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History of the Atom
Ancient Ideas:
The word atom comes from atomos, all matter would be
divided into units and sub units
...
The Greek philosopher Aristotle believed that matter
could be divided infinitely without changing its properties
...
He thought that matter could only
be divided until you got to the smallest particle
...
J Thomson (1856-1940)
Thomson used a cathode ray tube with a magnet and
discovered that the green beam it produced was made
up of negatively charged material
...
He discovered the ‘plum pudding’
models
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Dalton did experiments
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He also stated that all
atoms of the same element will be exactly
the same and that atoms of different
elements can combine to form
compounds
...
His model was
created solely on experiments that were
macroscopic, or seen with the unaided eye
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Thus developed the planetary model of the atom
which put all the protons in the nucleus and the
electrons orbited around the nucleus like planets
around the sun
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What is the Atomic Model
like today?

Niels Henrik David
Bohr (1885-1962)
In 1913, Bohr proposed his shell
model of the atom to explain how
electrons can have stable orbits
around the nucleus
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The energy of an electron
depends on the size of the orbit
and is lower for smaller orbits
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Electrons are in constant motion around
the nucleus, protons and neutrons jiggle
within the nucleus, and quarks jiggle
within the protons and neutrons
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Benjamin Pearson



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