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that was present. This was an extraordinary result, a wild mira-
cle of the highest order, because from her observations Marie
was forced to conclude that the rays being emitted from uranium
and thorium were not the result of a chemical reaction,
but came directly from the element itself. At the beginning of
the twenty-first century, with our detailed understanding of the
atom and nuclear physics, it is difficult for us to appreciate
the tremendous importance of Marie's discovery, which ulti-
mately led to the most profound changes in our understanding
of science.
The history of the atom goes back nearly two and a half thou-
sand years to ancient Greece, around 400 BC , when the Greek
philosopher Democritus made a significant contribution to
metaphysics with his atomic theory of the universe. According
to him all things originated from a vortex of tiny, indivisible par-
ticles - atoms. Objects differed only in the shape, position and
arrangement of these atoms: atoms of a liquid were smooth
and round while atoms of a solid were jagged so that they
could catch on to each other and hold fast. Democritus
coined the word atom, which in Greek ( atomos ) means 'undi-
vided' because, according to his theory, atoms could not be
destroyed. Two and a half thousand years went by before
the theory developed much further, then, at the very end of the
eighteenth century, Antoine Lavoisier catalogued ninety-two
naturally occurring types of matter. These were the elements -
oxygen, hydrogen, helium, carbon, nitrogen, uranium, lead,
gold, silver, calcium, zinc, silicon and so on and so on. The
building blocks of which everything is made - you, me, your
car, my house, our dog and the whole shebang.
But how could the elements be classified? What characteris-
tics did they all have in common which would allow them to be
ordered? This was the great question of the day. Well, they were
all made of atoms, the smallest and most fundamental particle
of matter which, it was thought, could not be subdivided. So in
1805 John Dalton suggested that each element should be recog-
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