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The Development of Atomic
Theory
SCH12U
September 7 2011
Mr. Dvorsky
• Nearly 2500 years ago Greek
philosophers (i.e. Democritus) expressed
a belief matter is composed of tiny
indivisible particles called atoms (atomos
is the Greek word for “indivisible”)
• These conclusions were not based on any
evidence; they were derived from
philosophical reasoning.
JJ Thompson –
Discovery of Electrons
Source of
Electrical
Potential
Stream of negative
particles (electrons)
Metal Plate
Gas-filled
glass tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Metal plate
A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58
Thomson’s Experiment
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voltage
source
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vacuum tube
metal disks
Thomson’s Experiment
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voltage
source
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vacuum tube
metal disks
Thomson’s Experiment
ON
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OFF
voltage
source
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Passing an electric current makes a beam appear
to move from the negative to the positive end
Thomson’s Experiment
ON
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OFF
voltage
source
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Thomson’s Experiment
ON
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OFF
voltage
source
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By adding an electric field…
he found that the moving pieces were negative.
J.J. Thomson
J.J. Thomson
• He proved that atoms
of any element can be
made to emit tiny
negative particles.
• From this he concluded
that ALL atoms must
contain these negative
particles.
• He knew that atoms did
not have a net negative
charge and so there
must be balancing the
negative charge.
Thomson
• In 1910 proposed
the Plum Pudding
model
– Negative electrons
were embedded into
a positively charged
spherical cloud.
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
Spherical cloud of
Positive charge
Electrons
Plum-Pudding Model
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
Thomson Model of the Atom
• J.J. Thomson discovered the electron and knew that
electrons could be emitted from matter (1897).
• William Thomson proposed that atoms consist of
small, negative electrons embedded in a massive,
positive sphere.
• The electrons were like currants in a plum pudding.
• This is called the ‘plum pudding’ model of the atom.
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electrons
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Rutherford, see animation
-fired alpha particles at a very thin piece of
foil.
-The alpha particles to pass through without
changing direction (very much)
Because....
-The positive charges were spread out
evenly.
Alone they were not enough to stop the
Because he thought the mass was
evenly distributed in the atom.
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Because, he thought the
mass was evenly
distributed in the atom
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Explanation of Alpha-Scattering Results
Alpha particles
Nucleus
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Plum-pudding atom
Nuclear atom
Thomson’s model
Rutherford’s model
The Rutherford Atom
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 323
• Bohr suggested the planetary model of the
atom could be rescued if one assumption
is made: certain special “states of motion”
of the electron corresponding to electron
shells would not result in radiation and
therefore the electron can exist
indefinitely.
Bohr was saying, in effect, is that the atom can
exist
only in certain discrete energy states: the energy
of
the atom is quantized. Bohr noted that this
quantization nicely explained the observed
emission
spectrum of the hydrogen atom. The electron is
normally in its smallest allowed orbit,
corresponding
to n = 1; upon excitation in an electrical discharge
or
These higher excited states of the atom are
unstable, so after a very short time (around
10—
9 sec) the electron falls into lower orbits and
finally into the innermost one, which
corresponds to the atom's ground state. The
energy lost on each jump is given off as a
photon, and the frequency of this light
provides
a direct experimental measurement of the
difference in the energies of the two states,
according to the Planck-Einstein