| Sir William Crookes discovers properties of cathode rays, such as travelling in straight lines from the cathode, causing glass to fluoresce, imparting a negative charge to objects they strike, being deflected by electric fields and magnets to suggest a negative charge, and causing pinwheels in their path to spin indicating they have mass.
1886
E. Goldstein uses a CRT to study "canal rays" which have electrical and magnetic properties opposite of an electron.
1895
Wilhelm Roentgen of Germany discovers X-rays. While studying electrical discharge through rarified gas, he finds that invisible rays came from the positive electrode and would darken a photographic plate through an opaque wrapping.
1896
Antoine Henri Becquerel of France discovers natural radioactivity when invisible rays from uranium ore darken a photographic plate.
1898
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.
1899
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.
1900
Max Planck of Germany develops quantum theory, which explains 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.
1901
Max Planck publishes "On the Law of Distribution of Energy in the Normal Spectrum."
1902
Ernest Rutherford and Frederick Soddy publish their 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.
1903
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.
1904
Ernest Rutherford discovers that alpha rays are heavy positively charged particles. In 1908, he is awarded a Nobel Prize for his work.
1905
Albert Einstein publishes "Fundamental Ideas and Problems of the Theory of Relativity," which addresses convertibility of matter and energy (E=mc2).
1910
Robert Andrews Millikan accurately measures the electric charge and the mass of an electron in his famous oil drop experiment.
1911
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.
1912
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.
1913
Niels Bohr of Denmark publishes theory of atomic structure combining nuclear theory with quantum theory.
1914
Using x-ray tubes, H. G. J. Moseley determines charges on the nuclei of most atoms. Accordingly, the periodic table was reorganized 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.
1915
Albert Einstein publishes general theory of relativity.
1919
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.
1921
Ernest Rutherford and James Chadwick of Great Britain achieve transmutation of all elements except carbon, oxygen, lithium and beryllium.
1922
P. M. S. Blackett of Great Britain begins experiments on transmutation of elements.
1923
Supporting Einstein 's theory, de Broglie discovers that electrons have a dual nature--similar to both particles and waves.
1929
Ernest O. Lawrence of the United States conceives idea for the first cyclotron (atom smasher).
John D. Cockcroft 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.
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Atomic Discovey 
