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Structure of the Nuclear Atom
Cathode-ray tubes are found in TVs, computer monitors, and many other devices with electronic displays.
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Structure of the Nuclear Atom >
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Subatomic Particles
Subatomic Particles
What are three kinds of subatomic particles?
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Three kinds of subatomic particles are electrons, protons, and neutrons.
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>A Crumb 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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>A Crumb Subatomic Particles
Cathode Ray Tube
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>A Crumb Subatomic Particles
A cathode ray is deflected by a magnet.
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>A Crumb Subatomic Particles
A cathode ray is deflected by electrically charged plates.
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>A Crumb Subatomic Particles
Thomson concluded that a cathode ray is a stream of tiny, negatively-charged particles moving at high speed. He called them electrons.
He found it didn’t matter what gas was in the tube, or what metal was used for the electrodes.
Thomson measured the mass-to-charge ratio to show that the particles were all the same, and electrons are parts of the atoms of all elements.
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>A Crumb Subatomic Particles
Oil Drop Experiment
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U.S. physicist Robert Millikan determined the charge carried by the electron.
Using this value and the mass-to-charge ratio from the cathode ray experiment, the mass of the electron was determined.
• The electron carries exactly one unit of negative charge.• Mass is 1/1840 the mass of a Hydrogen atom.
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>A Crumb Subatomic Particles
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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>A Crumb Subatomic Particles
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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>A Crumb Subatomic Particles4.2
Matter and Electrical Charge
Atoms have no net charge (i.e. they are neutral) - you do not get a shock every time you touch something.
Electrical charges are carried by particles of matter.
Electrical charges exist in whole number multiples of a basic unit.
When a given number of negative particles combine with an equal number of positive particles, an electrically neutral particle is formed.
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>A Crumb Subatomic Particles
This table summarizes the properties of electrons, protons, and neutrons.
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>A Crumb The Atomic Nucleus
J.J. Thompson and others supposed the atom was filled with positively charged material and the electrons were evenly distributed throughout.
“Plum pudding” 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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Structure of the Atomic Nucleus
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>A Crumb The Atomic Nucleus
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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>A Crumb The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
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>A Crumb The Atomic Nucleus
Alpha particles scatter from the gold foil.
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>A Crumb The Atomic Nucleus
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.
Rutherord’s atomic model known as the nuclear atom
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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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Section Quiz
-or-Continue to: Launch:
Assess students’ understanding of the concepts in Section
4.2 Section Quiz
4.2.
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4.2 Section Quiz
1. Which of the following is NOT an example of a subatomic particle?
a. proton
b. molecule
c. electron
d. neutron
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Slide 21 of 25
4.2 Section Quiz
2. The nucleus of an atom consists of
a. electrons only.
b. protons only.
c. protons and neutrons.
d. electrons and neutrons.
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Slide 22 of 25
4.2 Section Quiz
3. Most of the volume of the atom is occupied by the
a. electrons.
b. neutrons.
c. protons and neutrons.
d. protons.
