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Chapter 3 Atoms and Elements

3.4 The Atom

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Dalton’s Atomic Theory

In Dalton’s Atomic Theory, atoms

• are tiny particles of matter.

• of an element are similar and different from other elements.

• of two or more different elements combine to form compounds.

• are rearranged to form new combinations in a chemical reaction.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

John Dalton (1766-1844)

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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.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Rutherford’s Gold-Foil Experiment

In Rutherford’s gold-foil experiment, positively

charged particles

• were aimed at atoms of gold.

• mostly went straight through the atoms.

• were deflected only occasionally.

Conclusion:

There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close.

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Rutherford’s Gold-Foil Experiment

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Ernest Rutherford Gold foil experiment. A beam of positively charged alpha particles hits the gold foil. Most particles passed straight, some slightly deflected and some deflected back. The reason for deflection were the positions of the nucleus.

Separation of alpha, beta, and gamma particles by applying an electric field.

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Ernest Rutherford (1871-1937)

Rutherford’s interpretation of the gold foil experiment done by Geiger and Marsden.

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Modern View of the Atom

For an atom, which always has no net electrical charge, the number of negatively charged electrons around the nucleus equals the number of positively charged protons in the nucleus.

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Structure of the Atom

An atom consists• of a nucleus that

contains protons and neutrons.

• of electrons in a large empty space around the nucleus.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Atomic Mass Scale

On the atomic mass scale for subatomic particles,

• 1 atomic mass unit (amu) has a mass 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, 0.000549 amu.

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Particles in the Atom

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Scientists have been able to obtain computer-enhanced images of the outer surface of atoms using the scanning tunneling microscope (STM) and the atomic force microscope (AFM).

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Learning Check

Identify each statement as describing a

1) proton 2) neutron 3) electron

A. found outside the nucleus

B. has a positive charge

C. is neutral

D. found in the nucleus

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Solution

Identify each statement as describing a

1) proton 2) neutron 3) electron

A. 3 found outside the nucleus

B. 1 has a positive charge

C. 2 is neutral

D. 1, 2 found in the nucleus