Editing Atom (section)

=== Discovery of protons and neutrons===
{{Main|Atomic nucleus|Discovery of the neutron}}
Back in 1815, [[William Prout]] observed that the atomic weights of many elements were multiples of hydrogen's atomic weight, which is in fact true for all of them if one takes [[isotopes]] into account. In 1898, [[J. J. Thomson]] found that the positive charge of a hydrogen ion is equal to the negative charge of an electron, and these were then the smallest known charged particles.<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> Thomson later found that the positive charge in an atom is a positive multiple of an electron's negative charge.<ref>J. J. Thomson (1907). ''The Corpuscular Theory of Matter''. p. 26–27: "In an unelectrified atom there are as many units of positive electricity as there are of negative; an atom with a unit of positive charge is a neutral atom which has lost one corpuscle, while an atom with a unit of negative charge is a neutral atom to which an additional corpuscle has been attached."</ref> In 1913, [[Henry Moseley]] discovered that the frequencies of X-ray emissions from an [[excited state|excited]] atom were a mathematical function of its [[atomic number]] and hydrogen's nuclear charge. In 1919, [[Ernest Rutherford|Rutherford]] bombarded [[nitrogen]] gas with [[alpha particle]]s and detected [[hydrogen]] ions being emitted from the gas, and concluded that they were produced by alpha particles hitting and splitting the nuclei of the nitrogen atoms.<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}}</ref>

These observations led Rutherford to conclude that the hydrogen nucleus is a singular particle with a positive charge equal to the electron's negative charge.<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> He named this particle "[[proton]]" 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 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 number of protons in an atom (which Rutherford called the "[[atomic number]]"<ref>Eric Scerri (2020). ''The Periodic Table: Its Story and Its Significance'', p. 185</ref><ref>Helge Kragh (2012). ''Niels Bohr and the Quantum Atom'', p. 33</ref>) was found to be equal to the element's ordinal number on the [[periodic table]] and therefore provided a simple and clear-cut way of distinguishing the elements from each other. The atomic weight of each element is higher than its proton number, so Rutherford hypothesized that the surplus weight was carried by unknown particles with no electric charge and a mass equal to that of the proton.

In 1928, [[Walter Bothe]] observed that [[beryllium]] emitted a highly penetrating, electrically neutral radiation when bombarded with alpha particles. It was later discovered that this radiation could knock hydrogen atoms out of [[paraffin wax]]. Initially it was thought to be high-energy [[gamma radiation]], since gamma radiation had a similar effect on electrons in metals, but [[James Chadwick]] found that the [[ionization]] effect was too strong for it to be due to electromagnetic radiation, so long as energy and momentum were conserved in the interaction. In 1932, Chadwick exposed various elements, such as hydrogen and nitrogen, to the mysterious "beryllium radiation", and by measuring the energies of the recoiling charged particles, he deduced that the radiation was actually composed of electrically neutral particles which could not be massless like the gamma ray, but instead were required to have a mass similar to that of a proton. Chadwick now claimed these particles as Rutherford's neutrons.<ref>{{cite journal|author=James Chadwick |year=1932|url=http://web.mit.edu/22.54/resources/Chadwick.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://web.mit.edu/22.54/resources/Chadwick.pdf |archive-date=9 October 2022 |url-status=live |title=Possible Existence of a Neutron|doi=10.1038/129312a0|journal=Nature|page=312|volume=129|bibcode = 1932Natur.129Q.312C|issue=3252|s2cid=4076465|doi-access=free}}</ref>