American physical chemist best known for his contributions to chemical thermodynamics, the electron-pair model of the covalent bond, the electronic theory of acids (acid) and bases (base), the separation and study of deuterium and its compounds, and his work on phosphorescence and the triplet state (in which the quantum number for total spin angular momentum is 1).

Lewis spent his youth in Lincoln, Neb. Initially educated at home by his parents, at age 13 he entered the preparatory school of the University of Nebraska in Lincoln. He continued at the university through his sophomore year before transferring to Harvard University in 1893, from which he received a bachelor's degree in chemistry in 1896. After a year of teaching at Phillips Academy in Andover, Mass., he returned to Harvard to complete a master's degree in 1898, followed by a doctorate the next year under the supervision of Theodore Richards (Richards, Theodore William) for a dissertation on the electrochemistry of zinc and cadmium amalgams.

After graduation, Lewis remained at Harvard as an instructor for a year. He then pursued postgraduate work in the laboratories of Wilhelm Ostwald (Ostwald, Wilhelm) and Walther Nernst (Nernst, Walther Hermann) in Germany, before he returned for another three years as an instructor at Harvard and then a year in the Philippine Islands as superintendent of weights and measures. In 1905 Lewis joined the faculty of the Massachusetts Institute of Technology in Cambridge, and in 1912 he was appointed permanent dean of the college of chemistry and chair of the department of chemistry at the University of California at Berkeley, where he remained until his death at age 70 of an apparent heart attack while working in his laboratory. During his 34-year tenure at Berkeley, Lewis succeeded in molding its chemistry department into one of the best in the United States.

Though his appointment as chair at Berkeley relieved him of all classroom teaching duties, Lewis was well known for his insightful and often witty comments during student and staff research seminars. A brilliant conversationalist, with an almost unlimited supply of jokes and bon mots, Lewis was also addicted to the use of limericks and puns. He preferred to write his books and papers by dictating them to his assistants and collaborators, having fully composed his carefully crafted sentences in his head. When dictating, he would pace up and down the room while smoking an imported cigar—a habit picked up during his stay in the Philippines.

In 1912 Lewis married Mary Hinckley Sheldon, by whom he had three children, a daughter and two sons.

Lewis's major area of research was the field of chemical thermodynamics. In 1899 there was still a large gap between thermodynamic theory and practice. There was a complete theory of chemical equilibria, developed 20 years earlier by the American physicist Josiah Willard Gibbs (Gibbs, J Willard), which showed that chemical equilibrium was determined by the free energies (free energy) of the reacting substances. On the other hand, there was a vast amount of unorganized data on the enthalpies (enthalpy) of reaction of chemical substances, collected earlier in the century by such chemists as Julius Thomsen (Thomsen, Julius) of Denmark and Pierre-Eugène-Marcellin Berthelot (Berthelot, Pierre-Eugène-Marcellin) of France. In addition, a series of empirical laws, dealing with the behaviour of ideal gases and dilute solutions, were developed that formed the substance of the newer physical chemistry (chemistry) championed by such chemists as Ostwald, Svante Arrhenius (Arrhenius, Svante August) in Sweden, Jacobus van 't Hoff (Hoff, Jacobus Henricus van 't) in The Netherlands, and Nernst. Lewis set himself the task of closing this gap between theory and practice. This required that he either directly measure the missing free-energy values for chemical substances or supplement the existing enthalpy data with entropy values, which would allow their calculation. Second, it was also necessary to find some way of extending the empirical laws to include the behaviour of real gases and concentrated solutions.

In pursuit of the first of these goals, Lewis initiated a vigorous experimental program designed to measure the missing free-energy and entropy values. In pursuit of the second goal, he successively introduced the concepts of fugacity (1901), activity coefficient (1907), and ionic strength (1921; a measure of the average electrostatic interactions among ions in a solution). These efforts culminated in 1923 in the publication of Thermodynamics and the Free Energy of Chemical Substances, written in collaboration with chemist Merle Randall.

A second important thread in Lewis's research centred on his speculations on the role of the newly discovered electron in chemical bonding. Though his first attempts in this area date as early as 1902, he did not publish on the subject until 1913—and then only to comment critically on attempts of others to formulate similar theories. In 1916 Lewis finally published his own model, which equated the classical chemical bond with the sharing of a pair of electrons between the two bonded atoms. Most students know of Lewis today because of “electron dot diagrams,” which he introduced in this paper to symbolize the electronic structures of atoms and molecules. Now known as Lewis structures, they are discussed in virtually every introductory chemistry book.

Shortly after publication of his 1916 paper, Lewis became involved with military research. He did not return to the subject of chemical bonding until 1923, when he masterfully summarized his model in a short monograph entitled "Valence and the Structure of Atoms and Molecules." His renewal of interest in this subject was largely stimulated by the activities of the American chemist Irving Langmuir (Langmuir, Irving), who between 1919 and 1921 popularized and elaborated Lewis's model. Many current terms relating to the chemical bond, such as covalent and the octet rule, were actually introduced by Langmuir rather than Lewis.

The 1920s saw a rapid adoption and application of Lewis's model of the electron-pair bond in the fields of organic and coordination chemistry. In organic chemistry (chemistry), this was primarily due to the efforts of the British chemists Arthur Lapworth, Robert Robinson (Robinson, Sir Robert), Thomas Lowry, and Christopher Ingold; while in coordination chemistry, Lewis's bonding model was promoted through the efforts of the American chemist Maurice Huggins and the British chemist Nevil Sidgwick (Sidgwick, Nevil Vincent). Though Lewis occasionally published on his bonding model throughout the 1920s, he stopped writing on the subject after 1933 and left the task of reconciling the model with the newer quantum mechanics of Austrian physicist Erwin Schrödinger (Schrödinger, Erwin) and German physicist Werner Heisenberg (Heisenberg, Werner) in the hands of the American chemist Linus Pauling (Pauling, Linus). Pauling transformed it into the valence bond model and made it the subject of his classic book, The Nature of the Chemical Bond (1939).

Between 1933 and 1934, Lewis published more than 26 papers dealing with the separation and study of the properties of deuterium and its compounds. This was followed by a brief period of interest in neutron refraction (1936–37) and by his classic work on the electronic theory of acids and bases (1938). Now universally known as the Lewis acid-base definitions, these concepts define an acid as an electron-pair acceptor and a base as an electron-pair donor. First proposed, almost as a passing thought, in his 1923 monograph on chemical bonding, discussions of Lewis acids and bases are now found in most introductory chemistry textbooks. Almost simultaneously with his work on acid-base theory, Lewis also began his classic research on the triplet state and its role in determining the nature of the fluorescence, phosphorescence, and colours of organic dyes (dye), which continued until his death.

Lewis occasionally published speculative papers dealing with fundamental problems in theoretical physics. While still a student at Harvard, he had postulated that light could exert a pressure on dilute matter in outer space, and he later introduced the term photon to describe the particulate nature of electromagnetic radiation. In 1909 he published the first American paper to deal with Albert Einstein (Einstein, Albert)'s recently proposed theory of relativity. Later papers dealt with vector analysis, rational units, quantum field theory, statistical mechanics, and the thermodynamics of glacier formation. Some of these speculations were discussed in his third and final book, The Anatomy of Science (1926).

One of the great puzzles of Lewis's career is the absence of a Nobel Prize. It has been suggested that he should have shared the 1934 Nobel Prize for Chemistry with American Harold Urey (Urey, Harold C.) for his contributions to the separation and study of deuterium and its compounds and that, had he lived longer, he most certainly would have shared the 1954 Nobel Prize for Chemistry with Pauling for his contributions to the theory of the chemical bond.