Transcript Atomic Theory The Atom

Atomic Theory
The Atom
Copyright © 2008 b Pearson Education, Inc.
Publishing as Benjamin Cummings
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Dalton’s Atomic Theory
First to propose “theories” on the atom in 1803
*There were 4 postulates
*Problems with the first two
*He never ran experiments or provided evidence,
and therefore; they were not widely accepted by
the science community
*biggest issue was that JD thought the atom was
the smallest particle. Little did he know that
there was much more
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Thomson’s Cathode Ray
experiment and plum
pudding model
• Passed an electric current through gases at
low pressure and sealed the gas fitted with
electrodes
• The resulting glowing beam or cathode ray
traveled from the cathode to the anode and
was deflected by a magnet; therefore had a
(-) charge.
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Thompson
The positive side of the magnet attracted
the ray and the negative side repelled it
and therefore it was determined that there
were particles inside the atom that were
negative
 He determined the charge of an electron
 He called them electrons

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Millikan’s Oil Drop
Experiment (1909)
Determine the mass to charge ratio of an
electron and used Thompson’s findings to
calculate the mass of an electron
 Millikan Oil Drop Experiment

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Rutherford’s Gold-Foil
Experiment
In Rutherford’s gold-foil
experiment
• Positively charged particles were
aimed at atoms of gold.
• Most went straight through the
atoms.
• Only a few were deflected.
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Rutherford’s Gold-Foil
Experiment (1910)
Rutherford Goild Foil Experiment
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Rutherford’s Gold Foil
Conclusions
• There must be a small, dense,
positively charged nucleus in the
atom that deflects positive particles
that come close.
• The atom is mostly empty space
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James Chadwick
Worked with Rutherford who was convinced
there was another particle. After all the
atomic number and atomic mass are not the
same which indicates there must have been
an additional particle
 1932-Chadwick discovered by bombarding
beryllium with alpha particles. The particle
released was the same mass but no charge.

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Subatomic Particles
Atoms contains subatomic particles
• Protons have a positive (+)
charge.
• Electrons have a negative (-)
charge.
• Like charges repel and unlike
charges attract.
• Neutrons are neutral.
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Structure of the Atom
An atom consists
• Of a nucleus that
contains protons
and neutrons.
• Of electrons in the
large empty space
around the
nucleus.
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Atomic Mass Scale
On the atomic mass scale for subatomic particles
• 1 atomic mass unit (amu) is equal to 1/12 of the
mass of the carbon-12 atom.
• A proton has a mass of about 1 (1.007) amu.
• A neutron has a mass of about 1 (1.008) amu.
• An electron has a very small mass (0.00055 amu).
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Particles in the Atom
Table 4.5
Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
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