The Age of Discovery

500 B.C.
Leucippus, born ca. 500 BCE, and his pupil, Democritus, born ca. 460 BCE, are credited with postulating the theory of Atoms and Void.

Isaac Newton proposes a mechanical universe with small solid masses in motion

J. Plucker builds one of the first gas discharge tubes ("cathode ray tube").

James Clerk Maxwell proposes that the void is filled with electric and magnetic fields.

G. J. Stoney proposes that electricity is made of discrete negative particles he calls "electrons".

Sir William Crookes discovers properties of cathode rays such as, travelling in straight lines from the cathode; causing glass to fluoresce; imparting a negativeve charge to objects they strike; being deflected by electric fields and magnets to suggest a negative charge; causing pinwheels in their path to spin indicating they have mass.

E. Goldstein uses a CRT to study "canal rays" which have electrical and magnetic properties opposite of an electron.

Wilhelm Roentgen of Germany discovers x-rays. Studying electrical discharge through rarified gas, he finds that invisible rays came from the positive electrode. They would darken a photographic plate through an opaque wrapping.

Antoine Henri Becquerel of France discovers natural radioactivity when the invisible rays from uranium ore darken a photographic plate.

Using a CRT Joseph J. Thomson of Great Britain measures the ratio of charge to mass of electrons, by the deflection of cathode rays in electric and magnetic fields. He determines the charge to mass ratio (e/m) of an electron =1.759 x 108 coulombs/gram.    

Pierre and Marie Curie of France discover radium and polonium, the elements that constitute most of the radioactivity in uranium ore.

Ernest Rutherford of New Zealand distinguishes two kinds of rays from radium and its products. Some are stopped by a thin (20 micron) aluminum foil. These he names alpha rays; the more penetrating rays he calls beta rays.

Max Planck of Germany develops quantum theory, which deals with matter and energy on the subatomic level.

Frederick Soddy of Great Britain observes spontaneous disintegration of radioactive elements into variants he calls "isotopes" or totally new elements. He also discovers "half-life," and makes initial calculations on energy released during decay.

Max Planck publishes Laws of Radiation.

Ernest Rutherford and Frederick Soddy publish theory of radioactive decay: The atoms of a radioactive element emit charged particles (alpha or beta) and in doing so change into atoms of a different element.

Antoine Henri Becquerel shares Nobel Prize for Physics with Pierre and Marie Curie for 1898 discovery of natural radioactivity.
Nagaoka postulates a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.

Ernest Rutherford discovers that alpha rays are heavy positively charged particles. In 1908 he is awarded a Nobel Prize for his work.

Albert Einstein of Germany publishes theory of relativity regarding convertibility of matter and energy (E=mc2).

Robert Andrews Millikan of the United States, accurately measures the electric charge (e=1.602 x 10-19 coulomb) and the mass (m = 9.11 x 10-28 gram) of an electron in his famous oil drop experiment.

Ernest Rutherford develops theory of the structure of atoms.

Marie Curie receives a second Nobel Prize, this time for Chemistry, for the isolation of radium and polonium and for her investigation of their chemical properties. [see 1902]

Frederick Soddy proposes a theory of isotopes of elements. Kasimir Fajans, in Germany, independently and prior to Soddy, explains radioactive decay and the isotopes of elements.

Niels Bohr of Denmark publishes theory of atomic structure combining nuclear theory with quantum theory.

Using x-ray tubes, H. G. J. Moseley determines charges on the nuclei of most atoms. Accordingly, the periodic table was reorganizaed to be based upon atomic number instead of atomic mass.

H.G. Wells publishes novel, The World Set Free, in which an atomic war in 1956 destroys the major cities of the world. (See Related Links on top right for on-line version of the book.)

Albert Einstein publishes general theory of relativity.

Ernest Rutherford studies visible scintillations when a zinc sulfide film is struck by alpha particles. Occasional scintillations are brighter when the alpha particles pass through rarified nitrogen. Rutherford conjectures that it happens when an atom of nitrogen is changed into oxygen by an alpha particle, the first observation of artificial transmutation of an element.
Francis Aston discovers existence of isotopes through the use of a mass spectrograph.

Ernest Rutherford and James Chadwick of Great Britain achieve transmutation of all elements except carbon, oxygen, lithium and beryllium.

P. M. S. Blackett of Great Britain begins experiments on transmutation of elements.

Supporting Einstein 's theory, de Broglie discovers that electrons have a dual nature--similar to both particles and waves.

Ernest O. Lawrence of the United States conceives idea for the first cyclotron (atom smasher).

John D. Crockcroft and E. T. S. Walton of Great Britain, working in Ernest Rutherford's Cavendish Laboratory at Cambridge University develop a high voltage apparatus ("linear accelerator") for accelerating protons. With this they study nuclear reactions (atomic transmutation) and are awarded a Nobel Prize in 1931.

Schrödinger views electrons as continuous clouds and introduces "wave mechanics" as a mathematical model of the atom.

Albert Einstein urges all scientists to refuse military work.

Harold C. Urey of the United States and associates discover deuterium (heavy hydrogen) which is present (0.014%) in all natural hydrogen compounds including water.

John D. Crockcroft of Great Britain develops high-voltage apparatus for atomic transmutation.

James Chadwick proves the existence of neutrons, using alpha particles striking a beryllium foil. He determines their mass by measuring the recoil tracks of known atoms of the rarified gas in his cloud chamber.

John Cockcroft and E. T. S. Walton of Great Britain split the atom on a linear accelerator built at Ernest Rutherford's Cavendish Laboratory at Cambridge University. Their experiment proves Albert Einstein's theory of relativity.

September 12: Leo Szilard , a Hungarian physicist who took refuge in London from Nazi Germany, reads about a speech in which Lord Rutherford ridiculed the idea of using transformation of atoms as a source of power. Szilard realizes that, "if we could find an element which is split by neutrons, and which would emit two neutrons when it absorbs one neutron, such an element could sustain a nuclear chain reaction."

Frederic and Irene Joliot-Curie of France discover artificial radioactivity, i.e. the radioactivity of atoms produced in transmutation experiments.

Enrico Fermi of Italy irradiates uranium with neutrons. He believes he has produced the first transuranic element, but unknowingly achieves the world's first nuclear fission.

Otto Hahn and Fritz Strassmann of Germany split the uranium atom by bombarding it with neutrons and show that the elements barium and krypton are formed.

Lise Meitner
conducts experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction.

Werner von Braun is appointed the technical director of the German rocket research center at Peenemünde.

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