Editing Gilbert N. Lewis (section)

== Scientific achievements ==
{{more footnotes needed|sect|date=August 2020}}

===Thermodynamics===
Most of Lewis’ lasting interests originated during his Harvard years. The most important was thermodynamics, a subject in which Richards was very active at that time. Although most of the important thermodynamic relations were known by 1895, they were seen as isolated equations, and had not yet been rationalized as a logical system, from which, given one relation, the rest could be derived. Moreover, these relations were inexact, applying only to ideal chemical systems. These were two outstanding problems of theoretical thermodynamics. In two long and ambitious theoretical papers in 1900 and 1901, Lewis tried to provide a solution. Lewis introduced the thermodynamic concept of [[activity (chemistry)|activity]] and coined the term "[[fugacity]]".<ref>{{cite journal |last1=Lewis |first1=Gilbert Newton |title=The law of physico-chemical change |journal=Proceedings of the American Academy of Arts and Sciences |date=June 1901 |volume=37 |issue=3 |pages=49–69 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101050586062;view=1up;seq=57 |doi=10.2307/20021635 |jstor=20021635 }} ; the term "fugacity" is coined on p. 54.</ref><ref>{{cite journal |last1=Lewis |first1=Gilbert Newton |title=Outlines of a new system of thermodynamic chemistry |journal=Proceedings of the American Academy of Arts and Sciences |date=1907 |volume=43 |issue=7 |pages=259–293 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101050586120;view=1up;seq=275|doi=10.2307/20022322 |jstor=20022322 }} ; the term "activity" is defined on p. 262.</ref><ref>{{cite journal |last1=Pitzer |first1=Kenneth S. |title=Gilbert N. Lewis and the thermodynamics of strong electrolytes |journal=Journal of Chemical Education |date=February 1984 |volume=61 |issue=2 |pages=104–107 |doi=10.1021/ed061p104 |bibcode=1984JChEd..61..104P |url=https://escholarship.org/content/qt4x23w27c/qt4x23w27c.pdf?t=p0fw58 |doi-access=free }}</ref> His new idea of fugacity, or "escaping tendency",<ref>{{cite journal |last1=Lewis |first1=Gilbert Newton |title=A new conception of thermal pressure and a theory of solutions |journal=Proceedings of the American Academy of Arts and Sciences |date=1900 |volume=36 |issue=9 |pages=145–168 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101050586054;view=1up;seq=165|doi=10.2307/20020988 |jstor=20020988 }} The term "escaping tendency" is introduced on p. 148, where it is represented by the Greek letter ''ψ'' ; ''ψ'' is defined for ideal gases on p. 156.</ref> was a function with the dimensions of [[pressure]] which expressed the tendency of a substance to pass from one chemical phase to another. Lewis believed that fugacity was the fundamental principle from which a system of real thermodynamic relations could be derived. This hope was not realized, though fugacity did find a lasting place in the description of real gases.

Lewis’ early papers also reveal an unusually advanced awareness of [[Josiah Willard Gibbs|J. W. Gibbs's]] and [[Pierre Duhem|P. Duhem's]] ideas of free energy and [[thermodynamic potential]]. These ideas were well known to physicists and mathematicians, but not to most practical chemists, who regarded them as abstruse and inapplicable to chemical systems. Most chemists relied on the familiar thermodynamics of heat (enthalpy) of [[Marcellin Berthelot|Berthelot]], [[Wilhelm Ostwald|Ostwald]], and [[Jacobus Henricus van 't Hoff|Van ’t Hoff]], and the [[calorimetry|calorimetric]] school. Heat of reaction is not, of course, a measure of the tendency of chemical changes to occur, and Lewis realized that only free energy and entropy could provide an exact chemical thermodynamics. He derived free energy from fugacity; he tried, without success, to obtain an exact expression for the [[entropy]] function, which in 1901 had not been defined at low temperatures. Richards too tried and failed, and not until Nernst succeeded in 1907 was it possible to calculate entropies unambiguously. Although Lewis’ fugacity-based system did not last, his early interest in [[Thermodynamic free energy|free energy]] and entropy proved most fruitful, and much of his career was devoted to making these useful concepts accessible to practical chemists.

At Harvard, Lewis also wrote a theoretical paper on the thermodynamics of [[blackbody radiation]] in which he postulated that light has a pressure. He later revealed that he had been discouraged from pursuing this idea by his older, more conservative colleagues, who were unaware that [[Wilhelm Wien]] and others were successfully pursuing the same line of thought. Lewis’ paper remained unpublished; but his interest in radiation and [[quantum mechanics|quantum theory]], and (later) in [[theory of relativity|relativity]], sprang from this early, aborted effort. From the start of his career, Lewis regarded himself as both chemist and physicist.

===Valence theory===
[[Image:Lewis-cubic-notes.jpg|300px|right|thumb|Lewis' '''[[cubical atom]]s''' (as drawn in 1902)]]

About 1902 Lewis started to use unpublished drawings of [[cubical atom]]s in his lecture notes, in which the corners of the cube represented possible [[electron]] positions. Lewis later cited these notes in his classic 1916 paper on chemical bonding, as being the first expression of his ideas.

A third major interest that originated during Lewis’ Harvard years was his valence theory. In 1902, while trying to explain the laws of valence to his students, Lewis conceived the idea that atoms were built up of a concentric series of cubes with electrons at each corner. This “cubic atom” explained the cycle of eight elements in the periodic table and was in accord with the widely accepted belief that chemical bonds were formed by transfer of electrons to give each atom a complete set of eight. This electrochemical theory of valence found its most elaborate expression in the work of [[Richard Abegg]] in 1904,<ref>{{cite journal |last1=Abegg |first1=R. |title=Die Valenz und das periodische System. Versuch einer Theorie der Molekularverbindungen |journal=Zeitschrift für Anorganische Chemie |date=1904 |volume=39 |issue=1 |pages=330–380 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.b3959087;view=1up;seq=344 |trans-title=Valency and the periodic table. Attempt at a theory of molecular compounds |language=de|doi=10.1002/zaac.19040390125 }}</ref> but Lewis’ version of this theory was the only one to be embodied in a concrete atomic model. Again Lewis’ theory did not interest his Harvard mentors, who, like most American chemists of that time, had no taste for such speculation. Lewis did not publish his theory of the cubic atom, but in 1916 it became an important part of his theory of the shared electron pair bond.

In 1916, he published his classic paper on chemical bonding "''The Atom and the Molecule''"<ref>{{cite journal |last1=Lewis |first1=Gilbert N. |title=The atom and the molecule |journal=Journal of the American Chemical Society |date=April 1916 |volume=38 |issue=4 |pages=762–785 |doi=10.1021/ja02261a002 |bibcode=1916JAChS..38..762L |s2cid=95865413 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.hs1t2w;view=1up;seq=772}}</ref> in which he formulated the idea of what would become known as the [[covalent bond]], consisting of a [[shared pair]] of electrons, and he defined the term odd molecule (the modern term is [[free radical]]) when an electron is not shared. He included what became known as [[Lewis structure|Lewis dot structure]]s as well as the [[cubical atom]] model. These ideas on [[chemical bond]]ing were expanded upon by [[Irving Langmuir]] and became the inspiration for the studies on the nature of the chemical bond by [[Linus Pauling]].

===Acids and bases===
{{Main|Lewis acids and bases}}

In 1923, he formulated the electron-pair theory of [[acid–base reaction]]s. In this theory of [[acid]]s and [[base (chemistry)|base]]s, a "Lewis acid" is an ''electron-pair acceptor'' and a "Lewis base" is an ''electron-pair donor''.<ref>{{cite book |last1=Lewis |first1=Gilbert Newton |title=Valence and the Structure of Atoms and Molecules |date=1923 |publisher=Chemical Catalog Company |location=New York|page=142 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.b3219599;view=1up;seq=148|quote = We are inclined to think of substances as possessing acid or basic properties, without having a particular solvent in mind.  It seems to me that with complete generality we may say that ''a basic substance is one which has a lone pair of electrons which may be used to complete the stable group of another atom'', and that ''an acid substance is one which can employ a lone pair from another molecule'' in completing the stable group of one of its own atoms.  In other words, the basic substance furnishes a pair of electrons for a chemical bond, the acid substance accepts such a pair.}}</ref> This year he also published a monograph on his theories of the chemical bond.<ref>Lewis, G. N. (1926) ''Valence and the Nature of the Chemical Bond''. Chemical Catalog Company.</ref>

Based on work by [[Josiah Willard Gibbs|J. Willard Gibbs]], it was known that chemical reactions proceeded to an [[Chemical equilibrium|equilibrium]] determined by the [[Thermodynamic free energy|free energy]] of the substances taking part. Lewis spent 25 years determining free energies of various substances. In 1923 he and [[Merle Randall]] published the results of this study,<ref>Lewis, G. N. and [[Merle Randall]] (1923) ''Thermodynamics and the Free Energies of Chemical Substances''. McGraw-Hill.</ref> which helped formalize modern [[chemical thermodynamics]].

===Heavy water===
Lewis was the first to produce a pure sample of deuterium oxide ([[heavy water]]) in 1933<ref>{{Cite journal| pages = 341| year = 1933 | doi = 10.1063/1.1749300| last2 =  MacDonald| first1 = G. N.| volume = 1| last1 = Lewis| journal = The Journal of Chemical Physics | first2 = R. T.| title = Concentration of H<sub>2</sub> Isotope| issue = 6|bibcode = 1933JChPh...1..341L }}</ref> and the first to study survival and growth of life forms in heavy water.<ref>{{Cite journal| title = The biochemistry of water containing hydrogen isotope| last1 = Lewis| first1 = G. N.| journal = Journal of the American Chemical Society| volume = 55| issue = 8| pages = 3503–3504| year = 1933 | doi = 10.1021/ja01335a509| bibcode = 1933JAChS..55.3503L}}</ref><ref>{{Cite journal| doi = 10.1126/science.79.2042.151| pmid = 17788137| year = 1934| last1 = Lewis | first1 = G. N.| title = The biology of heavy water| volume = 79| issue = 2042| pages = 151–153| journal = Science   |bibcode = 1934Sci....79..151L | s2cid = 4106325}}</ref> By accelerating [[deuteron]]s (deuterium [[atomic nucleus|nuclei]]) in [[Ernest Lawrence|Ernest O. Lawrence's]] [[cyclotron]], he was able to study many of the properties of atomic nuclei.<ref>{{Cite book|url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/deuteron|title = Deuteron - an overview &#124; ScienceDirect Topics| chapter=Radioactivity Hall of Fame–Part VIII | date=2007 | pages=497–528 | publisher=Elsevier | isbn=978-0-444-52715-8 }}</ref> During the 1930s, he was mentor to [[Glenn T. Seaborg]], who was retained for post-doctoral work as Lewis' personal research assistant. Seaborg went on to win the 1951 [[Nobel Prize]] in Chemistry and have the element [[seaborgium]] named in his honor while he was still alive.

=== O<sub>4</sub> Tetraoxygen ===
In 1924, by studying the [[magnetism|magnetic]] properties of solutions of [[oxygen]] in [[liquid]] [[nitrogen]], Lewis found that O<sub>4</sub> molecules were formed.<ref>{{Cite journal|last=Lewis|first=Gilbert N.|date=1924-09-01|title=The magnetism of oxygen and the molecule O<sub>4</sub>|journal=Journal of the American Chemical Society|volume=46|issue=9|pages=2027–2032|doi=10.1021/ja01674a008|bibcode=1924JAChS..46.2027L |issn=0002-7863}}</ref> This was the first evidence for [[tetraoxygen|tetratomic oxygen]].

=== Relativity and quantum physics ===

In 1908 he published the first of several papers on [[theory of relativity|relativity]], in which he derived the [[mass]]-[[energy]] relationship in a different way from [[Albert Einstein]]'s derivation.<ref name=":9">{{Cite journal|author=Lewis, G. N.|year=1908|title=A revision of the Fundamental Laws of Matter and Energy|journal=Philosophical Magazine|volume=16|issue=95|pages=705–717|doi=10.1080/14786441108636549|title-link=s:A revision of the Fundamental Laws of Matter and Energy}}</ref> In 1909, he and [[Richard C. Tolman]] combined his methods with [[principle of relativity|special relativity]].<ref>{{Cite journal|author=Lewis, G. N. & [[Richard C. Tolman]]|year=1909|title=The Principle of Relativity, and Non-Newtonian Mechanics|journal=Proceedings of the American Academy of Arts and Sciences|volume=44|issue=25|pages=709–26|doi=10.2307/20022495|jstor=20022495|title-link=s:The Principle of Relativity, and Non-Newtonian Mechanics}}</ref> In 1912 Lewis and [[Edwin Bidwell Wilson]] presented a major work in mathematical physics that not only applied [[synthetic geometry]] to the study of [[spacetime]], but also noted the identity of a spacetime [[squeeze mapping]] and a [[Lorentz transformation]].<ref>{{cite journal|last1=Wilson|first1=Edwin B.|last2=Lewis|first2=Gilbert N.|year=1912|title=The Space-time Manifold of Relativity. The Non-Euclidean Geometry of Mechanics and Electromagnetics|journal=Proceedings of the American Academy of Arts and Sciences|volume=48|issue=11|pages=387–507|doi=10.2307/20022840|jstor=20022840}}</ref><ref>[https://web.archive.org/web/20091027012400/http://ca.geocities.com/cocklebio/synsptm.html Synthetic Spacetime], a digest of the axioms used, and theorems proved, by Wilson and Lewis. Archived by [[WebCite]]</ref>

In 1926, he coined the term "[[photon]]" for the smallest unit of radiant energy (light). Actually, the outcome of his letter to ''[[Nature (journal)|Nature]]'' was not what he had intended.<ref>
{{cite journal|author=Lewis, G.N.|year=1926|title=The conservation of photons|url=https://www.nature.com/articles/118874a0|journal=[[Nature (journal)|Nature]]|volume=118|issue=2981|pages=874–875|bibcode=1926Natur.118..874L|doi=10.1038/118874a0|s2cid=4110026}}</ref> In the letter, he proposed a photon being a structural element, not [[energy]]. He insisted on the need for a new variable, ''the number of photons''. Although his theory differed from the [[photon|quantum theory of light]] introduced by [[Albert Einstein]] in 1905, his name was adopted for what Einstein had called a '''light quantum''' (Lichtquant in German).

===Other achievements===
In 1921, Lewis was the first to propose an empirical equation describing the failure of [[strong electrolyte]]s to obey the [[law of mass action]], a problem that had perplexed physical chemists for twenty years.<ref>{{cite journal|last1=Lewis|first1=Gilbert N.|last2=Randall|first2=Merle|date=1921|title=The activity coefficient of strong electrolytes|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015018181431;view=1up;seq=1144|journal=Journal of the American Chemical Society|volume=43|issue=5|pages=1112–1154|doi=10.1021/ja01438a014|bibcode=1921JAChS..43.1112L }} The term "ionic strength" is introduced on p. 1140.</ref> His empirical equations for what he called [[ionic strength]] were later confirmed to be in accord with the [[Debye–Hückel equation]] for strong electrolytes, published in 1923.

Over the course of his career, Lewis published on many other subjects besides those mentioned in this entry, ranging from the nature of [[light]] quanta to the [[economics]] of price stabilization. In the last years of his life, Lewis and graduate student [[Michael Kasha]], his last research associate, established that [[phosphorescence]] of [[organic chemistry|organic]] molecules involves emission of light from one electron in an excited [[triplet state]] (a state in which two electrons have their [[Spin (physics)|spin vector]]s oriented in the ''same'' direction, but in different orbitals) and measured the [[paramagnetism]] of this triplet state.<ref>{{cite journal|last1=Lewis|first1=Gilbert N.|last2=Kasha|first2=M.|year=1944|title=Phosphorescence and the Triplet State|journal=Journal of the American Chemical Society|volume=66|issue=12|pages=2100–2116|doi=10.1021/ja01240a030|bibcode=1944JAChS..66.2100L }}</ref>