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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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• 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.

Thomson’s Cathode Ray experiment and plum pudding model

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

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

Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

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