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ATOMIC THEORY
History of the Atom
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http://www.youtube.com/watch?v=njGz69B_pUg&
feature=related
John Dalton
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http://www.youtube.com/watch?v=wsYDL6EjV4k
Suggested that elements were made up of atoms.
A given compound always contains the same
combination of atoms.
Prepared the first table of atomic masses.
Law of Multiple Proportions: When 2 elements form a
series of compounds (i.e., CO and CO2), the ratios of
the masses of the second element that combine with 1 g
of the first element can always be reduced to small
whole numbers. (See Sample Exercise 2.1, p.45)
Amadeo Avogadro
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Avogadro’s Hypothesis: at the same temperature
and pressure, equal volumes of different gases
contain the same number of particles.
J.J. Thomson
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http://www.youtube.com/watch?v=_SY7qBrVxsU
&feature=related
Cathode-ray tubes (cathode = negative electrode)
The ray emanating from the cathode was repelled
by the negative pole of an applied electrical field.
So what did Thomson propose the ray was
composed of?
Determined the charge-to-mass ratio of an electron:
c/m = -1.76 x 108 C/g
Thomson (cont.)
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Since electrons could be produced from electrodes
of various metals, all atoms must contain
electrons, so they must have some positive
charge.
Postulated that an atom consisted of a diffuse
cloud of positive charge with the negative
electrons embedded randomly around it. (Plum
Pudding Model)
Robert Millikan
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Oil Drop Experiment:
http://www.youtube.com/watch?v=91E6KvCvRf0
Altered the voltage on charged plates to stop the fall
of charged oil drops.
Using the voltage and the mass of the oil drop, he
was able to determine the charge on an oil drop.
Using the charge on the oil drop and Thomson’s
charge-to-mass ratio, Millikan calculated the mass of
an electron as 9.11 x 10-31 kg.
Ernest Rutherford
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Gold Foil Experiment:
http://www.youtube.com/watch?v=mHcjiiP0HB4&fe
ature=related
Tested Thomson’s plum pudding model by directing α
particles at a thin sheet of gold foil.
Expected result: particles would pass through
Actual result: most passed through, but some were
deflected and reflected
These results indicated a concentration of positive
charge (nucleus), with electrons moving around it at
some distance.
Niels Bohr
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Quantum Model:
http://www.youtube.com/watch?v=Cn6v5ygyZHQ
The electron in a hydrogen atom moves around the
nucleus only in certain allowed circular orbits (much
like the planetary model)
Bohr also postulated that atoms can become
“excited” by an electrical current, absorbing energy
and then releasing that energy in the form of light,
with each element emitting certain, unique
wavelengths (or spectrum).
http://www.youtube.com/watch?v=xUXnmPL9SE&NR=1
Schrodinger
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Said electrons were more like waves than
particles
James Chadwick
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Discovered the neutron
This accounted for the previously unexplained
difference between an element’s atomic number
and its atomic mass
http://www.youtube.com/watch?v=HnmEI94URK
8&feature=related
Werner Heisenberg
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Discovered both Bohr AND Schrodinger were correct:
an electron is a particle with wave-like properties
Photon: a “particle” of electromagnetic radiation
Also said you cannot know both an electron’s
location AND its spin at the same time.
Heisenberg’s Uncertainty Principle:
http://www.youtube.com/watch?v=Fqr4MFg6GHI&fe
ature=related
Atomic Structure