Editing Atom (section)

== History of atomic theory ==
{{Main|History of atomic theory}}

=== In philosophy ===
{{Main|Atomism}}
The basic idea that matter is made up of tiny indivisible particles is an old idea that appeared in many ancient cultures. The word ''atom'' is derived from the [[ancient Greek]] word ''atomos'',{{efn|a combination of the negative term "a-" and "τομή" (''tomḗ''), the term for "cut"}} which means "uncuttable". But this ancient idea was based in philosophical reasoning rather than scientific reasoning. Modern atomic theory is not based on these old concepts.<ref>{{cite book|last1=Pullman|first1=Bernard|title=The Atom in the History of Human Thought|date=1998|publisher=Oxford University Press|location=Oxford, England|isbn=978-0-19-515040-7|pages=31–33|url=https://books.google.com/books?id=IQs5hur-BpgC&q=Leucippus+Democritus+atom&pg=PA56|access-date=25 October 2020|archive-date=5 February 2021|archive-url=https://web.archive.org/web/20210205165029/https://books.google.com/books?id=IQs5hur-BpgC&q=Leucippus+Democritus+atom&pg=PA56|url-status=live}}</ref><ref>[[#refMelsen1952|Melsen (1952). ''From Atomos to Atom'', pp. 18–19]]</ref> In the early 19th century, the scientist [[John Dalton]] found evidence that matter really is composed of discrete units, and so applied the word ''atom'' to those units.<ref>[[#refPullman1998|Pullman (1998). ''The Atom in the History of Human Thought'', p. 201]]</ref>

=== Dalton's law of multiple proportions ===
[[File:Daltons symbols.gif|thumb|right|Various atoms and molecules from ''A New System of Chemical Philosophy'' (John Dalton 1808).]]
In the early 1800s, John Dalton compiled experimental data gathered by him and other scientists and discovered a pattern now known as the "[[law of multiple proportions]]". He noticed that in any group of chemical compounds which all contain two particular chemical elements, the amount of Element A per measure of Element B will differ across these compounds by ratios of small [[whole numbers]]. This pattern suggested that each element combines with other elements in multiples of a basic unit of weight, with each element having a unit of unique weight. Dalton decided to call these units "atoms".<ref>Pullman (1998). ''The Atom in the History of Human Thought'', p. 199: "The constant ratios, expressible in terms of integers, of the weights of the constituents in composite bodies could be construed as evidence on a macroscopic scale of interactions at the microscopic level between basic units with fixed weights. For Dalton, this agreement strongly suggested a corpuscular structure of matter, even though it did not constitute definite proof."</ref>

For example, there are two types of [[tin oxide (disambiguation)|tin oxide]]: one is a grey powder that is 88.1% tin and 11.9% [[oxygen]], and the other is a white powder that is 78.7% tin and 21.3% oxygen. Adjusting these figures, in the grey powder there is about 13.5&nbsp;g of oxygen for every 100&nbsp;g of tin, and in the white powder there is about 27&nbsp;g of oxygen for every 100&nbsp;g of tin. 13.5 and 27 form a ratio of 1:2. Dalton concluded that in the grey oxide there is one atom of oxygen for every atom of tin, and in the white oxide there are two atoms of oxygen for every atom of tin ([[tin(II) oxide|SnO]] and [[tin dioxide|SnO<sub>2</sub>]]).<ref>[[#refDalton1817|Dalton (1817). ''A New System of Chemical Philosophy'' vol. 2, p. 36]]</ref><ref>[[#refMelsen1952|Melsen (1952). ''From Atomos to Atom'', p. 137]]</ref>

Dalton also analyzed [[iron oxide]]s. There is one type of iron oxide that is a black powder which is 78.1% iron and 21.9% oxygen; and there is another iron oxide that is a red powder which is 70.4% iron and 29.6% oxygen. Adjusting these figures, in the black powder there is about 28&nbsp;g of oxygen for every 100&nbsp;g of iron, and in the red powder there is about 42&nbsp;g of oxygen for every 100&nbsp;g of iron. 28 and 42 form a ratio of 2:3. Dalton concluded that in these oxides, for every two atoms of iron, there are two or three atoms of oxygen respectively. These substances are known today as [[iron(II) oxide]] and [[iron(III) oxide]], and their formulas are FeO and Fe<sub>2</sub>O<sub>3</sub> respectively. Iron(II) oxide's formula is normally written as FeO, but since it is a crystalline substance we could alternately write it as Fe<sub>2</sub>O<sub>2</sub>, and when we contrast that with Fe<sub>2</sub>O<sub>3</sub>, the 2:3 ratio for the oxygen is plain to see.<ref>[[#refDalton1817|Dalton (1817). ''A New System of Chemical Philosophy'' vol. 2, p. 28]]</ref><ref>[[#refMillington1906|Millington (1906). ''John Dalton'', p. 113]]</ref>

As a final example: [[nitrous oxide]] is 63.3% [[nitrogen]] and 36.7% oxygen, [[nitric oxide]] is 44.05% nitrogen and 55.95% oxygen, and [[nitrogen dioxide]] is 29.5% nitrogen and 70.5% oxygen. Adjusting these figures, in nitrous oxide there is 80&nbsp;g of oxygen for every 140&nbsp;g of nitrogen, in nitric oxide there is about 160&nbsp;g of oxygen for every 140&nbsp;g of nitrogen, and in nitrogen dioxide there is 320&nbsp;g of oxygen for every 140&nbsp;g of nitrogen. 80, 160, and 320 form a ratio of 1:2:4. The respective formulas for these oxides are [[nitrous oxide|N<sub>2</sub>O]], [[nitric oxide|NO]], and [[nitrogen dioxide|NO<sub>2</sub>]].<ref>[[#refDalton1808|Dalton (1808). ''A New System of Chemical Philosophy'' vol. 1, pp. 316–319]]</ref><ref>[[#refHolbrowEtAl2010|Holbrow et al. (2010). ''Modern Introductory Physics'', pp. 65–66]]</ref>

=== Discovery of the electron ===
In 1897, [[J. J. Thomson]] discovered that [[cathode ray]]s can be deflected by electric and magnetic fields, which meant that cathode rays are not a form of light but made of electrically charged particles, and their charge was negative given the direction the particles were deflected in.<ref>{{cite journal |author=J. J. Thomson |url=http://web.lemoyne.edu/~GIUNTA/thomson1897.html |title=Cathode rays |journal=Philosophical Magazine |volume=44 |issue=269 |pages=293–316 |year=1897}}</ref> He measured these particles to be 1,700 times lighter than [[hydrogen]] (the lightest atom).<ref>In his book ''The Corpuscular Theory of Matter'' (1907), Thomson estimates electrons to be 1/1700 the mass of hydrogen.</ref> He called these new particles ''corpuscles'' but they were later renamed ''[[electron]]s'' since these are the particles that carry electricity.<ref>[http://library.thinkquest.org/C0111709/English/DC-Circuts/mechanism.html "The Mechanism Of Conduction In Metals"] {{Webarchive|url=https://web.archive.org/web/20121025004809/http://library.thinkquest.org/ |date=25 October 2012 }}, Think  Quest.</ref> Thomson also showed that electrons were identical to particles given off by [[Photoelectric effect|photoelectric]] and radioactive materials.<ref name="Thomson">{{cite journal|last=Thomson|first=J.J.|title=On bodies smaller than atoms|journal=The Popular Science Monthly|pages=323–335|date=August 1901|url=https://books.google.com/books?id=3CMDAAAAMBAJ&pg=PA323|access-date=21 June 2009|archive-date=1 December 2016|archive-url=https://web.archive.org/web/20161201152039/https://books.google.com/books?id=3CMDAAAAMBAJ&pg=PA323|url-status=live}}</ref> Thomson explained that an electric current is the passing of electrons from one atom to the next, and when there was no current the electrons embedded themselves in the atoms. This in turn meant that atoms were not indivisible as scientists thought. The atom was composed of electrons whose negative charge was balanced out by some source of positive charge to create an electrically neutral atom. Ions, Thomson explained, must be atoms which have an excess or shortage of electrons.<ref>J. J. Thomson (1907). ''On the Corpuscular Theory of Matter'', p. 26: "The simplest interpretation of these results is that the positive ions are the atoms or groups of atoms of various elements from which one or more corpuscles have been removed [...] while the negative electrified body is one with more corpuscles than the unelectrified one."</ref>

=== Discovery of the nucleus ===
[[File:Geiger-Marsden experiment expectation and result.svg|thumb|right|The [[Rutherford scattering experiments]]: The extreme scattering of some alpha particles suggested the existence of a nucleus of concentrated charge.]]
{{Main|Rutherford scattering experiments}}
The electrons in the atom logically had to be balanced out by a commensurate amount of positive charge, but Thomson had no idea where this positive charge came from, so he tentatively proposed that it was everywhere in the atom, the atom being in the shape of a sphere. This was the mathematically simplest hypothesis to fit the available evidence, or lack thereof. Following from this, Thomson imagined that the balance of electrostatic forces would distribute the electrons throughout the sphere in a more or less even manner.<ref>J. J. Thomson (1907). ''The Corpuscular Theory of Matter'', p. 103: "In default of exact knowledge of the nature of the way in which positive electricity occurs in the atom, we shall consider a case in which the positive electricity is distributed in the way most amenable to mathematical calculation, i.e., when it occurs as a sphere of uniform density, throughout which the corpuscles are distributed."</ref> Thomson's model is popularly known as the [[plum pudding model]], though neither Thomson nor his colleagues used this analogy.<ref name=HonGoldstein2013>{{cite journal |author1=Giora Hon |author2=Bernard R. Goldstein |date=6 September 2013 |title=J. J. Thomson's plum-pudding atomic model: The making of a scientific myth |journal=Annalen der Physik |volume=525 |issue=8–9 |pages=A129–A133 |doi= 10.1002/andp.201300732 |bibcode=2013AnP...525A.129H |url=https://onlinelibrary.wiley.com/doi/10.1002/andp.201300732 | issn=0003-3804}}</ref> Thomson's model was incomplete, it was unable to predict any other properties of the elements such as [[emission spectra]] and [[valency (chemistry)|valencies]]. It was soon rendered obsolete by the discovery of the [[atomic nucleus]].

Between 1908 and 1913, [[Ernest Rutherford]] and his colleagues [[Hans Geiger]] and [[Ernest Marsden]] performed a series of experiments in which they bombarded thin foils of metal with a beam of [[alpha particles]]. They did this to measure the scattering patterns of the alpha particles. They spotted a small number of alpha particles being deflected by angles greater than 90°. This shouldn't have been possible according to the Thomson model of the atom, whose charges were too diffuse to produce a sufficiently strong electric field. The deflections should have all been negligible. Rutherford proposed that the positive charge of the atom is concentrated in a tiny volume at the center of the atom and that the electrons surround this nucleus in a diffuse cloud. This nucleus carried almost all of the atom's mass. Only such an intense concentration of charge, anchored by its high mass, could produce an electric field that could deflect the alpha particles so strongly.<ref name=Heilbron2003p64-68>[[#refHeilbron2003|Heilbron (2003). ''Ernest Rutherford and the Explosion of Atoms'', pp. 64–68]]</ref>

=== Bohr model ===
{{Main|Bohr model}}
[[File:Bohr atom animation 2.gif|right|thumb|The Bohr model of the atom, with an electron making instantaneous "quantum leaps" from one orbit to another with gain or loss of energy. This model of electrons in orbits is obsolete.]]
A problem in classical mechanics is that an accelerating charged particle radiates [[electromagnetic radiation]], causing the particle to lose [[kinetic energy]]. Circular motion counts as acceleration, which means that an electron orbiting a central charge should spiral down into that nucleus as it loses speed. In 1913, the physicist [[Niels Bohr]] proposed a new model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits, and could jump between these orbits only in discrete changes of energy corresponding to absorption or radiation of a [[photon]].<ref name=stern20050516 /> This quantization was used to explain why the electrons' orbits are stable and why elements absorb and emit electromagnetic radiation in discrete spectra.<ref name=bohr19221211 /> Bohr's model could only predict the emission spectra of hydrogen, not atoms with more than one electron.

=== 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>

=== The current consensus model ===
[[File:S-p-Orbitals.svg|thumb|right|The modern model of atomic orbitals draws zones where an electron is most likely to be found at any moment.]]
In 1925, [[Werner Heisenberg]] published the first consistent mathematical formulation of quantum mechanics ([[matrix mechanics]]).<ref name="Pais">{{cite book|last=Pais|first=Abraham|year=1986|location=New York|title=Inward Bound: Of Matter and Forces in the Physical World|publisher=Oxford University Press|isbn=978-0-19-851971-3|pages=[https://archive.org/details/inwardboundofmat00pais_0/page/228 228–230]|url=https://archive.org/details/inwardboundofmat00pais_0/page/228}}</ref> One year earlier, [[Louis de Broglie]] had proposed that all particles behave like waves to some extent,<ref>{{cite book |title=Introducing Quantum Theory |author1=McEvoy, J. P. |author2=Zarate, Oscar  |publisher=Totem Books |year=2004 |isbn=978-1-84046-577-8 |pages=110–114}}</ref> and in 1926 [[Erwin Schrödinger]] used this idea to develop the [[Schrödinger equation]], which describes electrons as three-dimensional [[waveform]]s rather than points in space.<ref>{{cite web |last=Kozłowski |first=Miroslaw |year=2019 |title=The Schrödinger equation A History |url=https://www.researchgate.net/publication/332241721}}</ref> A consequence of using waveforms to describe particles is that it is mathematically impossible to obtain precise values for both the [[point (geometry)|position]] and [[momentum]] of a particle at a given point in time. This became known as the [[uncertainty principle]], formulated by Werner Heisenberg in 1927.<ref name="Pais" /> In this concept, for a given accuracy in measuring a position one could only obtain a range of probable values for momentum, and vice versa.<ref>{{cite web|author=Chad Orzel|url=https://www.youtube.com/watch?v=TQKELOE9eY4|title=What is the Heisenberg Uncertainty Principle?|website=TED-Ed|date=16 September 2014|via=YouTube|archive-url=https://web.archive.org/web/20150913185956/https://www.youtube.com/watch?v=TQKELOE9eY4|archive-date=13 September 2015|url-status=live}}</ref> Thus, the planetary model of the atom was discarded in favor of one that described [[atomic orbital]] zones around the nucleus where a given electron is most likely to be found.<ref name=brown2007 /><ref name=harrison2000 /> This model was able to explain observations of atomic behavior that previous models could not, such as certain structural and [[Spectral line|spectral]] patterns of atoms larger than hydrogen.