1789: Antoine Lavoisier noticed that the mass of the substances present after a chemical reaction is the same as the reactants prior to the reaction. This idea became known as "The Law of Conservation Mass."

1808: John Dalton took this law, some experiments performed by Joseph Proust, and some of his own experiments and devised an atomic theory which stated that: (a) each element is made up of tiny particles known as atoms; (b) atoms of a specific element are identified; (c) chemical compounds form when atoms combine with each other; and (d) chemical reactions cause a reorganization of the atom.

1895: Sir J.J. Thomson discovers the electron, the extremely light, negatively charge particles orbiting inside the atom which give it is chemical properties.

1900: Max Planck discovers that heat energy is not continuously variable, as classical physics assumes. There is the smallest common coin in the currency, the quantum, and all transactions are in multiples of it.

1905: Einstein realizes that light has to be understood not only as waves, but as quantum particles, later known as photons.

1910: Ernest Rutherford shows that the electrons orbit around a tiny nucleus in which almost the entire mass of the atom is concentrated.

1913: Niels Bohr realizes that quantum theory applies to matter itself. The orbits of the electrons around the nucleus are limited to several separate whole number possibilities, so that the atom can exist only in a number of distinct and definite states.
(The incomplete, so-called "old quantum theory.")

1924: French Prince Louis-Victor Pierre Raymond deBroglie suggests that, just as radiation can be treated as particles, so the particles of matter can be treated as a wave formation.

1925: Werner Heisenberg abandons electron orbits as unobservable. Max Born finds instead a mathematical formulation in terms of matrices based on the effects they produce upon the absorption and emission of light.

1926: Erwin Schrodinger finds the mathematical equation for the wave interpretation and proves that wave and matrix mechanics are mathematically equivalent.

1927: Werner Heisenberg demonstrates that all statements about the movement of a particle are governed by the uncertainty relationship: the more accurately you know its position, the less accurately you know its velocity and vice versa.

1928: Niels Bohr relates Heisenberg's particle theory and Schrodinger's wave theory through the complementarity principle, according to which the behavior of an electron can be understood completely only by descriptions in both wave and particle form. Uncertainty plus complementarity become established as the pillars of the Copenhagen interpretation of quantum mechanics.

1932: James Chadwick discovers the neutron, a particle that can be used to explore the nucleus because it carries no electrical charge and can penetrate it undeflected.

Werner Heisenberg opens the new era of nuclear physics by using neutron theory to apply quantum mechanics to the structure of the nucleus.

1937: Niels Bohr explains the properties of a nucleus by analogy with a drop of liquid.

1939: Otto Hahn and Dr. F. Strassmann in Berlin identify the substance produced by Fermi's bombardment as barium that has only about half the atomic weight of uranium.

Lise Meitner and Otto Frisch in Sweden apply Bohr's liquid drop model to the uranium nucleus and realize that it has turned into barium (and krypton) under bombardment by splitting into two parts with the release of huge quantums of energy.

Bohr and John Wheeler at Princeton realize that fission also produces free neutrons.

Frederic Joliot in Paris and Enrico Fermi in New York demonstrate the release of two or more free neutrons with each fission which proves the possibility of a chain reaction.

World War II begins and Germany at once commences research into the military possibilities of fission.

Albert Einstein, urged by Leo Szilard, writes a letter to President Roosevelt about the dangers the United States would incur if Germany developed an atomic weapon first.

1940: Otto Frisch and Rudolf Peierls in Birmingham calculate wrongly the minimum amount of uranium needed to sustain an effective chain reaction.

1942: The Allies' atomic bomb program, known as The Manhattan Project, begins.

Fermi in Chicago achieves the first self-sustaining chain reaction in a prototype reactor.

1945: The Allies' advance into Germany presumably halts the German atomic program.

The atomic bomb is successfully tested in the United States in July and, in the following month, is used on Hiroshima and Nagasaki, Japan.