A MINIATURE SOLAR SYSTEM
(1890s-1919)
Events: Atomic Discoveries, 1890s-1939

The modern effort to uncover the inner structure of the atom began with the discovery of the electron by the English physicist J. J. Thomson (right) in 1897.  Thomson proved that cathode rays were not some sort of undefined process occurring in "ether" but were in fact composed of extremely small, negatively charged particles.  Dubbed electrons, their exact charge and mass were soon determined by John Townsend and Robert Millikan.  

At the same time, discoveries relating to the curious phenomenon of radioactivity had also begun to propel atomic research forward.  In 1896, the French physicist Antoine Becquerel detected the three basic forms of radioactivity, which were soon named alpha, beta, and gamma by Ernest Rutherford, a student of Thomson from New Zealand.  Also in 1896, the husband-and-wife team of Marie and Pierre Curie began work in Paris on the emission of radiation by uranium and thorium.  The Curies soon announced their discoveries of radium and polonium; they also proved that beta particles were negatively charged.  In 1900, Becquerel realized that beta particles and electrons were the same things.  

In the first decade of the 20th-century, Rutherford began to pull all of this information into a coherent whole.  In 1903, he proposed that radioactivity was caused by the breakdown of atoms; in 1908, he correctly identified alpha particles as being the nuclei of atoms of helium; and in 1911, along with the German physicist Hans Geiger, Rutherford postulated that electrons orbit an atom's nucleus, much as the planets orbit the sun.  The second fundamental atomic particle, the proton, was identified by Rutherford in 1919.  

It was the Danish physicist Niels Bohr (right), however, who combined Rutherford's atomic concepts with Max Planck's quantum theory to produce the first modern model of the atom.  In 1913, Bohr demonstrated that electrons moved around an atom's nucleus in certain discrete energy "shells," and that radiation is emitted or absorbed when an electron moves from one shell to another.  The following year Henry Moseley, an English physicist, showed that each element could be identified by its unique "atomic number."  

By the 1910s, then, scientists investigating the inner structure of the atom had come to believe, among other things, that energy exists within the atom, latent and bound up with the structure of the atom. Considered in light of Albert Einstein's 1905 theoretical formula E=mc² (energy equals mass times the square of the velocity of light) stating that matter and energy were equivalent, this belief held breathtaking possibilities. For if Einstein (right) were correct that matter and energy were different forms of the same thing, it followed that anyone unlocking the secrets of how these minute particles were held together—and how they could be broken apart—could produce a massive release of energy.