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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 (above) 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 (left), 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.
Next
Sources and notes for this page.
The text for this page is original to the
Department of Energy's Office
of History and Heritage Resources. The information in this
page is derived from the essays on the history of "Chemistry" and
"Physics" in Roy Porter and Marilyn Ogilvie, eds., The Biographical
Dictionary of Scientists (New York: Oxford University Press, 2000),
26-27, 59-60. The photographs of J. J. Thomson and Niels Bohr are
courtesy the Fermi National Accelerator
Laboratory. Click here for more
information on the comic book image. The illustration of
Ernest Rutherford's concept of an atom
is modified from a graphic produced by the Lawrence
Berkeley National Laboratory. The portrait of Albert
Einstein is courtesy the Library of
Congress; it was taken in 1947 by Oren Jack Turner; its copyright
was not renewed.
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