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Historical Development
of an Atom
Democritus 470-380 B.C
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Democritus was a Greek philosopher (470380 B.C.) who is the father of modern
atomic thought.
He proposed that matter could NOT be
divided into smaller pieces forever
(uncuttable)
He believed that matter was made of small,
hard, particles, that he called “atomos”
Thought atoms are made of a single
material formed into different shapes and
sizes
Only had a mental model or the atom –
nothing recorded
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/elements (experiments)
John Dalton 1807
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Model Drawing
Solid throughout
 Same throughout
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J.J. Thompson 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.
Cathode ray experiment
“plum pudding model” – said the atom was
like plum pudding with pieces of plums
dispersed throughout
J.J. Thompson 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/energy
levels
Every atom has a specific
number of electron shells.
Said electrons can jump levels
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.
Chadwick 1932
p=protons n = neutrons
Pnnn
Pnnnn
ppp
Electron Cloud
Theory
Erwin Schrodinger - 1925
 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
*this is the theory we use today!
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Electron Cloud Model
Current
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Model Drawing
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Electrons are located in clouds