Chemistry 4.2

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Chemistry 4.2

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Structure of the Nuclear Atom >

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4.2 Subatomic Particles

Three kinds of subatomic particles are electrons, protons, and neutrons.

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>Structure of the Nuclear Atom Subatomic Particles

Electrons

In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron. Electrons are negatively charged subatomic particles.

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Thomson performed experiments that involved passing electric current through gases at low pressure.

The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode.

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Cathode Ray Tube

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A cathode ray is deflected by a magnet.

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A cathode ray is deflected by electrically charged plates.

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>Structure of the Nuclear Atom

Let’s go to the videotape

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>Structure of the Nuclear Atom Thomson’s Conclusions

Thomson found:

1.The charge to mass ratio of the particle

2.Various metals produced the exact same cathode ray (same charge to mass)

3.Various gases produced the exact same cathode ray (same charge to mass)

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Thomson concluded that a cathode ray is a fundamental particle contained in all matter.

It is made of a stream of electrons. Electrons are parts of the atoms of all elements.

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>Structure of the Nuclear Atom Robert Millikan

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Let’s go to the videotape!!

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>Structure of the Nuclear Atom Robert Millikan

Millikan determined:

1.The charge on an electron.

2.The fundamental charge of matter.

3.He calculated the mass of an electron from Thomson’s charge to mass ratio.

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>Structure of the Nuclear Atom Conclusions

•Atom is divisible

•One of the basic subatomic particles is the negatively charged electron

•Atom is electrically neutral, so it must contain positive charges to balance out the electrons

•Electrons have an extremely small mass, therefore there must be other massive particles in the atom

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Protons and Neutrons

In 1886, Eugen Goldstein (1850–1930) observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles.

Such positively charged subatomic particles are called protons.

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In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron.

Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.

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Table 4.1 summarizes the properties of electrons, protons, and neutrons.

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>Structure of the Nuclear Atom The Atomic Nucleus

J.J. Thompson and others supposed the atom was filled with positively charged material and the electrons were evenly distributed throughout.

This was called the “plum pudding model,” but you can think of it as the “chocolate chip cookie model.”

This model of the atom turned out to be short-lived, however, due to the work of Ernest Rutherford (1871–1937).

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Rutherford’s Gold-Foil Experiment

In 1911, Rutherford and his coworkers at the University of Manchester, England, directed a narrow beam of alpha particles at a very thin sheet of gold foil.

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Rutherford’s Gold-Foil Experiment

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Alpha particles scatter from the gold foil.

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Video

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The Rutherford Atomic Model

Rutherford concluded that the atom is mostly empty space. All the positive charge and almost all of the mass are concentrated in a small region called the nucleus.

The nucleus is the tiny central core of an atom and is composed of protons and neutrons.

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Structure of the Nuclear Atom >

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The Atomic Nucleus

In the nuclear atom, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atom.

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Chemistry 4.2