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Chemistry 4.2
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Structure of the Nuclear Atom
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Subatomic Particles
Three kinds of subatomic particles are
electrons, protons, and neutrons.
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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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Subatomic Particles
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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Subatomic Particles
Cathode Ray Tube
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Subatomic Particles
A cathode ray is deflected by a magnet.
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Subatomic Particles
A cathode ray is deflected by electrically
charged plates.
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Let’s go to the
videotape
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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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Subatomic Particles
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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Robert Millikan
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Let’s go to the videotape!!
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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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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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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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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 shortlived, however, due to the work of Ernest
Rutherford (1871–1937).
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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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The Atomic Nucleus
Rutherford’s Gold-Foil Experiment
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The Atomic Nucleus
Alpha particles scatter from the gold foil.
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Video
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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.
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The Atomic Nucleus
Rutherford suggested that the nucleus
contained a particle with a positive charge the
proton. Atoms of different elements have
different numbers of protons giving their
nuclei different charges.
That meant the hydrogen nucleus (it has one
proton) was an elementary particle.
Rutherford named it the proton, from the
Greek word "protos," meaning "first."
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Subatomic Particles
In 1932, the English physicist James Chadwick
who had worked with Rutherford (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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Chadwick fired the neutrons at a block of
paraffin wax, which has a high concentration
of hydrogen and is therefore rich in protons.
Some of the neutrons collided with protons in
the wax and knocked them out. Chadwick
could then detect these protons and measure
their energy. Using his knowledge of energy
and momentum, he was able to work out the
mass of the neutrons from the range of
energies of the protons that they knocked out.
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He found that its mass was slightly more than
that of a proton. Chadwick, like Rutherford,
used an ingenious method to probe into what
cannot be seen.
To the Videotape!!!!!
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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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Subatomic Particles
Table 4.1 summarizes the properties of
electrons, protons, and neutrons.
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4.2 Section Quiz
Assess students’ understanding
of the concepts in Section 4.2.
Continue to:
-or-
Launch:
Section Quiz
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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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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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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.
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END OF SHOW