title: Timeline of physical chemistry
text: # Timeline of physical chemistry
  
  The timeline of physical chemistry lists the sequence of physical chemistry theories and discoveries in chronological order.
  
  ## Timeline details
  
  | Date | Person | Contribution |
  | - | - | - |
  | 1088 | Shen Kuo | First person to write of the magnetic needle compass and that it improved the accuracy of navigation by helping to employ the astronomical concept of True North at all times of the day, thus making the first, recorded, scientific observation of the magnetic field (as opposed to a theory grounded in superstition or mysticism). |
  | 1187 | Alexander Neckham | First in Europe to describe the magnetic compass and its use in navigation. |
  | 1269 | Pierre de Maricourt | Published the first extant treatise on the properties of magnetism and compass needles. |
  | 1550 | Gerolamo Cardano | Wrote about electricity in De Subtilitate distinguishing, perhaps for the first time, between electrical and magnetic forces. |
  | 1600 | William Gilbert | In De Magnete, expanded on Cardano's work (1550) and coined the New Latin word electricus from ἤλεκτρον (elektron), the Greek word for "amber" (from which the ancients knew an electric spark could be created by rubbing it with silk). Gilbert undertook a number of careful electrical experiments, in the course of which he discovered that many substances other than amber, such as sulphur, wax, glass, etc., were capable of manifesting electrostatic properties. Gilbert also discovered that a heated body lost its electricity and that moisture prevented the electrification of all bodies, due to the now well-known fact that moisture impairs the electrical insulation of such bodies. He also noticed that electrified substances attracted all other substances indiscriminately, whereas a magnet only attracted iron. The many discoveries of this nature earned for Gilbert the title of founder of the electrical sciences. |
  | 1646 | Sir Thomas Browne | The first usage of the word electricity is ascribed to his work Pseudodoxia Epidemica. |
  | 1660 | Otto von Guericke | Invented an early electrostatic generator. By the end of the 17th Century, researchers had developed practical means of generating electricity by friction by the use of an electrostatic generator, but the development of electrostatic machines did not begin in earnest until the 18th century, when they became fundamental instruments in the studies of the new science of electricity. |
  | 1667 | Johann Joachim Becher | Stated the now-defunct scientific theory that postulated the existence of a fire-like element called "phlogiston" that was contained within combustible bodies and released during combustion. The theory was an attempt to explain processes such as combustion and the rusting of metals, which are now understood as oxidation, and which was ultimately disproved by Antoine Lavoisier in 1789. |
  | 1675 | Robert Boyle | Discovered that electric attraction and repulsion can act across a vacuum and does not depend upon the air as a medium. He also added resin to the then-known list of "electrics". |
  | 1678 | Christiaan Huygens | Stated his theory to the French Academy of Sciences that light is a wave-like phenomenon. |
  | 1687 | Sir Isaac Newton | Published Philosophiæ Naturalis Principia Mathematica, by itself considered to be among the most influential books in the history of science, laying the groundwork for most of classical mechanics. In this work, Newton described universal gravitation and the three laws of motion, which dominated the scientific view of the physical universe for the next three centuries. Newton showed that the motions of objects on Earth and of celestial bodies are governed by the same set of natural laws by demonstrating the consistency between Kepler's laws of planetary motion and his theory of gravitation, thus removing the last doubts about heliocentrism and advancing the scientific revolution. In mechanics, Newton enunciated the principles of conservation of both momentum and angular momentum. (Eventually, it was determined that Newton's laws of classical mechanics were a limiting case of the more general theory of quantum mechanics for macroscopic objects (in the same way that Newton's laws of motion are a limiting case of Einstein's Theory of Relativity).) |
  | 1704 | Sir Isaac Newton | In his work Opticks, Newton contended that light was made up of numerous small particles. This hypothesis could explain such features as light's ability to travel in straight lines and reflect off surfaces. However, this proposed theory was known to have its problems: although it explained reflection well, its explanation of refraction and diffraction was less satisfactory. In order to explain refraction, Newton postulated an "Aethereal Medium" transmitting vibrations faster than light, by which light, when overtaken, is put into "Fits of easy Reflexion and easy Transmission", which he supposed caused the phenomena of refraction and diffraction. |
  | 1708 | Brook Taylor | Obtained a remarkable solution of the problem of the "centre of oscillation" fundamental to the development of wave mechanics which, however, remained unpublished until May 1714. |
  | 1715 | Brook Taylor | In Methodus Incrementorum Directa et Inversa (1715), he added a new branch to the higher mathematics, now designated the "calculus of finite differences". Among other ingenious applications, he used it to determine the form of movement of a vibrating string, first successfully reduced by him to mechanical principles. The same work contained the celebrated formula known as Taylor's theorem, the importance of which remained unrecognized until 1772, when J. L. Lagrange realized its powers and termed it "le principal fondement du calcul différentiel" ("the main foundation of differential calculus"). Taylor's work thereby provided the cornerstone of the calculus of wave mechanics. |
  | 1722 | René Antoine Ferchault de Réaumur | Demonstrated that iron was transformed into steel through the absorption of some substance, now known to be carbon. |
  | 1729 | Stephen Gray | Conducted a series of experiments that demonstrated the difference between conductors and non-conductors (insulators). From these experiments he classified substances into two categories: "electrics", like glass, resin and silk, and "non-electrics", like metal and water. Although Gray was the first to discover and deduce the property of electrical conduction, he incorrectly stated that "electrics" conducted charges while "non-electrics" held the charge. |
  | 1732 | C. F. du Fay | Conducted several experiments and concluded that all objects, except metals, animals, and liquids, could be electrified by rubbing them and that metals, animals and liquids could be electrified by means of an "electric machine" (the name used at the time for electrostatic generators), thus discrediting Gray's "electrics" and "non-electrics" classification of substances (1729). |
  | 1737 | C. F. du Fay and Francis Hauksbee the younger | Independently discovered what they believed to be two kinds of frictional electricity: one generated from rubbing glass, the other from rubbing resin. From this, Du Fay theorized that electricity consists of two "electrical fluids": "vitreous" and "resinous", that are separated by friction, and that neutralize each other when combined. This two-fluid theory would later give rise to the concept of positive and negative electrical charges devised by Benjamin Franklin. |
  | 1740 | Jean le Rond d'Alembert | In Mémoire sur la réfraction des corps solides, explains the process of refraction. |
  | 1740s | Leonhard Euler | Disagreed with Newton's corpuscular theory of light in the Opticks, which was then the prevailing theory. His 1740s papers on optics helped ensure that the wave theory of light proposed by Christiaan Huygens would become the dominant mode of thought, at least until the development of the quantum theory of light. |
  | 1745 | Pieter van Musschenbroek | At Leiden University, he invented the Leyden jar, a type of capacitor (also known as a "condensor") for electrical energy in large quantities. |
  | 1747 | William Watson | While experimenting with a Leyden jar (1745), he discovered the concept of an electrical potential (voltage) when he observed that a discharge of static electricity caused the electric current earlier observed by Stephen Gray to occur. |
  | 1752 | Benjamin Franklin | Identified lightning with electricity when he discovered that lightning conducted through a metal key could be used to charge a Leyden jar, thus proving that lightning was an electric discharge and current (1747). He is also attributed with the convention of using "negative" and "positive" to denote an electrical charge or potential. |
  | 1766 | Henry Cavendish | The first to recognize hydrogen gas as a discrete substance, by identifying the gas from a metal–acid reaction as "flammable air" and further finding in 1781 that the gas produces water when burned. |
  | 1771 | Luigi Galvani | Invented the voltaic cell. Galvani made this discovery when he noted that two different metals (copper and zinc for example) were connected together and then both touched to different parts of a nerve of a frog leg at the same time, a spark was generated which made the leg contract. Although he incorrectly assumed that the electric current was proceeding from the frog as some kind of "animal electricity", his invention of the voltaic cell was fundamental to the development of the electric battery. |
  | 1772 | Antoine Lavoisier | Showed that diamonds are a form of carbon, when he burned samples of carbon and diamond then showed that neither produced any water and that both released the same amount of carbon dioxide per gram. |
  | 1772 | Carl Wilhelm Scheele | Showed that graphite, which had been thought of as a form of lead, was instead a type of carbon. |
  | 1772 | Daniel Rutherford | Discovered and studied nitrogen, calling it noxious air or fixed air because this gas constituted a fraction of air that did not support combustion. Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air or phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as "mephitic air" or azote, from the Greek word άζωτος (azotos) meaning "lifeless". Animals died in it, and it was the principal component of air in which animals had suffocated and flames had burned to extinction. |
  | 1772 | Carl Wilhelm Scheele | Produced oxygen gas by heating mercuric oxide and various nitrates by about 1772. Scheele called the gas 'fire air' because it was the only known supporter of combustion, and wrote an account of this discovery in a manuscript he titled Treatise on Air and Fire, which he sent to his publisher in 1775. However, that document was not published until 1777. |
  | 1778 | Carl Scheele and Antoine Lavoisier | Discovered that air is composed mostly of nitrogen and oxygen. |
  | 1781 | Joseph Priestley | The first to utilize the electric spark to produce an explosion of hydrogen and oxygen, mixed in the proper proportions, to produce pure water. |
  | 1784 | Henry Cavendish | Discovered the inductive capacity of dielectrics (insulators) and, as early as 1778, measured the specific inductive capacity for beeswax and other substances by comparison with an air condenser. |
  | 1784 | Charles-Augustin de Coulomb | Devised the torsion balance, by means of which he discovered what is known as Coulomb's law: the force exerted between two small electrified bodies varies inversely as the square of the distance; not as Franz Aepinus in his theory of electricity had assumed, merely inversely as the distance. |
  | 1788 | Joseph-Louis Lagrange | Stated a re-formulation of classical mechanics that combines conservation of momentum with conservation of energy, now called Lagrangian mechanics, and which would be critical to the later development of a quantum mechanical theory of matter and energy. |
  | 1789 | Antoine Lavoisier | In his text Traité Élémentaire de Chimie (often considered to be the first modern chemistry text), stated the first version of the law of conservation of mass, recognized and named oxygen (1778) and hydrogen (1783), abolished the phlogiston theory, helped construct the metric system, wrote the first extensive list of elements, and helped to reform chemical nomenclature. |
  | 1798 | Louis Nicolas Vauquelin | In 1797 received samples of crocoite ore from which he produced chromium oxide (CrO3) by mixing crocoite with hydrochloric acid. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious gemstones, such as ruby or emerald. |
  | 1798 | Louis Nicolas Vauquelin | Discovered beryllium in emerald (beryl) when he dissolved the beryl in sodium hydroxide, separating the aluminium hydroxide and beryllium compound from the silicate crystals, and then dissolving the aluminium hydroxide in another alkali solution to separate it from the beryllium. |
  | 1800 | William Nicholson and Johann Ritter | Used electricity to decompose water into hydrogen and oxygen, thereby discovering the process of electrolysis, which led to the discovery of many other elements. |
  | 1800 | Alessandro Volta | Invented the voltaic pile, or "battery", specifically to disprove Galvani's animal electricity theory. |
  | 1801 | Johann Wilhelm Ritter | Discovered ultraviolet light. |
  | 1803 | Thomas Young | Double-slit experiment supports the wave theory of light and demonstrates the effect of interference. |
  | 1806 | Alessandro Volta | Employing a voltaic pile of approximately 250 cells, or couples, decomposed potash and soda, showing that these substances were respectively the oxides of potassium and sodium, which metals previously had been unknown. These experiments were the beginning of electrochemistry. |
  | 1807 | John Dalton | Published his Atomic Theory of Matter. |
  | 1807 | Sir Humphry Davy | First isolates sodium from caustic soda and potassium from caustic potash by the process of electrolysis. |
  | 1808 | Sir Humphry Davy, Joseph Louis Gay-Lussac, and Louis Jacques Thénard | Boron isolated through the reaction of boric acid and potassium. |
  | 1809 | Sir Humphry Davy | First publicly demonstrated the electric arc light. |
  | 1811 | Amedeo Avogadro | Proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules, regardless of the nature of the gas—a key step in the development of the Atomic Theory of Matter. |
  | 1817 | Johan August Arfwedson and Jöns Jakob Berzelius | Arfwedson, then working in the laboratory of Berzelius, detected the presence of a new element while analyzing petalite ore. This element formed compounds similar to those of sodium and potassium, though its carbonate and hydroxide were less soluble in water and more alkaline. Berzelius gave the alkaline material the name "lithos", from the Greek word λιθoς (transliterated as lithos, meaning "stone"), to reflect its discovery in a solid mineral, as opposed to sodium and potassium, which had been discovered in plant tissues. |
  | 1819 | Hans Christian Ørsted | Discovered the deflecting effect of an electric current traversing a wire upon a suspended magnetic needle, thus deducing that magnetism and electricity were somehow related to each other. |
  | 1821 | Augustin-Jean Fresnel | Demonstrated via mathematical methods that polarization could be explained only if light was entirely transverse, with no longitudinal vibration whatsoever. This finding was later very important to Maxwell's equations and to Einstein's Theory of Special Relativity. His use of two plane mirrors of metal, forming with each other an angle of nearly 180°, allowed him to avoid the diffraction effects caused (by the apertures) in the experiment of F. M. Grimaldi on interference. This allowed him to conclusively account for the phenomenon of interference in accordance with the wave theory. With François Arago he studied the laws of the interference of polarized rays. He obtained circularly polarized light by means of a rhombus of glass, known as a Fresnel rhomb, having obtuse angles of 126° and acute angles of 54°. |
  | 1821 | André-Marie Ampère | Announced his celebrated theory of electrodynamics, relating the force one current exerts upon another by way of its electromagnetic effects. |
  | 1821 | Thomas Johann Seebeck | Discovered the thermoelectric effect. |
  | 1827 | Georg Simon Ohm | Discovered the relationship between voltage, current, and resistance, making possible the development of electric circuitry and power transmission. |
  | 1831 | Macedonio Melloni | Used a thermopile to detect infrared radiation. |
  | 1831 | Michael Faraday | Discovered electromagnetic induction, making possible the invention of the electric motor and generator. |
  | 1833 | William Rowan Hamilton | Stated a reformulation of classical mechanics that arose from Lagrangian mechanics, a previous reformulation of classical mechanics introduced by Joseph-Louis Lagrange in 1788, but which can be formulated without recourse to Lagrangian mechanics using symplectic spaces (see Mathematical Formalism). As with Lagrangian mechanics, Hamilton's equations provide a new and equivalent way of looking at classical mechanics. Generally, these equations do not provide a more convenient way of solving a particular problem. Rather, they provide deeper insights into both the general structure of classical mechanics and its connection to quantum mechanics as understood through Hamiltonian mechanics, as well as its connection to other areas of science. |
  | 1833 | Michael Faraday | Announced his important law of electrochemical equivalents, viz.: "The same quantity of electricity—that is, the same electric current—decomposes chemically equivalent quantities of all the bodies which it traverses; hence the weights of elements separated in these electrolytes are to each other as their chemical equivalents." |
  | 1834 | Heinrich Lenz | Applied an extension of the law of conservation of energy to the non-conservative forces in electromagnetic induction to give the direction of the induced electromotive force (emf) and current resulting from electromagnetic induction. The law provides a physical interpretation of the choice of sign in Faraday's law of induction (1831), indicating that the induced emf and the change in flux have opposite signs. |
  | 1834 | Jean-Charles Peltier | Discovered what is now called the Peltier effect: the heating effect of an electric current at the junction of two different metals. |
  | 1838 | Michael Faraday | Using Volta's battery, Farraday discovered "cathode rays" when, during an experiment, he passed current through a rarefied air filled glass tube and noticed a strange light arc starting at the anode (positive electrode) and ending at the cathode (negative electrode). |
  | 1839 | Alexandre Edmond Becquerel | Observed the photoelectric effect via an electrode in a conductive solution exposed to light. |
  | 1852 | Edward Frankland | Initiated the theory of valency by proposing that each element has a specific "combining power", e.g. some elements such as nitrogen tend to combine with three other elements (e.g. $NO_3)$ while others may tend to combine with five (e.g. $PO_5),$ and that each element strives to fulfill its combining power (valency) quota. |
  | 1857 | Heinrich Geissler | Invented the Geissler tube. |
  | 1858 | Julius Plücker | Published the first of his classical researches on the action of magnets on the electric discharge of rarefied gases in Geissler tubes. He found that the discharge caused a fluorescent glow to form on the glass walls of the vacuum tube, and that the glow could be made to shift by applying a magnetic field to the tube. It was later shown by Johann Wilhelm Hittorf that the glow was produced by rays emitted from one of the electrodes (the cathode). |
  | 1859 | Gustav Kirchhoff | Stated the "black body problem", i.e. how does the intensity of the electromagnetic radiation emitted by a black body depend on the frequency of the radiation and the temperature of the body? |
  | 1865 | Johann Josef Loschmidt | Estimated the average diameter of the molecules in air by a method that is equivalent to calculating the number of particles in a given volume of gas. This latter value, the number density of particles in an ideal gas, is now called the Loschmidt constant in his honour, and is approximately proportional to the Avogadro constant. The connection with Loschmidt is the root of the symbol L sometimes used for the Avogadro constant, and German language literature may refer to both constants by the same name, distinguished only by the units of measurement. |
  | 1868 | Norman Lockyer and Edward Frankland | On October 20 observed a yellow line in the solar spectrum, which he named the "D3 Fraunhofer line" because it was near the known D1 and D2 lines of sodium. He correctly concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and Frankland named the element with the Greek word for the Sun, ἥλιος, "helios". |
  | 1869 | Dmitri Mendeleev | Devises the Periodic Table of the Elements. |
  | 1869 | Johann Wilhelm Hittorf | Studied discharge tubes with energy rays extending from a negative electrode, the cathode. These rays, which he discovered but were later called cathode rays by Eugen Goldstein, produced a fluorescence when they hit a tube's glass walls and, when interrupted by a solid object, cast a shadow. |
  | 1869 | William Crookes | Invented the Crookes tube. |
  | 1873 | Willoughby Smith | Discovered the photoelectric effect in metals not in solution (i.e., selenium). |
  | 1873 | James Clerk Maxwell | Published his theory of electromagnetism in which light was determined to be an electromagnetic wave (field) that could be propagated in a vacuum. |
  | 1877 | Ludwig Boltzmann | Suggested that the energy states of a physical system could be discrete. |
  | 1879 | William Crookes | Showed that cathode rays (1838), unlike light rays, can be bent in a magnetic field. |
  | 1885 | Johann Balmer | Discovered that the four visible lines of the hydrogen spectrum could be assigned integers in a series |
  | 1886 | Henri Moissan | Isolated elemental fluorine after almost 74 years of effort by other chemists. |
  | 1886 | Oliver Heaviside | Coined the term "inductance". |
  | 1886 | Eugen Goldstein | Goldstein had undertaken his own investigations of discharge tubes and had named the light emissions studied by others "kathodenstrahlen", or cathode rays. In 1886, he discovered that discharge tubes with a perforated cathode also emit a glow at the cathode end. Goldstein concluded that in addition to the already-known cathode rays (later recognized as electrons) moving from the negatively charged cathode toward the positively charged anode, there is another ray that travels in the opposite direction. Because these latter rays passed through the holes, or channels, in the cathode, Goldstein called them "kanalstrahlen", or canal rays. He determined that canal rays are composed of positive ions whose identity depends on the residual gas inside the tube. It was another of Helmholtz's students, Wilhelm Wien, who later conducted extensive studies of canal rays, and in time this work would become part of the basis for mass spectrometry. |
  | 1887 | Albert A. Michelson and Edward W. Morley | Conducted what is now called the "Michelson–Morley" experiment, in which they disproved the existence of a luminiferous aether and that the speed of light remained constant relative to all inertial frames of reference. The full significance of this discovery was not understood until Albert Einstein published his Theory of Special Relativity. |
  | 1887 | Heinrich Hertz | Discovered the production and reception of electromagnetic (EM) radio waves. His receiver consisted of a coil with a spark gap, where a spark would be seen upon detection of EM waves transmitted from another spark gap source. |
  | 1888 | Johannes Rydberg | Modified the Balmer formula to include the other series of lines, producing the Rydberg formula |
  | 1891 | Alfred Werner | Proposed a theory of affinity and valence in which affinity is an attractive force issuing from the center of the atom which acts uniformly from there towards all parts of the spherical surface of the central atom. |
  | 1892 | Heinrich Hertz | Showed that cathode rays (1838) could pass through thin sheets of gold foil and produce appreciable luminosity on glass behind them. |
  | 1893 | Alfred Werner | Showed that the number of atoms or groups associated with a central atom (the "coordination number") is often 4 or 6; other coordination numbers up to a maximum of 8 were known, but less frequent. |
  | 1893 | Victor Schumann | Discovered the vacuum ultraviolet spectrum. |
  | 1895 | Sir William Ramsay | Isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10% rare earth elements) with mineral acids. |
  | 1895 | Wilhelm Röntgen | Discovered X-rays with the use of a Crookes tube. |
  | 1896 | Henri Becquerel | Discovered "radioactivity" a process in which, due to nuclear disintegration, certain elements or isotopes spontaneously emit one of three types of energetic entities: alpha particles (positive charge), beta particles (negative charge), and gamma particles (neutral charge). |
  | 1897 | J. J. Thomson | Showed that cathode rays (1838) bend under the influence of both an electric field and a magnetic field. To explain this he suggested that cathode rays are negatively charged subatomic electrical particles or "corpuscles" (electrons), stripped from the atom; and in 1904 proposed the "plum pudding model" in which atoms have a positively charged amorphous mass (pudding) as a body embedded with negatively charged electrons (raisins) scattered throughout in the form of non-random rotating rings. Thomson also calculated the mass-to-charge ratio of the electron, paving the way for the precise determination of its electrical charge by Robert Andrews Millikan (1913). |
  | 1900 | Max Planck | To explain black-body radiation (1862), he suggested that electromagnetic energy could only be emitted in quantized form, i.e. the energy could only be a multiple of an elementary unit E = hν, where h is Planck's constant and ν is the frequency of the radiation. |
  | 1901 | Frederick Soddy and Ernest Rutherford | Discovered nuclear transmutation when they found that radioactive thorium was converting itself into radium through a process of nuclear decay. |
  | 1902 | Gilbert N. Lewis | To explain the octet rule (1893), he developed the "cubical atom" theory in which electrons in the form of dots were positioned at the corner of a cube and suggested that single, double, or triple "bonds" result when two atoms are held together by multiple pairs of electrons (one pair for each bond) located between the two atoms (1916). |
  | 1904 | J. J. Thomson | Articulated the plum pudding model of the atom that was later experimentally disproved by Rutherford (1907). |
  | 1904 | Richard Abegg | Noted the pattern that the numerical difference between the maximum positive valence, such as +6 for $H_2SO_4,$ and the maximum negative valence, such as −2 for $H_2S,$ of an element tends to be eight (Abegg's rule). |
  | 1905 | Albert Einstein | Determined the equivalence of matter and energy. |
  | 1905 | Albert Einstein | First to explain the effects of Brownian motion as caused by the kinetic energy (i.e., movement) of atoms, which was subsequently, experimentally verified by Jean Baptiste Perrin, thereby settling the century-long dispute about the validity of John Dalton's atomic theory. |
  | 1905 | Albert Einstein | Published his Special Theory of Relativity. |
  | 1905 | Albert Einstein | Explained the photoelectric effect (1839), i.e. that shining light on certain materials can function to eject electrons from the material, he postulated as based on Planck's quantum hypothesis (1900), that light itself consists of individual quantum particles (photons). |
  | 1907 | Ernest Rutherford | To test the plum pudding model (1904), he fired positively charged alpha particles at gold foil and noticed that some bounced back, thus showing that atoms have a small-sized positively charged atomic nucleus at its center. |
  | 1909 | Geoffrey Ingram Taylor | Demonstrated that interference patterns of light were generated even when the light energy introduced consisted of only one photon. This discovery of the wave–particle duality of matter and energy was fundamental to the later development of quantum field theory. |
  | 1909 and 1916 | Albert Einstein | Showed that, if Planck's law of black-body radiation is accepted, the energy quanta must also carry momentum p = h / λ, making them fully fledged particles, albeit with no "rest mass". |
  | 1911 | Lise Meitner and Otto Hahn | Performed an experiment that showed that the energies of electrons emitted by beta decay had a continuous rather than discrete spectrum. This was in apparent contradiction to the law of conservation of energy, as it appeared that energy was lost in the beta decay process. A second problem was that the spin of the nitrogen-14 atom was 1, in contradiction to the Rutherford prediction of 1⁄2. These anomalies were later explained by the discoveries of the neutrino and the neutron. |
  | 1912 | Henri Poincaré | Published an influential mathematical argument in support of the essential nature of energy quanta. |
  | 1913 | Robert Andrews Millikan | Publishes the results of his "oil drop" experiment, in which he precisely determines the electric charge of the electron. Determination of the fundamental unit of electric charge made it possible to calculate the Avogadro constant (which is the number of atoms or molecules in one mole of any substance) and thereby to determine the atomic weight of the atoms of each element. |
  | 1913 | Niels Bohr | To explain the Rydberg formula (1888), which correctly modeled the light emission spectra of atomic hydrogen, Bohr hypothesized that negatively charged electrons revolve around a positively charged nucleus at certain fixed "quantum" distances and that each of these "spherical orbits" has a specific energy associated with it such that electron movements between orbits requires "quantum" emissions or absorptions of energy. |
  | 1911 | Ștefan Procopiu | Performed experiments in which he determined the correct value of electron's magnetic dipole moment, $μ_B$ = $9.27×10^{−21}$ $erg⋅Oe^{−1}.$ |
  | 1916 | Gilbert N. Lewis | Developed the Lewis dot structures that ultimately led to a complete understanding of the electronic covalent bond that forms the fundamental basis for our understanding of chemistry at the atomic level; he also coined the term "photon" in 1926. |
  | 1916 | Arnold Sommerfeld | To account for the Zeeman effect (1896), i.e. that atomic absorption or emission spectral lines change when the light is first shone through a magnetic field, he suggested there might be "elliptical orbits" in atoms in addition to spherical orbits. |
  | 1918 | Ernest Rutherford | Noticed that, when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle, which he decided must be the protons hypothesized by Eugen Goldstein (1886). |
  | 1919 | Irving Langmuir | Building on the work of Lewis (1916), he coined the term "covalence" and postulated that coordinate covalent bonds occur when the electrons of a pair come from the same atom, thus explaining the fundamental nature of chemical bonding and molecular chemistry. |
  | 1922 | Arthur Compton | Found that X-ray wavelengths increase due to scattering of the radiant energy by "free electrons". The scattered quanta have less energy than the quanta of the original ray. This discovery, now known as the "Compton effect" or "Compton scattering", demonstrates the "particle" concept of electromagnetic radiation. |
  | 1922 | Otto Stern and Walther Gerlach | Stern–Gerlach experiment detects discrete values of angular momentum for atoms in the ground state passing through an inhomogeneous magnetic field leading to the discovery of the spin of the electron. |
  | 1923 | Louis de Broglie | Postulated that electrons in motion are associated with waves the lengths of which are given by the Planck constant h divided by the momentum mv = p of the electron: λ = h / mv = h / p. |
  | 1924 | Satyendra Nath Bose | His work on quantum mechanics provides the foundation for Bose–Einstein statistics, the theory of the Bose–Einstein condensate, and the discovery of the boson. |
  | 1925 | Friedrich Hund | Outlined the "rule of maximum multiplicity" which states that, when electrons are added successively to an atom, as many levels or orbits are singly occupied as possible before any pairing of electrons with opposite spin occurs, and also made the distinction that the inner electrons in molecules remain in their atomic orbitals and only the valence electrons need occupy the molecular orbitals involving both nuclei of the atoms participating in a covalent bond. |
  | 1925 | Werner Heisenberg | Developed the matrix mechanics formulation of quantum mechanics. |
  | 1925 | Wolfgang Pauli | Outlined the "Pauli exclusion principle" which states that no two identical fermions may occupy the same quantum state simultaneously. |
  | 1926 | Gilbert Lewis | Coined the term photon, which he derived from the Greek word for light, φως (transliterated phôs). |
  | 1926 | Erwin Schrödinger | Used De Broglie's electron wave postulate (1924) to develop a "wave equation" that represents mathematically the distribution of a charge of an electron distributed through space, being spherically symmetric or prominent in certain directions, i.e. directed valence bonds, which gave the correct values for spectral lines of the hydrogen atom. |
  | 1927 | Charles Drummond Ellis (along with James Chadwick and colleagues) | Finally established clearly that the beta decay spectrum is really continuous, ending all controversies. |
  | 1927 | Walter Heitler | Used Schrödinger's wave equation (1926) to show how two hydrogen atom wavefunctions join together, with plus, minus, and exchange terms, to form a covalent bond. |
  | 1927 | Robert Mulliken | In 1927 Mulliken worked, in coordination with Hund, to develop a molecular orbital theory where electrons are assigned to states that extend over an entire molecule and in 1932 introduced many new molecular orbital terminologies, such as σ bond, π bond, and δ bond. |
  | 1928 | Paul Dirac | In the Dirac equations, Paul Dirac integrated the principle of special relativity with quantum electrodynamics and thereby hypothesized the existence of the positron. |
  | 1928 | Linus Pauling | Outlined the nature of the chemical bond in which he used Heitler's quantum mechanical covalent bond model (1927) to describe the quantum mechanical basis for all types of molecular structure and bonding, thereby suggesting that different types of bonds in molecules can become equalized by the rapid shifting of electrons, a process called "resonance" (1931), such that resonance hybrids contain contributions from the different possible electronic configurations. |
  | 1929 | John Lennard-Jones | Introduced the linear combination of atomic orbitals approximation for the calculation of molecular orbitals. |
  | 1930 | Wolfgang Pauli | In a famous letter written by him, Pauli suggested that, in addition to electrons and protons, atoms also contained an extremely light neutral particle which he called the "neutron". He suggested that this "neutron" was also emitted during beta decay and had simply not yet been observed. Later it was determined that this particle was actually the almost massless neutrino. |
  | 1931 | Walther Bothe and Herbert Becker | Found that, if the very energetic alpha particles emitted from polonium fell on certain light elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced. At first this radiation was thought to be gamma radiation, although it was more penetrating than any gamma rays known, and the details of experimental results were very difficult to interpret on this basis. Some scientists began to hypothesize the possible existence of another fundamental, atomic particle. |
  | 1932 | Irène Joliot-Curie and Frédéric Joliot | Showed that if the unknown radiation generated by alpha particles fell on paraffin or any other hydrogen-containing compound, it ejected protons of very high energy. This was not in itself inconsistent with the proposed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a hypothesis. |
  | 1932 | James Chadwick | Performed a series of experiments showing that the gamma ray hypothesis for the unknown radiation produced by alpha particles was untenable, and that the new particles must be the neutrons hypothesized by Enrico Fermi. Chadwick suggested that, in fact, the new radiation consisted of uncharged particles of approximately the same mass as the proton, and he performed a series of experiments verifying his suggestion. |
  | 1932 | Werner Heisenberg | Applied perturbation theory to the two-electron problem and showed how resonance arising from electron exchange could explain exchange forces. |
  | 1932 | Mark Oliphant | Building upon the nuclear transmutation experiments of Ernest Rutherford done a few years earlier, fusion of light nuclei (hydrogen isotopes) was first observed by Oliphant in 1932. The steps of the main cycle of nuclear fusion in stars were subsequently worked out by Hans Bethe throughout the remainder of that decade. |
  | 1932 | Carl D. Anderson | Experimentally proves the existence of the positron. |
  | 1933 | Leó Szilárd | First theorized the concept of a nuclear chain reaction. He filed a patent for his idea of a simple nuclear reactor the following year. |
  | 1934 | Enrico Fermi | Studies the effects of bombarding uranium isotopes with neutrons. |
  | 1934 | N. N. Semyonov | Develops the total quantitative chain chemical reaction theory. The idea of the chain reaction, developed by Semyonov, is the basis of various high technologies using the incineration of gas mixtures. The idea was also used for the description of the nuclear reaction. |
  | 1935 | Hideki Yukawa | Published his hypothesis of the Yukawa Potential and predicted the existence of the pion, stating that such a potential arises from the exchange of a massive scalar field, such as would be found in the field of the pion. Prior to Yukawa's paper, it was believed that the scalar fields of the fundamental forces necessitated massless particles. |
  | 1936 | Carl D. Anderson | Discovered muons while studying cosmic radiation. |
  | 1937 | Carl Anderson | Experimentally proved the existence of the pion. |
  | 1938 | Charles Coulson | Made the first accurate calculation of a molecular orbital wavefunction with the hydrogen molecule. |
  | 1938 | Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Robert Frisch | Hahn and Strassmann sent a manuscript to Naturwissenschaften reporting they had detected the element barium after bombarding uranium with neutrons. Simultaneously, they communicated these results to Meitner. Meitner, and her nephew Frisch, correctly interpreted these results as being nuclear fission. Frisch confirmed this experimentally on 13 January 1939. |
  | 1939 | Leó Szilárd and Enrico Fermi | Discovered neutron multiplication in uranium, proving that a chain reaction was indeed possible. |
  | 1942 | Kan-Chang Wang | First proposed the use of beta capture to experimentally detect neutrinos. |
  | 1942 | Enrico Fermi | Created the first artificial self-sustaining nuclear chain reaction, called Chicago Pile-1 (CP-1), in a racquets court below the bleachers of Stagg Field at the University of Chicago on December 2, 1942. |
  | 1945 | Manhattan Project | First nuclear fission explosion. |
  | 1947 | G. D. Rochester and C. C. Butler | Published two cloud chamber photographs of cosmic ray-induced events, one showing what appeared to be a neutral particle decaying into two charged pions, and one which appeared to be a charged particle decaying into a charged pion and something neutral. The estimated mass of the new particles was very rough, about half a proton's mass. More examples of these "V-particles" were slow in coming, and they were soon given the name kaons. |
  | 1948 | Sin-Itiro Tomonaga and Julian Schwinger | Independently introduced perturbative renormalization as a method of correcting the original Lagrangian of a quantum field theory so as to eliminate an infinite series of counterterms that would otherwise result. |
  | 1951 | Clemens C. J. Roothaan and George G. Hall | Derived the Roothaan–Hall equations, putting rigorous molecular orbital methods on a firm basis. |
  | 1952 | Manhattan Project | First explosion of a thermonuclear bomb. |
  | 1952 | Herbert S. Gutowsky | Physical chemistry of solids investigated by NMR: structure, spectroscopy and relaxation |
  | 1952 | Charles P. Slichter | Introduced Chemical shifts, NQR in solids, the first NOE experiments |
  | 1952 | Albert W. Overhauser | First investigation of dynamic polarization in solids/NOE-Nuclear Overhauser Effect |
  | 1953 | Charles H. Townes (collaborating with James P. Gordon, and Herbert J. Zeiger) | Built and reported the first ammonia maser; received a Nobel prize in 1964 for his experimental success in producing coherent radiation by atoms and molecules. |
  | *1958—1959 | Edward Raymond Andrew, A. Bradbury, and R. G. Eades; and independently, I. J. Lowe | Described the technique of magic angle spinning. |
  | 1956 | P. Kuroda | Predicted that self-sustaining nuclear chain reactions should occur in natural uranium deposits. |
  | 1956 | Clyde L. Cowan and Frederick Reines | Experimentally proved the existence of the neutrino. |
  | 1957 | William Alfred Fowler, Margaret Burbidge, Geoffrey Burbidge, and Fred Hoyle | In their 1957 paper Synthesis of the Elements in Stars, they explained how the abundances of essentially all but the lightest chemical elements could be explained by the process of nucleosynthesis in stars. |
  | 1961 | Claus Jönsson | Performed Young's double-slit experiment (1909) for the first time with particles other than photons by using electrons and with similar results, confirming that massive particles also behaved according to the wave–particle duality that is a fundamental principle of quantum field theory. |
  | 1974 | Pier Giorgio Merli | Performed Young's double-slit experiment (1909) using a single electron with similar results, confirming the existence of quantum fields for massive particles. |
  | 1995 | Eric Cornell, Carl Wieman and Wolfgang Ketterle | The first "pure" Bose–Einstein condensate was created by Eric Cornell, Carl Wieman, and co-workers at JILA. They did this by cooling a dilute vapor consisting of approximately two thousand rubidium-87 atoms to below 170 nK using a combination of laser cooling and magnetic evaporative cooling. About four months later, an independent effort led by Wolfgang Ketterle at MIT created a condensate made of sodium-23. Ketterle's condensate had about a hundred times more atoms, allowing him to obtain several important results such as the observation of quantum mechanical interference between two different condensates. |
  