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Subatomic Physics
Preview
Section 1 The Nucleus
Section 2 Nuclear Decay
Section 3 Nuclear Reactions
Section 4 Particle Physics
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Section 1
Subatomic Physics
TEKS
Section 1
The student is expected to:
5A research and describe the historical
development of the concepts of gravitational,
electromagnetic, weak nuclear, and strong
nuclear forces
5H describe evidence for and effects of the
strong and weak nuclear forces in nature
8C describe the significance of mass-energy
equivalence and apply it in explanations of
phenomena such as nuclear stability, fission,
and fusion
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Subatomic Physics
What do you think?
• What holds a nucleus together?
• What particles exist within the nucleus?
• What force(s) exist between these particles?
• Are these forces attractive or repulsive?
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Section 1
Subatomic Physics
Section 1
The Nucleus
• The chemical symbol for an
element is written like the one
shown to the left. What
information is provided by this
symbol?
– The atomic number (Z) or
number of protons is 13.
– The mass number (A) or number
of protons + neutrons is 27.
– The number of neutrons (N) is
14 (27 – 13).
– The element is aluminum.
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Subatomic Physics
Section 1
Isotopes
• Isotopes are atoms of the same element with
different atomic masses.
– The number of neutrons is different.
• Most carbon nuclei have 6 protons and 6
neutrons and an atomic mass of 12.
– Called carbon-12
– Others have 5 neutrons (carbon-11), 7 neutrons
(carbon-13), or 8 neutrons (carbon-14).
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Subatomic Physics
Isotopes
Click below to watch the Visual Concept.
Visual Concept
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Section 1
Subatomic Physics
Section 1
Nuclear Mass
• The density of the nucleus is
approximately 2.3  1017 kg/m3.
• Mass is measured in unified
mass units (u).
– 1 u is one-twelfth the mass of one
atom of carbon-12.
• 1 u = 1.6605  10-27 kg
• Protons and neutrons each
have a mass of approximately
1 u.
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Subatomic Physics
Section 1
Nuclear Mass
• Find the energy equivalent of 1 u in both J and eV.
(For c, use the value 2.9979  108 m/s.)
– Answers:
• 1.4924  10-44 J, 931.47  106 eV or 931.47 MeV
• With more significant figures, 1 u = 931.49 MeV.
• The mass of subatomic particles is often expressed in
MeV.
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Subatomic Physics
Section 1
Nuclear Mass
• This table provides the mass and rest energy of atomic
particles in kilograms, unified mass units, and MeV.
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Subatomic Physics
Section 1
Nuclear Stability
• What type of electric force would
exist in the nucleus shown?
– Protons would repel other protons
very strongly because the distance
between them is small.
– Neutrons would produce no forces.
• What holds the nucleus together?
– A force called the strong force:
a powerful attractive force between
all particles in the nucleus
• Does not depend on charge
• Exists only over a very short range
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Subatomic Physics
Section 1
Nuclear Stability
• As more protons are added to the nucleus, more
repulsion exists.
– Larger and larger nuclei require more neutrons, and
more strong force, to maintain stability.
• Look at a periodic table to find out which
elements have approximately a 1:1 ratio between
neutrons and protons, and which elements have
the highest ratio of neutrons to protons.
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Subatomic Physics
© Houghton Mifflin Harcourt Publishing Company
Section 1
Subatomic Physics
Section 1
Nuclear Stability and Ratio of Neutrons and
Protons
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Visual Concept
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Subatomic Physics
Section 1
Binding Energy
• The nucleons (protons and neutrons) have a greater
mass when unbound than they do after binding to form a
nucleus.
– Called binding energy
– This energy is released when the binding occurs, and must be
absorbed to separate the nucleons.
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Subatomic Physics
Section 1
Classroom Practice Problem
• The mass of the individual particles in an atom is
the mass of the protons, neutrons, and
electrons.
– For the mass of the protons and electrons combined,
simply multiply the atomic number times the mass of a
hydrogen atom (1 electron bound to 1 proton).
• Find the binding energy (in u and MeV) for a
helium atom with two protons and two neutrons.
The atomic mass of helium-4 is 4.002602 u.
– Answer: 0.030378 u or 28.297 MeV
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Subatomic Physics
Section 1
Now what do you think?
• What holds a nucleus together?
•
•
•
•
What particles exist within the nucleus?
What forces exist between these particles?
Are these forces attractive or repulsive?
What happens to each of these forces when the
particles are farther and farther apart?
• What is meant by the term binding energy?
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Subatomic Physics
TEKS
Section 2
The student is expected to:
8D give examples of applications of atomic
and nuclear phenomena such as radiation
therapy, diagnostic imaging, and nuclear
power and examples of applications of
quantum phenomena such as digital cameras
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Subatomic Physics
Section 2
What do you think?
• Often scientists use radioactive carbon
dating to determine the age of fossils.
• What does the term radioactive mean?
• Are all atoms radioactive?
• If not, how are radioactive atoms different from those
that are not radioactive?
• How can radioactivity be used to determine the
age of a fossil?
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Subatomic Physics
Section 2
Nuclear Decay
• When nuclei are unstable, particles and
photons are emitted.
– The process is called radioactivity.
– It occurs because the nucleus has too many or
too few neutrons.
– Three types of radiation can occur:
• Alpha
• Beta
• Gamma
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Subatomic Physics
Section 2
Alpha Decay ()
• An alpha particle (2 protons and 2 neutrons) is emitted
from the nucleus.
–  particles are helium-4 nuclei.
– A new element is formed by alpha decay.
– Example of alpha decay:
238
92
U 
 23490Th + 24 He
• The uranium atom has changed into a thorium atom by
ejecting an alpha particle.
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Subatomic Physics
Section 2
Radioactive Decay
• These rules are used to determine the daughter nucleus
when a parent nucleus decays.
• Note how these rules apply in the alpha decay of
uranium-238:
– 238 = 234 + 4
– 92 = 90 + 2
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238
92
U 

234
90
4
2
Th + He
Subatomic Physics
Section 2
Beta Decay
• An electron or positron is emitted from the
nucleus.
– A positron is the same as an electron but with an
opposite charge.
– A positron is the antiparticle of an electron.
• Since there are no electrons or positrons in the
nucleus, how can beta decay occur?
– A neutron is transformed into a proton and an electron,
and then the electron is ejected.
– A proton is transformed into a positron and a neutron,
and then the positron is ejected.
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Subatomic Physics
Section 2
Beta Decay
• It was discovered that, during beta decay, momentum and
energy were not conserved.
– The ejected electron did not have as much forward momentum as
the recoiling nucleus.
• In 1930, Wolfgang Pauli proposed the existence of a
particle that was not detectable at the time.
• In 1956, Pauli’s neutrino () was detected.
• The neutrino and its antiparticle, the antrineutrino (), are
emitted during beta decay.
– Electrons are accompanied by antineutrinos.
– Positrons are accompanied by neutrinos.
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Subatomic Physics
Section 2
Beta Decay
• What new element is formed by the beta decay
of carbon-14?
14
6
C 
 N + e +
14
7
0
-1
• The new element is nitrogen-14.
• The electron is shown with a mass number of
zero and an atomic number of -1.
– The total of the mass numbers and atomic numbers
are still equal.
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Subatomic Physics
Section 2
Gamma Decay
• During alpha and beta decay, the nucleons left
behind are often in an excited state.
• When returning to ground state, the nucleus
emits electromagnetic radiation in the form of a
gamma ray.
• The nucleus remains unchanged except for its
energy state.
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Subatomic Physics
Types of Radioactive Decay
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Section 2
Subatomic Physics
Alpha, Beta, and Gamma Radiation
Click below to watch the Visual Concept.
Visual Concept
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Section 2
Subatomic Physics
Section 2
Nuclear Decay Series
• During nuclear decay, the daughter may be
unstable as well, causing further decays.
• What element would be formed by thorium-232
undergoing 6 alpha and 4 beta decays?
– Answer: lead-208
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Subatomic Physics
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Section 2
Subatomic Physics
Section 2
Classroom Practice Problems
• Find the missing item (X) in these reactions:
228
88
Ra 
 X + -10 e+
– Answer: 228
88
220
86
0
Ra 
 228
Ac
+
89
-1 e + 
Rn 

– Answer: 220
86
216
84
Po + X
Rn 

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216
84
4
2
Po + He
Subatomic Physics
Section 2
Measuring Nuclear Decay
• The rate of decay is different for each nucleus.
N/t = -N
– N is the number of nuclei, t is the time, and  is the
decay constant.
•  differs for every element.
– The rate of decay is called the activity.
– The negative sign occurs because the number of
nuclei is decreasing.
– SI unit: becquerel (Bq) or decays/s
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Subatomic Physics
Section 2
Half-Life
• Half life is the time required for half of the nuclei to
decay.
– Half-lives can be very short (nanoseconds) or very long (millions
of years).
• Half-life is inversely related to the decay constant.
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Subatomic Physics
Half-Life
• Carbon-14 is radioactive with a
half-life of 5715 years.
• The figure shows a decay curve
for carbon-14.
– Does the total number of nuclei
change?
• No
– How much time has passed at
T1/2?
• 5715 years
– How much time has passed at
2T1/2?
• 11 430 years
– How many blue circles will there
be at 3T1/2 ?
• one
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Section 2
Subatomic Physics
Section 2
Radioactive Carbon Dating
• All living things have about the same ratio of
carbon-14 to carbon-12.
– Carbon-14 is radioactive, and carbon-12 is not.
– After death, the ratio drops because the carbon-14
decays into nitrogen-14, while the carbon-12 is stable
and remains.
– When the ratio is half the starting ratio, 5715 years
have passed since death occurred.
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Subatomic Physics
Section 2
Classroom Practice Problems
• A sample of barium-144 contains
5.0  10 9 atoms. The half-life is about 12 s.
–
–
–
–
What is the decay constant of barium-144?
How many atoms would remain after 12 s?
How many atoms would remain after 24 s?
How many atoms would remain after 36 s?
• Answers:
– 0.058 s-1
– 2.5  109 atoms, 1.2  109 atoms, 6.2  108 atoms
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Subatomic Physics
Half-Life
Click below to watch the Visual Concept.
Visual Concept
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Section 2
Subatomic Physics
Section 2
Now what do you think?
• Often scientists use radioactive carbon
dating to determine the age of fossils.
• What does the term radioactive mean?
• Are all atoms radioactive?
• If not, how are the radioactive atoms different from
those that are not radioactive?
• How can radioactivity be used to determine the
age of a fossil?
© Houghton Mifflin Harcourt Publishing Company
Subatomic Physics
TEKS
Section 3
The student is expected to:
8C describe the significance of mass-energy
equivalence and apply it in explanations of
phenomena such as nuclear stability, fission,
and fusion
8D give examples of applications of atomic
and nuclear phenomena such as radiation
therapy, diagnostic imaging, and nuclear
power and examples of applications of
quantum phenomena such as digital cameras
© Houghton Mifflin Harcourt Publishing Company
Subatomic Physics
Section 3
What do you think?
• Nuclear power and nuclear weapons are
important and frequently-discussed issues in the
world today.
• How does a nuclear reactor produce energy?
• What is nuclear about it?
• What problems are associated with nuclear power?
• Do atomic bombs and hydrogen bombs differ in the
way they produce energy?
• If so, how are they different?
© Houghton Mifflin Harcourt Publishing Company
Subatomic Physics
Nuclear Changes
• For nuclear changes to
occur naturally, energy
must be released.
– Binding energy must
increase.
– Lighter elements must
combine, and heavier
elements must reduce in
size.
– The greatest stability is for
atoms with mass numbers
between 50 and 60.
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Section 3
Subatomic Physics
Section 3
Fission
• Fission occurs when a large nucleus absorbs a
neutron and splits into two or more smaller
nuclei.
• Example of fission:
1
0
n+
235
92
*
U 
 236
U

 X + Y + neutrons
92
– It only occurs for heavy atoms.
– The * indicates an an unstable state that lasts for
about a trillionth of a second.
– X and Y can be different combinations of atoms that
have a total atomic number of 92.
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Section 3
Subatomic Physics
Fission
1
0
n+
235
92
U 

140
56
Ba +
93
36
1
0
Kr + 3 n
• A typical fission reaction is shown above.
• The products, Ba and Kr, have more binding energy than
the uranium.
• As a result, energy is released.
– Each fission yields about 100 million times the energy released
when burning a molecule of gasoline.
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Subatomic Physics
Chain Reaction
• On the average, 2.5
neutrons are released
with each fission.
• These neutrons are
then absorbed and
cause more fissions.
– A chain reaction
occurs.
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Section 3
Subatomic Physics
Nuclear Fission
Click below to watch the Visual Concept.
Visual Concept
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Section 3
Subatomic Physics
Section 3
Nuclear Reactors
• Reactors manage the fission rate by inserting
control rods to absorb some of the neutrons.
• Nuclear power plants and navy vessels use
fission reactions as an energy source.
– Reactors produce radioactive waste, and disposal is
one difficulty.
– Presently 20% of the U.S. electric power is generated
by nuclear reactors.
• Atomic bombs use uncontrolled fission.
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Subatomic Physics
Section 3
Fusion
• Light elements can combine and release energy
as well.
– Hydrogen atoms have less binding energy per
nucleon than helium atoms.
• Fusion is the source of a star’s energy.
– Hydrogen atoms fuse to form helium atoms.
– Much energy is released with each fusion.
• Hydrogen bombs use uncontrolled fusion.
– First tested in 1952 but never used in war
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Subatomic Physics
Section 3
Fusion as an Energy Source
• Fusion reactors are being developed.
• Advantages of fusion reactors:
– The fuel source, hydrogen from water, is cheap.
– The products of fusion are clean and are not
radioactive.
• Disadvantages of fusion:
– It requires extremely high temperatures of roughly
108 K to force atoms to fuse.
– It is difficult to keep the hydrogen atoms contained at
this temperature.
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Subatomic Physics
Nuclear Fusion
Click below to watch the Visual Concept.
Visual Concept
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Section 3
Subatomic Physics
Section 3
Now what do you think?
• Nuclear power and nuclear weapons are
important and frequently-discussed issues in the
world today.
– How does a nuclear reactor produce energy?
• What is nuclear about it?
– What problems are associated with nuclear power?
– Do atomic bombs and hydrogen bombs differ in the
way they produce energy?
• If so, how are they different?
© Houghton Mifflin Harcourt Publishing Company
Subatomic Physics
TEKS
Section 4
The student is expected to:
5A research and describe the historical
development of the concepts of gravitational,
electromagnetic, weak nuclear, and strong
nuclear forces
5H describe evidence for and effects of the
strong and weak nuclear forces in nature
© Houghton Mifflin Harcourt Publishing Company
Subatomic Physics
Section 4
What do you think?
• When the idea of the atom was first conceived,
it was thought to be a fundamental particle,
indivisible and indestructible. We now know
differently.
• List every particle you can think of that is smaller
than an atom.
• If you know the properties of these particles, list them as
well.
• Which of the particles on your list are fundamental?
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Subatomic Physics
Section 4
Fundamental Forces
• There are four fundamental interactions or forces in
nature:
–
–
–
–
strong
electromagnetic
weak
gravitational
• They exert force using the
exchange of mediating
particles.
– Photons are the mediating
particle exchanged between
electrons.
• This causes repulsion.
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Subatomic Physics
Section 4
Fundamental Forces
• Strong force
– Holds protons and neutrons together in the nucleus
• Electromagnetic force
– Creates forces between charged particles
– Holds atoms and molecules together
• Weak force
– A nuclear force that controls radioactive decay
• Gravitational force
– The weakest force
– Gravitons (the mediating particle) not yet discovered
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Subatomic Physics
Fundamental Forces
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Section 4
Subatomic Physics
Classification of Particles
• All particles are classified as
leptons, hadrons, or mediating
particles.
– Over 300 particles are
known.
• Leptons are thought to be
fundamental.
– Electrons are leptons.
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Section 4
Subatomic Physics
Classification of Particles
• Hadrons are composed of
smaller particles called
quarks.
– Quarks are thought to be
fundamental.
– Protons and neutrons are
hadrons.
• Two types of hadrons:
baryons and mesons
• Hadrons interact through all
four of the fundamental
forces, while leptons do not
participate in strong force
interactions.
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Section 4
Subatomic Physics
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Section 4
Section 4
Subatomic Physics
Classification of Particles
• Protons and neutrons are
baryons.
• What combination of up and
down quarks would make a
proton and a neutron?
– Two up quarks (+4/3) and one
down quark (-1/3) gives a
proton a charge of +1.
– One up quark (+2/3) and two
down quarks (-2/3) gives a
neutron a charge of zero.
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Subatomic Physics
Section 4
Combinations of Quarks
• Baryons and mesons are distinguished by their internal
structure.
• The particles above are a proton, a neutron, a pion, and
a kaon.
• Mesons are unstable, and are not constituents of
everyday matter.
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Subatomic Physics
Section 4
The Standard Model of Particle Physics
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Visual Concept
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Subatomic Physics
Section 4
The Standard Model
• The Standard Model is the current model used in particle physics.
• How many fundamental particles are there in the standard model?
– Six quarks, six leptons, and an antiparticle for each (24 total)
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Subatomic Physics
Evolution of the Four Forces
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Section 4
Subatomic Physics
Quarks and their Charges
Click below to watch the Visual Concept.
Visual Concept
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Section 4
Subatomic Physics
Section 4
Now what do you think?
• When the idea of the atom was first conceived, it
was thought to be a fundamental particle,
indivisible and indestructible. We now know
differently.
• List every particle you can think of that is smaller than
an atom.
• If you know the properties of these particles, list them as well.
• Which of the particles on your list are fundamental?
• What are the four fundamental forces and what
particle mediates each?
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