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The Atomic Theory of Matter
c.460 BCE - c.370 BCE
The Atomic Theory, a cornerstone of modern science, was
proposed by an early Greek thinker, Democritus.
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Atomic Weights
1808
John Dalton, an English chemist, stated that each atom
of any given element is identical to every other atom
of that element, including weight.
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The Periodic Table
1871
Dmitry Mendeleyev, a Russian chemist, revealed the basic
importance of atomic weights and of nuclear structure.
His work also showed the significance of structure in
comprehending the behavior and properties of matter.
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Cathode Rays
1887
Sir William Crookes, an English chemist and physicist,
pioneered work on cathode rays.
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X-Rays
1895
While studying cathode rays, German physicist Wilhelm
Röentgen noticed some glowing barium platinocyanide
across the room from his experiment. This led to the discovery
of X-rays. His work helped found a major new medical technique
and played an important role in revealing the secrets
of the atom and its nucleus.
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Radioactivity
1896
French physicist Antoine Henri Becquerel discovered radioactivity.
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The Electron
1897
English physicist Sir J. J. Thomson explained the nature
of the electron.
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Radium
1902
Marie Curie and her husband, Pierre, discovered the radioactive
elements polonium and radium. Their work confirmed the
existence of radioactivity.
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E=mc2
1905
For more than two centuries, scientists had unquestionably
believed that the basic quantities of measurement -- mass,
length, and time -- were absolute and unvarying. The German-born
physicist Albert Einstein showed that in fact they depended
very much on the relative motion between the observer
and whatever was being observed.
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The Nuclear Model
1909
Sir Ernest Rutherford's great contribution to modern science
was to show what happens to an element during radioactive
decay. This enabled him to construct the first nuclear
model of the atom, a cornerstone of present-day physics.
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The Electron Orbit
1913
Niels Bohr modified Rutherford's model of the atom to
incorporate the ideas of quantum physics. This required
a new mechanism for the way electrons emitted energy.
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Transformation of Atoms
1919
Rutherford's work, which he published in 1919, demonstrated
that atoms could be transformed from those of one element
into those of another by means of artificial tampering
with the nucleus. Far more important, his experiment demonstrated
that the nucleus of an atom could be breached.
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The Neutron
1932
British physicist Sir James Chadwick is best known for
discovering the neutron, one of the fundamental particles
making up the nucleus of atoms. The neutron differed from
all other particles then known by having no electrical
charge.
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The Atom is Split
1932
Sir John Douglas Cockroft and his colleague, Ernest T.
S. Walton, developed the Cockroft-Walton particle accelerator.
Using it in 1932, they managed to boost the speed of protons
to the point where the voltage was high enough to energize
each atom of lithium, their target metal, to form two
atoms of helium. This was the first example of man-made
nuclear transformation.
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Uranium and Fission
1938
German scientists Otto Hahn and Fritz Strassmann discovered
that a tiny portion of the uranium atom's mass could be
converted into an estimated 200 million electron volts
of potentially usable energy. This process was to be called
fission.
The
Atomic Age, one of history's most important events, took centuries
to arrive, as events in science and international affairs
evolved.The Atomic Theory, a cornerstone of modern science,
was proposed by an early Greek thinker, Democritus. He suggested
that everything in the universe was made up of particles so
tiny that nothing smaller could exist. 
