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Atomic Theory
The development of the scientific
model of the atom.
Earliest Atomic Theory
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400 B.C. – Democritus, a Greek philosopher,
developed the first atomic theory.
He believed that matter was made up of tiny
particles called atoms.
He also believed that matter could not be created,
destroyed, or further divided.
His theory was met with criticism from other
influential philosophers such as Aristotle.
His theory was eventually rejected because it was
not supported by experimental evidence.
Democritus’ Model of the Atom
1808-Dalton, an English school teacher,
proposed an atomic theory.
His theory stated:
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All matter is composed of extremely small particles
called atoms.
All atoms of a given element are identical. Atoms of
a specific element are different from those of any
other element.
Atoms cannot be created, destroyed, or divided into
smaller particles.
Different atoms combine in simple whole-number
ratios to form compounds.
In a chemical reaction, atoms are separated,
combined, or rearranged.
Dalton’s Model of the Atom
Dalton’s Symbols for the
Elements
Dalton’s theory was supported by
experimental evidence which led to the
general acceptance of his theory.
His theory has had to be revised,
however, as additional information has
been learned.
Late 1890’s- Thomson, an English
physicist, discovered the existence
of a small negatively charged
particle (the electron) through his
cathode ray tube experiment.
1909- Millikan, an American
physicist, determined the charge of
an electron through his Oil Drop
Experiment. The value he
determined is still the accepted
value used today.
As a result of these discoveries,
Thomson proposed a model of
the atom that became known as
the Plum Pudding model (or
Chocolate Chip Cookie model).
1911- Rutherford, a New
Zealand chemist, conducted
the Gold-Foil Experiment.
He recorded the following
observations during his experiment:
Most of the alpha particles passed through
the gold foil undeflected.
 A smaller percentage of the particles were
slightly deflected.
 A very small number of particles were
deflected straight back towards their
source.
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From these observations, he
concluded the following:
 The
atom is mostly empty space.
 There is a tiny, dense central core
in the atom. He called this the
nucleus.
 The nucleus is positively charged.
From this experiment he
concluded that the Plum
Pudding model was
incorrect.
1919- Goldstein concluded that the
nucleus contained positively
charged particles called protons.
This particle carries a charge equal
but opposite to that of an electron.
1932- Chadwick, an English physicist,
showed that the nucleus also contains
another subatomic particle.
This neutral particle was called the
neutron. Its mass is nearly equal to
that of a proton, but it carries no
charge.
As a result of this knowledge, a new model of
the atom was developed by Rutherford and
Bohr, a Danish physicist.
This model is sometimes called the Planetary
model because it describes the atoms as being
similar to the solar system.
The sun represents the nucleus and the planets
represent the electrons.
Summary of the Subatomic
Particles
Particle
Symbol
Location
Relative
Charge
Relative
Mass
Proton
p+
Nucleus
1+
1
Neutron
no
Nucleus
0
1
e-
In the space
surrounding
the nucleus
1-
1/1840
Electron