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Historical Development of
an Atom
Democritus 470-380 B.C
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Democritus was a Greek philosopher
(470-380 B.C.) who is the father of
modern atomic thought.
He proposed that matter could NOT be
divided into smaller pieces forever.
He believed that matter was made of
small, hard, particles, that he called
“atomos”
John Dalton 1807
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He built upon Democritus’s atomic
theory.
Matter is made up of atoms
Atoms cannot be divided into smaller
pieces.
All the atoms of an element are
exactly the same
Different elements have different
kinds of atoms
Compounds form by combining
atoms
John Dalton 1807
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Model Drawing
Solid throughout
 Same throughout
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J.J. Thomson 1898
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Particles smaller than the atom
exists
Discovered the electron, which is
negatively charged
If atoms have negative particles,
then atoms must also have positive
particles.
J.J. Thomson 1898
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Model Drawing
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Positive and negative particles
Ernest Rutherford 1911
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Rutherford performed the gold foil
experiment.
In his experiment a radio active
beam emitted (+) alpha particles
toward gold foil.
Most particles went through,
some were deflected, and others
bounced back!
This showed that atoms have a
very dense (+) center.
Ernest Rutherford 1911
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Rutherford showed that atoms
have (+) particles in the center and
are mostly empty space.
He called the center of the atoms
the nucleus.
All the mass of an atom and its
positive charge are located in the
nucleus.
Ernest Rutherford 1911
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Model Drawing
Positive nucleus
 Negative particles surround nucleus
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Niels Bohr 1913
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Niels Bohr improved on
Rutherford’s model.
He proposed that electrons move
around the nucleus in specific
layers or shells.
Every atom has a specific number
of electron shells.
Niels Bohr 1913
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Electrons move in orbits around
the nucleus
1st energy level = 2 electrons
2nd energy level = 8 electrons
3rd energy level = 18 electrons
4th energy level = 32 electrons
Niels Bohr 1913
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Model Drawing
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Electrons orbit the positive nucleus
on energy levels
James Chadwick 1932
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Chadwick discovered neutrons
He worked with Rutherford and
discovered particles with no charge
He called those particles neutrons
Neutrons are found in the nucleus
of an atom.
James Chadwick 1932
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Model Drawing
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Electrons orbit the positive nucleus
which contains protons and neutrons
Electron Cloud Model
Current
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Electrons do not follow fixed
orbits, but tend to occur more
frequently in certain areas around
the nucleus
it is impossible to know where an
electron is at any given time
Electron Cloud Model
Current
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Model Drawing
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Electrons are located in clouds