Editing Molecule (section)

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

The definition of the molecule has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise, defining molecules as the smallest [[list of particles#Molecules|particles]] of pure [[chemical substance]]s that still retain their [[chemical compound|composition]] and chemical properties.<ref>[http://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#molecule Molecule Definition] {{Webarchive|url=https://web.archive.org/web/20141013143129/http://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#molecule|date=13 October 2014}} ([[Frostburg State University]])</ref> This definition often breaks down since many substances in ordinary experience, such as [[rock (geology)|rocks]], [[salt (chemistry)|salts]], and [[metal]]s, are composed of large crystalline networks of [[chemical bond|chemically bonded]] atoms or [[ion]]s, but are not made of discrete molecules.

The modern concept of molecules can be traced back towards pre-scientific and Greek philosophers such as [[Leucippus]] and [[Democritus]] who argued that all the universe is composed of [[Atomic theory|atoms and voids]]. Circa 450 BC [[Empedocles]] imagined [[Classical element|fundamental elements]] ([[Fire (classical element)|fire]] ([[File:Fire_symbol_(alchemical).svg|20x20px]]), [[Earth (classical element)|earth]] ([[File:Earth_symbol_(alchemical).svg|20x20px]]), [[Air (classical element)|air]] ([[File:Air_symbol_(alchemical).svg|20x20px]]), and [[Water (classical element)|water]] ([[File:Water_symbol_(alchemical).svg|20x20px]])) and "forces" of attraction and repulsion allowing the elements to interact.

A fifth element, the incorruptible quintessence [[Aether (classical element)|aether]], was considered to be the fundamental building block of the heavenly bodies. The viewpoint of Leucippus and Empedocles, along with the aether, was accepted by [[Aristotle]] and passed to medieval and renaissance Europe.

In a more concrete manner, however, the concept of aggregates or units of bonded atoms, i.e. "molecules", traces its origins to [[Robert Boyle]]'s 1661 hypothesis, in his famous treatise ''[[The Sceptical Chymist]]'', that matter is composed of ''clusters of particles'' and that chemical change results from the rearrangement of the clusters. Boyle argued that matter's basic elements consisted of various sorts and sizes of particles, called "corpuscles", which were capable of arranging themselves into groups. In 1789, [[William Higgins (chemist)|William Higgins]] published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of [[valency bonds]]. If, for example, according to Higgins, the force between the ultimate particle of oxygen and the ultimate particle of nitrogen were 6, then the strength of the force would be divided accordingly, and similarly for the other combinations of ultimate particles.

[[Amedeo Avogadro]] created the word "molecule".<ref name="ley196606">{{Cite magazine |last=Ley |first=Willy |date=June 1966 |title=The Re-Designed Solar System |url=https://archive.org/stream/Galaxy_v24n05_1966-06#page/n93/mode/2up |department=For Your Information |magazine=Galaxy Science Fiction |pages=94–106}}</ref> His 1811 paper "Essay on Determining the Relative Masses of the Elementary Molecules of Bodies", he essentially states, i.e. according to [[J. R. Partington|Partington]]'s ''A Short History of Chemistry'', that:<ref>{{cite journal |last1=Avogadro |first1=Amedeo |date=1811 |title=Masses of the Elementary Molecules of Bodies |url=http://web.lemoyne.edu/~giunta/AVOGADRO.HTML |journal=Journal de Physique |volume=73 |pages=58–76 |access-date=25 August 2022 |archive-date=12 May 2019 |archive-url=https://web.archive.org/web/20190512182624/http://web.lemoyne.edu/~giunta/avogadro.html |url-status=live }}</ref>{{Blockquote|The smallest particles of gases are not necessarily simple atoms, but are made up of a certain number of these atoms united by attraction to form a single '''molecule'''.}}In coordination with these concepts, in 1833 the French chemist [[Marc Antoine Auguste Gaudin]] presented a clear account of Avogadro's hypothesis,<ref>{{cite journal |author=Seymour H. Mauskopf |date=1969 |title=The Atomic Structural Theories of Ampère and Gaudin: Molecular Speculation and Avogadro's Hypothesis |journal=Isis |volume=60 |issue=1 |pages=61–74 |doi=10.1086/350449 |jstor=229022 |s2cid=143759556}}</ref> regarding atomic weights, by making use of "volume diagrams", which clearly show both semi-correct molecular geometries, such as a linear water molecule, and correct molecular formulas, such as H<sub>2</sub>O:
[[File:Gaudins-volume-diagrams.jpg|center|thumb|350x350px|Marc Antoine Auguste Gaudin's volume diagrams of molecules in the gas phase (1833)]]
In 1917, an unknown American undergraduate chemical engineer named [[Linus Pauling]] was learning the [[Dalton model|Dalton hook-and-eye bonding method]], which was the mainstream description of bonds between atoms at the time. Pauling, however, was not satisfied with this method and looked to the newly emerging field of quantum physics for a new method. In 1926, French physicist [[Jean Perrin]] received the Nobel Prize in physics for proving, conclusively, the existence of molecules. He did this by calculating the [[Avogadro constant]] using three different methods, all involving liquid phase systems. First, he used a [[gamboge]] soap-like emulsion, second by doing experimental work on [[Brownian motion]], and third by confirming Einstein's theory of particle rotation in the liquid phase.<ref>Perrin, Jean, B. (1926). [https://www.nobelprize.org/prizes/physics/1926/perrin/lecture/ Discontinuous Structure of Matter] {{Webarchive|url=https://web.archive.org/web/20190529115507/https://www.nobelprize.org/prizes/physics/1926/perrin/lecture/ |date=29 May 2019 }}, Nobel Lecture, 11 December.</ref>

In 1927, the physicists [[Fritz London]] and [[Walter Heitler]] applied the new quantum mechanics to the deal with the saturable, nondynamic forces of attraction and repulsion, i.e., exchange forces, of the hydrogen molecule. Their valence bond treatment of this problem, in their joint paper,<ref>{{cite journal |last1=Heitler |first1=Walter |last2=London |first2=Fritz |date=1927 |title=Wechselwirkung neutraler Atome und homöopolare Bindung nach der Quantenmechanik |journal=Zeitschrift für Physik |volume=44 |issue=6–7 |pages=455–472 |bibcode=1927ZPhy...44..455H |doi=10.1007/BF01397394 |s2cid=119739102}}</ref> was a landmark in that it brought chemistry under quantum mechanics. Their work was an influence on Pauling, who had just received his doctorate and visited Heitler and London in Zürich on a [[Guggenheim Fellowship]].

Subsequently, in 1931, building on the work of Heitler and London and on theories found in Lewis' famous article, Pauling published his ground-breaking article "The Nature of the Chemical Bond"<ref>{{cite journal |last1=Pauling |first1=Linus |date=1931 |title=The nature of the chemical bond. Application of results obtained from the quantum mechanics and from a theory of paramagnetic susceptibility to the structure of molecules |journal=J. Am. Chem. Soc. |volume=53 |issue=4 |pages=1367–1400 |doi=10.1021/ja01355a027|bibcode=1931JAChS..53.1367P }}</ref> in which he used [[quantum mechanics]] to calculate properties and structures of molecules, such as angles between bonds and rotation about bonds. On these concepts, Pauling developed [[hybridization theory]] to account for bonds in molecules such as CH<sub>4</sub>, in which four sp³ hybridised orbitals are overlapped by [[hydrogen]]'s ''1s'' orbital, yielding four [[Sigma bond|sigma (σ) bonds]]. The four bonds are of the same length and strength, which yields a molecular structure as shown below:
[[File:Ch4_hybridization.svg|center|thumb|200x200px|A schematic presentation of hybrid orbitals overlapping hydrogens' s orbitals]]