- 440 BC Democritus speculates about fundamental indivisible particles---calls them "atoms"
- 1766 Henry Cavendish discovers and studies hydrogen
- 1778 Carl Scheele and Antoine Lavoisier discover that air is composed mostly of nitrogen and oxygen
- 1781 Joseph Priestley creates water by igniting hydrogen and oxygen
- 1800 William Nicholson and Anthony Carlisle use electrolysis to separate water into hydrogen and oxygen
- 1803 John Dalton introduces atomic ideas into chemistry and states that matter is composed of atoms of different weights
- 1811 Amedeo Avogadro claims that equal volumes of gases should contain equal numbers of molecules
- 1832 Michael Faraday states his laws of electrolysis
- 1871 Dmitri Ivanovich Mendeleev systematically examines the periodic table and predicts the existence of gallium, scandium, and germanium
- 1873 Johannes van der Waals introduces the idea of weak attractive forces between molecules
- 1885 Johann Balmer finds a mathematical expression for observed hydrogen line wavelengths
- 1887 Heinrich Hertz discovers the photoelectric effect
- 1894 Lord Rayleigh and William Ramsay discover argon by spectroscopically analyzing the gas left over after nitrogen and oxygen are removed from air
- 1895 William Ramsay discovers terrestrial helium by spectroscopically analyzing gas produced by decaying uranium
- 1896 Antoine Becquerel discovers the radioactivity of uranium
- 1896 Pieter Zeeman studies the splitting of sodium D lines when sodium is held in a flame between strong magnetic poles
- 1897 Joseph Thomson discovers the electron
- 1898 William Ramsay and Morris Travers discover neon, krypton, and xenon
- 1898 Marie Curie and Pierre Curie isolate and study radium and polonium
- 1899 Ernest Rutherford discovers that uranium radiation is composed of positively charged alpha particles and negatively charged beta particles
- 1900 Paul Villard discovers gamma-rays while studying uranium decay
- 1900 Johannes Rydberg refines the expression for observed hydrogen line wavelengths
- 1900 Max Planck states his quantum hypothesis and blackbody radiation law
- 1902 Philipp Lenard observes that maximum photoelectron energies are independent of illuminating intensity but depend on frequency
- 1902 Theodor Svedberg suggests that fluctuations in molecular bombardment cause the Brownian motion
- 1905 Albert Einstein explains the photoelectric effect
- 1906 Charles Barkla discovers that each element has a characteristic X-ray and that the degree of penetration of these X-rays is related to the atomic weight of the element
- 1909 Hans Geiger and Ernest Marsden discover large angle deflections of alpha particles by thin metal foils
- 1909 Ernest Rutherford and Thomas Royds demonstrate that alpha particles are doubly ionized helium atoms
- 1911 Ernest Rutherford explains the Geiger-Marsden experiment by invoking a nuclear atom model and derives the Rutherford cross section
- 1912 Max von Laue suggests using lattice solids to diffract X-rays
- 1912 Walter Friedrich and Paul Knipping diffract X-rays in zinc blende
- 1913 William Bragg and Lawrence Bragg work out the Bragg condition for strong X-ray reflection
- 1913 Henry Moseley shows that nuclear charge is the real basis for numbering the elements
- 1913 Niels Bohr presents his quantum model of the atom
- 1913 Robert Millikan measures the fundamental unit of electric charge
- 1913 Johannes Stark demonstrates that strong electric fields will split the Balmer spectral line series of hydrogen
- 1914 James Franck and Gustav Hertz observe atomic excitation
- 1914 Ernest Rutherford suggests that the positively charged atomic nucleus contains protons
- 1915 Arnold Sommerfeld develops a modified Bohr atomic model with elliptic orbits to explain relativistic fine structure
- 1916 Gilbert Lewis and Irving Langmuir formulate an electron shell model of chemical bonding
- 1917 Albert Einstein introduces the idea of stimulated radiation emission
- 1921 Alfred Lande introduces the Lande g-factor
- 1922 Arthur Compton studies X-ray photon scattering by electrons
- 1922 Otto Stern and Walter Gerlach show "space quantization"
- 1923 Louis de Broglie suggests that electrons may have wavelike properties
- 1924 Wolfgang Pauli states the quantum exclusion principle
- 1924 John Lennard-Jones proposes a semiempirical interatomic force law
- 1924 Satyendra Bose and Albert Einstein introduce Bose-Einstein statistics
- 1925 George Uhlenbeck and Samuel Goudsmit postulate electron spin
- 1925 Pierre Auger discovers the Auger autoionization process
- 1925 Werner Heisenberg, Max Born, and Pascual Jordan formulate quantum matrix mechanics
- 1926 Erwin Schrödinger states his nonrelativistic quantum wave equation and formulates quantum wave mechanics
- 1926 Erwin Schrödinger proves that the wave and matrix formulations of quantum theory are mathematically equivalent
- 1926 Oskar Klein and Walter Gordon state their relativistic quantum wave equation
- 1926 Enrico Fermi discovers the spin-statistics connection
- 1926 Paul Dirac introduces Fermi-Dirac statistics
- 1927 Clinton Davission, Lester Germer, and George Thomson confirm the wavelike nature of electrons
- 1927 Werner Heisenberg states the quantum uncertainty principle
- 1927 Max Born interprets the probabilistic nature of wavefunctions
- 1928 Chandrasekhara Raman studies optical photon scattering by electrons
- 1928 Paul Dirac states his relativistic electron quantum wave equation
- 1928 Charles G. Darwin and Walter Gordon solve the Dirac equation for a Coulomb potential
- 1929 Oskar Klein discovers the Klein paradox
- 1929 Oskar Klein and Y. Nishina derive the Klein-Nishina cross section for high energy photon scattering by electrons
- 1929 N.F. Mott derives the Mott cross section for the Coulomb scattering of relativistic electrons
- 1930 Paul Dirac introduces electron hole theory
- 1930 Erwin Schrödinger predicts the zitterbewegung motion
- 1930 Fritz London explains van der Waals forces as due to the interacting fluctuating dipole moments between molecules
- 1931 John Lennard-Jones proposes the Lennard-Jones interatomic potential
- 1931 Irene Joliot-Curie and Frédéric Joliot observe but misinterpret neutron scattering in paraffin
- 1931 Wolfgang Pauli puts forth the neutrino hypothesis to explain the apparent violation of energy conservation in beta decay
- 1931 Linus Pauling discovers resonance bonding and uses it to explain the high stability of symmetric planar molecules
- 1931 Paul Dirac shows that charge conservation can be explained if magnetic monopoles exist
- 1931 Harold Urey discovers deuterium using evaporation concentration techniques and spectroscopy
- 1932 John Cockcroft and Thomas Walton split lithium and boron nuclei using proton bombardment
- 1932 James Chadwick discovers the neutron
- 1932 Werner Heisenberg presents the proton-neutron model of the nucleus and uses it to explain isotopes
- 1932 Carl Anderson discovers the positron
- 1933 Max Delbruck suggests that quantum effects will cause photons to be scattered by an external electric field
- 1934 Irene Joliot-Curie and Frédéric Joliot bombard aluminum atoms with alpha particles to create artificially radioactive phosphorus-30
- 1934 Leo Szilard realizes that nuclear chain reactions may be possible
- 1934 Enrico Fermi formulates his theory of beta decay
- 1934 Lev Davidovich Landau tells Edward Teller that nonlinear molecules may have vibrational modes which remove the degeneracy of an orbitally degenerate state
- 1934 Enrico Fermi suggests bombarding uranium atoms with neutrons to make a 93 proton element
- 1934 Pavel Alekseyevich Cherenkov reports that light is emitted by relativistic particles traveling in a nonscintillating liquid
- 1935 Hideki Yukawa presents a theory of strong interactionss and predicts mesons
- 1935 Albert Einstein, Boris Podolsky, and Nathan Rosen put forth the EPR paradox
- 1935 Niels Bohr presents his analysis of the EPR paradox
- 1936 Eugene Wigner develops the theory of neutron absorption by atomic nuclei
- 1936 Hans Jahn and Edward Teller present their systematic study of the symmetry types for which the Jahn-Teller effect is expected
- 1937 H. Hellmann finds the Hellmann-Feynman theorem
- 1937 Seth Neddermeyer, Carl Anderson, J.C. Street, and E.C. Stevenson discover muons using cloud chamber measurements of cosmic rays
- 1939 Richard Feynman finds the Hellmann-Feynman theorem
- 1939 Otto Hahn and Fritz Strassman bombard uranium salts with thermal neutrons and discover barium among the reaction products
- 1939 Lise Meitner and Otto Frisch determine that nuclear fission is taking place in the Hahn-Strassman experiments
- 1942 Enrico Fermi makes the first controlled nuclear chain reaction
- 1942 Ernst Stuckelberg introduces the propagator to positron theory and interprets positrons as negative energy electrons moving backwards through spacetime
- 1943 Sin-Itiro Tomonaga publishes his paper on the basic physical principles of quantum electrodynamics
- 1947 Willis Lamb and Robert Retheford measure the Lamb-Retheford shift
- 1947 Cecil Powell, C.M.G. Lattes, and G.P.S. Occhialini discover the pi-meson by studying cosmic ray tracks
- 1947 Richard Feynman presents his propagator approach to quantum electrodynamics
- 1948 Hendrik Casimir predicts a rudimentary attractive Casimir force on a parallel plate capacitor
- 1951 Martin Deutsch discovers positronium
- 1952 David Bohm propose his interpretation of quantum mechanics
- 1953 R. Wilson observes Delbruck scattering of 1.33 MeV gamma-rays by the electric fields of lead nuclei
- 1954 Chen Ning Yang and Robert Mills investigate a theory of hadronic isospin by demanding local gauge invariance under isotopic spin space rotations---first non-Abelian gauge theory
- 1955 Owen Chamberlain, Emilio Segre, Clyde Wiegand, and Thomas Ypsilantis discover the antiproton
- 1956 Frederick Reines and Clyde Cowan detect antineutrinos
- 1956 Chen Ning Yang and Tsung Lee propose parity violation by the weak nuclear force
- 1956 Chien Shiung Wu discovers parity violation by the weak force in decaying cobalt
- 1957 Gerhart Luders proves the CPT theorem
- 1957 Richard Feynman, Murray Gell-Mann, Robert Marshak, and Ennackel Sudarshan propose a variational approximation (VA) Lagrangian for weak interactions
- 1958 Marcus Sparnaay experimentally confirms the Casimir effect
- 1959 Yakir Aharonov and David Bohm predict the Aharonov-Bohm effect
- 1960 R.G. Chambers experimentally confirms the Aharonov-Bohm effect
- 1961 Murray Gell-Mann and Yuval Ne'eman discover the Eightfold Way patterns---SU(3) group
- 1961 Jeffery Goldstone considers the breaking of global phase symmetry
- 1962 Leon Lederman shows that the electron neutrino is distinct from the muon neutrino
- 1963 Murray Gell-Mann and George Zweig propose the quark/aces model
- 1964 Peter Higgs considers the breaking of local phase symmetry
- 1964 John Stewart Bell shows that all local hidden variable theories must satisfy Bell's inequality
- 1964 Val Fitch and James Cronin observe CP violation by the weak force in the decay of K mesons
- 1967 Steven Weinberg puts forth his electroweak model of leptons
- 1969 J.C. Clauser, M. Horne, A. Shimony, and R. Holt propose a polarization correlation test of Bell's inequality
- 1970 Sheldon Glashow, John Iliopoulos, and Luciano Maiani propose the charm quark
- 1971 Gerard 't Hooft shows that the Glashow-Salam-Weinberg electroweak model can be renormalized
- 1972 S. Freedman and J.C. Clauser perform the first polarization correlation test of Bell's inequality
- 1973 David Politzer proposes the asymptotic freedom of quarks
- 1974 Burton Richter and Samuel Ting discover the psi meson implying the existence of the charm quark
- 1975 Martin Perl discovers the tauon
- 1977 S.W. Herb finds the upsilon resonance implying the existence of the beauty quark
- 1982 A. Aspect, J. Dalibard, and G. Roger perform a polarization correlation test of Bell's inequality that rules out conspiratorial polarizer communication
- 1983 Carlo Rubbia, Simon van der Meer, and the CERN UA-1 collaboration find the W and Z intermediate vector bosons
- 1989 The Z intermediate vector boson resonance width indicates three quark-lepton generations