page_title: Timeline of quantum mechanics
section_title: 1920–1929
hierarchical_section_title: Timeline of quantum mechanics / 20th century / 1920–1929
caption_attribution_description: Deutsch: Gedenktafel für Otto Stern und Walther Gerlach am Physikalischen Verein in FFM in der Robert Mayerstraße, gegenüber dem AFE Turm. Grund:Der hier stattgefundene Stern-Gerlach-Versuch English: A plaque at the Frankfurt institute commemorating the experiment Español: Placa commemorativa del experimento de Stern-Gerlach situada en el instituto de física FFM.
context_page_description: The timeline of quantum mechanics is a list of key events in the history of quantum mechanics, quantum field theories and quantum chemistry.
context_section_description: 1920 - Hendrik Kramers uses Bohr–Sommerfeld quantization to derive formulas for intensities of spectral transitions of the Stark effect. Kramers also includes the effect of fine structure, including corrections for relativistic kinetic energy and coupling between electron spin and orbit.
  1921–1922 – Frederick Soddy receives the Nobel Prize for 1921 in Chemistry one year later, in 1922, "for his contributions to our knowledge of the chemistry of radioactive substances, and his investigations into the origin and nature of isotopes"; he writes in his Nobel Lecture of 1922: "The interpretation of radioactivity which was published in 1903 by Sir Ernest Rutherford and myself ascribed the phenomena to the spontaneous disintegration of the atoms of the radio-element, whereby a part of the original atom was violently ejected as a radiant particle, and the remainder formed a totally new kind of atom with a distinct chemical and physical character."
  1922 – Arthur Compton finds 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, known as the Compton effect or Compton scattering, demonstrates the particle concept of electromagnetic radiation.
  1922 – Otto Stern and Walther Gerlach perform the Stern–Gerlach experiment, which 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.
  1922 – Bohr updates his model of the atom to better explain the properties of the periodic table by assuming that certain numbers of electrons (for example 2, 8 and 18) corresponded to stable "closed shells", presaging orbital theory.
  1923 – Pierre Auger discovers the Auger effect, where filling the inner-shell vacancy of an atom is accompanied by the emission of an electron from the same atom.
  1923 – Louis de Broglie extends wave–particle duality to particles, postulating that electrons in motion are associated with waves. He predicts that the wavelengths are given by Planck's constant h divided by the momentum of the mv = p of the electron: λ = h / mv = h / p.
  1923 – Gilbert N. Lewis creates the theory of Lewis acids and bases based on the properties of electrons in molecules, defining an acid as accepting an electron lone pair from a base.
  1924 – Satyendra Nath Bose explains Planck's law using a new statistical law that governs bosons, and Einstein generalizes it to predict Bose–Einstein condensate. The theory becomes known as Bose–Einstein statistics.
  1924 – Wolfgang Pauli outlines the "Pauli exclusion principle" which states that no two identical fermions may occupy the same quantum state simultaneously, a fact that explains many features of the periodic table.
  1925 – George Uhlenbeck and Samuel Goudsmit postulate the existence of electron spin.
  1925 – Friedrich Hund outlines Hund's 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 made the distinction that the inner electrons in molecules remained in atomic orbitals and only the valence electrons needed to be in molecular orbitals involving both nuclei.
  1925 – Werner Heisenberg, Max Born, and Pascual Jordan develop the matrix mechanics formulation of Quantum Mechanics.
  1926 – Lewis coins the term photon in a letter to the scientific journal Nature, which he derives from the Greek word for light, φως (transliterated phôs).
  1926 – Oskar Klein and Walter Gordon state their relativistic quantum wave equation, later called the Klein–Gordon equation.
  1926 – Enrico Fermi discovers the spin-statistics theorem connection.
  1926 – Paul Dirac introduces Fermi–Dirac statistics.
  1926 – Erwin Schrödinger uses 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 
caption: A plaque at the University of Frankfurt commemorating the Stern–Gerlach experiment.
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