus, leaving two excess protons and a net nuclear charge of 2+. After the neutron was discovered, scientists realized the helium nucleus in fact contained two protons and two neutrons.