The Atomic theory
A theory of the structure and behavior of atoms has taken more than two millenia to evolve, from the abstract musings of ancient Greek philosophers to the high-tech experiments of modern scientists. However, prior to the scientific revolution and the development of the scientific method starting in the 16th century, ideas about the atom were mainly speculative. It wasn't until the very end of the 19th century that technology became advanced enough to allow scientists a glimpse of the atom's constituent parts: the electron, nucleus, proton, and neutron.
Greek Origins
The idea that all matter is made up of tiny, indivisible
particles, or atoms, is believed to have originated with the Greek
philosopher Leucippus of Miletus and his
student Democritus of Abdera in the 5th
century B.C. (The word atom comes from the Greek word atomos,
which means indivisible.) These thinkers held that, in addition to being too
small to be seen, unchangeable, and indestructible, atoms were also completely
solid, with no internal structure, and came in an infinite variety of shapes
and sizes, which accounted for the different kinds of matter. Color, taste, and
other intangible qualities were also thought to be composed of atoms.
While the idea of the atom was supported by some later Greek
philosophers, it was fiercely attacked by others, including Aristotle, who argued against
the existence of such particles. During the Middle Ages in Europe, Roman
Catholic theologians were heavily influenced by Aristotle's ideas, and so
atomic philosophy was largely dismissed for centuries. However, the Greeks'
conception of the atom survived, both in Aristotle's works (his arguments
against) and in another classical work by the Roman author
Lucretius, De rerum
natura (On the Nature of Things), which was rediscovered in Europe at
the start of the Renaissance.
Modern Development
Modern atomic theory is generally said to begin with John
Dalton, an
English chemist and meteorologist who in 1808 published a book on the
atmosphere and the behavior of gases that was entitled A New System of
Chemical Philosophy. Dalton's theory of atoms rested on four
basic ideas: chemical elements were composed of atoms; the atoms of an element
were identical in weight; the atoms of different elements had different
weights; and atoms combined only in small whole-number ratios, such as 1:1,
1:2, 2:1, 2:3, to form compounds.
Not all of these ideas were new; the Greeks had already
introduced the idea that elements were composed of atoms and that atoms of
different elements had different physical properties. Dalton's particular
contribution, which distinguished his work from what had been done before, was
his method for actually determining atomic
weight.
In an essay published in 1805, Dalton had included a list of atomic weights for
21 elements. Dalton was also the first to propose standard symbols for the
elements.
Subatomic Structure
Dalton's work was mainly about the chemistry of atoms how
they combined to form new compounds rather than the physical, internal
structure of atoms, although he never denied the possibility of atoms' having a
substructure. Modern theories about the physical structure of atoms did not
begin until 1897, with J. J.
Thomson's discovery
of the electron.
Actually, what Thomson discovered was that cathode
rays were
streams of negatively charged particles with a mass about 1,000 times smaller
than a hydrogen atom. He claimed that these particles, which he called
corpuscles, were the things that atoms were made from. The term ?electron?
predated Thomson's discovery a few years earlier Irish physicist G. J. Stoney
had proposed that electricity was made of negative particles called electrons, and scientists had
adopted the word to refer to anything with an electric charge. However,
Thomson, who was a physicist at Cambridge University, was the first to suggest
that these particles were a building block of the atom.
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