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The Nucleus
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Chapter 8
electron
neutron
proton
The Atom and Nucleus
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Atoms are the smallest particles of ordinary matter.
Every atom has a central core called the nucleus.
The nucleus contains protons and neutrons that
provide nearly all the atom’s mass.
Moving around the nucleus are the much lighter
electrons.
In a neutral atom, the number of protons and
electrons are equal.
http://www.metacafe.com/watch/yt2x3F08_8B80/lec_1_mit_5_111_principles_of_chemical_science_fall_2005/
The Atom and Nucleus
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The chief properties (except mass) of atoms,
molecules, solids, and liquids can be traced to the
behavior of atomic electrons.
The nucleus is important, too.
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The continuing evolution of the universe is powered by
energy from nuclear reactions and transformations as well
as the heat from the Earth’s interior.
All the energy at our command has a nuclear origin,
except for the energy of the tides (gravity).
The Atom and Nucleus
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JJ THOMSON - in 1898, he
suggested that atoms were
simply positively charged
lumps of matter with
electrons embedded in
them, like raisins in a
fruitcake.
His idea was taken seriously
since he had played an
important part in discovering
electrons.
The Atom and Nucleus
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ERNEST RUTHERFORD – (1871-1937) developed
the model of the atom that had a positive charge
concentrated in a central nucleus with the electrons
some distance away.
Rutherford wanted to know what was inside an
atom, so he probed the atom using Alpha Particles.
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Alpha particles are emitted by certain substances.
(discussed later in this chapter)
Alpha particles are 8000 times heavier than an electron
and each one has a +2e charge.
Rutherford Model of the Atom
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Rutherford experiment:
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Alpha-emitting substance
was placed behind a lead
screen with a small hole in
it, so that a narrow beam of
alpha particles was
produced.
The beam was aimed at a
thin gold foil.
A zinc sulfide screen, which
emits a flash of visible light
when struck with an alpha
particle, was set on the
other side of the foil.
Results of Rutherford Experiment
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Rutherford expected the alpha
particles to go right through the foil
with little or no deflection.
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Follows from Thomson model
which the electric charge inside an
atom is uniformly spread thru its
volume.
What he found was that, although
most of the alpha particles were
not deflected much, a few were
scattered through very large
angles.
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Some even backward
Conclusions from the
Rutherford Experiment
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Since alpha particles are relatively heavy and since those in the
experiment were traveling at high speeds, strong forces HAD to
be exerted upon them to cause such large deflections.
Rutherford found that the only way to explain the deflections
was to picture an atom with a tiny nucleus in which positive
charge existed and nearly all the mass existed;
And the electrons were some distance away from the nucleus.
In other words, AN ATOM IS MOSTLY EMPTY SPACE.
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That’s why most alpha particle go right through a thin foil.
However, when an alpha particle gets close to a nucleus, the
strong electric field there causes the alpha particle to be deflected.
The electrons, being so light, have little effect on the alpha
particles.
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IF ALL THE ELECTRONS
AND NUCLEI IN OUR
BODIES COULD BE
PACKED CLOSELY
TOGETHER, WE WOULD
BE NO LARGER THAN
SPECKS JUST VISIBLE
WITH A MICROSCOPE!
Nuclear Structure
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Proton
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Positively charged particle
+e charge
Mass 1836 times that of an
electron.
Neutron
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Particle with no charge
Mass 1839 times that of an
electron.
Nuclear Structure
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Elements are the simplest substances in the
bulk matter around us.
Over 100 elements are known:
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11 are gases
5 are liquids (Cs, Ga, Hg, Fr, Br)
Remaining are solids at room temperature and
atmospheric pressure
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Most of the solid elements are metals.
Nuclear Structure
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Atomic number – in a neutral atom of any
element, the number of protons or electrons
(since p=e). (small blue number in upper right corner of each
element box on periodic table)
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The atomic number is an element’s most basic
property since this determines how many
electrons its atoms have and how they are
arranged, which in turn determines the physical
and chemical behavior of the element.
Nuclear Structure
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Mass number – Also called nucleon number is the
number of protons and neutrons in an atom.
Atomic mass is a weighted average of the atomic
masses of the different isotopes of an element.
Units are the atomic mass unit (amu) (this is the number
located at bottom of each element box on periodic table)
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Carbon-12 (98.89%) & Carbon-13 (1.11%)
98.89/100 x 12.000 amu + 1.11/100 x 13.003354 amu =
12.011 amu
The number of neutrons of an element is
determined by taking the mass number and
subtracting the atomic number.
What is the difference between atomic
mass and mass number?
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Mass number and atomic mass are easily
confused. Let’s clear up the difference.
Concentrate on the 2nd word of each: number and
mass
Mass number is the count of the number of
nucleons in an isotope and requires no units
because it is simply a count.
Atomic mass is a measure of the total mass of an
atom whose units are the amu (atomic mass units).
What is the difference between atomic
mass number and mass number?
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What is the atomic mass of oxygen?
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15.9994 amu
What is the mass number of oxygen?
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16
Isotopes
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All atoms of a given element have the same
number of protons in their nuclei, but not
necessarily the same number of neutrons.
Isotopes of an element have atoms with the
same atomic number but different atomic
masses.
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Their number of neutrons varies.
All elements have isotopes.
Isotopes of Hydrogen
•About 1 in every 7000 hydrogen
atoms is a deuterium atom.
•Only about 2 kg of tritium of natural
origin is present of the Earth, nearly all
in the oceans.
•Tritium decays to a helium isotope.
•Nuclear reactions in the atmosphere
caused by cosmic rays from space
continually replenish the Earth’s
tritium.
Nuclide
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A nucleus with a
particular composition
is called a nuclide.
Symbols for nuclides
are represented as
follows:
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X is chemical symbol of
element
A is mass number
Z is atomic number
A
Z
35
17
Cl
X
7
3
Li
Radioactivity
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In 1896, Henri Becquerel
discovered radioactivity.
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accidentally
Uranium
Subsequent to Becquerel’s discovery, Pierre
and Marie Curie found two other radioactive
elements called polonium and radium.
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Radium turned out to be thousands of times more
radioactive than uranium.
Radioactivity
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Chemical reactions, heating, nor cooling
change the ability of a radioactive material to
emit radiation.
Radioactivity must be associated with atomic
nuclei because these are the only parts of
atoms not affected by such treatments.
The radioactivity of an element is due to the
radioactivity of one or more of its isotopes.
A nucleus is said to decay when it emits an
alpha or beta particle or a gamma ray.
Radioactivity Decay
How unstable nuclei change into stable ones
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Early experimenters found that a
magnetic field splits the radiation
from a radioactive material such a
radium into three parts:
Alpha Particles were deflected one
way
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Beta Particles were deflected
another way
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helium nuclei (probes used in
Rutherford experiment)
Positive charge
2 protons, 2 neutrons
Electrons
Negatively charged
Gamma Rays were not deflected
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High energy electromagnetic waves
Alpha and Beta Particles and Gamma Rays
Tam7s6_7
Why Does a Nucleus Decay?
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A nucleus decays because it is UNSTABLE.
Nuclei are unstable for several reasons:
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Nucleus has excess energy
Nucleus is too large to be stable
Nucleus has too many protons relative to number
of neutrons
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Larger nuclei need more neutrons than protons in order
to overcome the electrical repulsion of the protons
Nucleus has too many neutrons relative to
number of protons
5 Kinds of Radioactive Decay
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Gamma Decay
Alpha Decay
Beta Decay
Electron Capture
Positron Emission
Alpha Particles and Alpha Decay
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Alpha Particles – radiation is deflected as
though it consists of positively charged
particles.
Alpha particles are, in fact, the nuclei of
helium atoms.
The nuclei contain 2 protons and 2 neutrons.
Alpha particles are least penetrating.
Alpha decay occurs in nuclei too large to be
stable.
Alpha Decay
238/92U → 234/90Th + 4/2He2+ [1]
99% of He on the Earth comes from Alpha Decay of Uraneum Deposits
Beta Particles and Beta Decay
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Beta Particles – radiation is deflected as
though it consists of negatively charged
particles and are, in fact, electrons.
Beta particles are more penetrating than an
alpha, but less penetrating than a gamma
ray.
Beta decay occurs when one of the neutrons
in a nucleus with too many neutrons
spontaneously turns into a proton with the
emission of an electron.
Electron Emission ( neutron to proton
ratio too high)
Carbon has 6 protons and 8 Neutrons
Nitrogen has 7 protons and 7 Neutrons
One of the Carbon neutrons changed to a proton
And Emitted an electron plus a neutrino
The change involves the conversion of a neutron into a proton
with the emission of an electron and an antineutrino (n → p+e−+ν¯)
Gamma Rays and Gamma Decay
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Gamma Rays – are em waves whose
frequencies are higher than those of x-rays.
They are not affected by a magnetic field.
In gamma decay, a gamma ray is emitted by
a nucleus that has more than its normal
amount of energy.
The composition of the nucleus does not
change in gamma decay, unlike alpha and
beta decay.
Positron Emission
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In a nucleus with too few neutrons for
stability, one of the protons may become a
neutron with the emission of a positron.
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A positron is an electron that has a positive
charge rather than a negative one.
Electron Capture
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In a nucleus with too few neutrons for
stability, one of the electrons in the atom is
absorbed by one of the protons to form a
neutron in a process called electron
capture. (The reverse of Beta decay)
The Electron comes from one of the atoms
orbiting electrons
5 Kinds of Radioactive Decay
Half-life
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The half-life of a
radionuclide is the
period of time needed
for half of any initial
amount of the nuclide
to decay.
Radiation Hazards
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All ionizing radiation is harmful to living
tissue, although to varying degrees.
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Radiation hazards are easy to underestimate
because there is usually a delay between an
exposure and possible health consequences.
Radiation dosage is measured in sieverts
(Sv) where 1 Sv is the amount of any
radiation that has the same biological effects
as those produced when 1 kg of body tissue
absorbs 1 joule of x-rays or gamma rays.
Radiation Hazards
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The largest portion of
human exposure to
radiation is from radon,
which is naturally
occurring in the
environment.
Medical x-rays and
nuclear medicines are
second.
Sources of radiation exposure (U.S.)
Nuclear Energy
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The atomic nucleus is the energy source of
the reactors that produce some of the world’s
electricity.
It is also the energy source of destructive
weapons.
Nearly all the energy that keeps the sun and
stars shining comes from the nucleus.
Units of Mass and Energy
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The atomic mass unit (u) is the mass unit in atomic
physics since kg are much too large.
The atomic mass unit = 1 u = 1.66 x 10-27 kg
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1u is approximately equal to the mass of hydrogen (1.008u)
The electronvolt (eV) is the energy unit used in
atomic physics which is the energy gained by an
electron accelerated by a potential difference of 1
volt.
One electronvolt = 1 eV = 1.60 x 10-19 J
The energy equivalent (E=mc2) of a rest mass of 1u
is 931 MeV.
Binding Energy
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Binding energy is the missing energy that keeps a nucleus
together.
When a nucleus forms, energy is given off due to the action of
the forces that hold the neutrons and protons together.
The resulting nucleus therefore has LESS mass than the total
mass of the particles before interacting.
The Binding Energy is the energy equivalent of the missing mass
of a nucleus.
Greater binding energy means more energy must be supplied to
break up the nucleus.
Without binding energy, no nuclei more complex than the single
proton of hydrogen would be stable.
Binding Energy
Since the energy equivalent of 1u
of mass is 931 MeV, the energy
that corresponds to the missing
deuterium mass of 0.0024 u is:
Missing Energy = (0.0024u)(931MeV/u) = 2.2 MeV
Binding Energy of Deuterium
Binding Energy
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Nuclear binding energies are very high.
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2.2MeV for deuterium to 1640MeV for
209
83
Bi
Larger nuclei are all unstable and decay
radioactively.
To understand how large binding energies are, we
can compare them in terms of kJ/kg.
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A typical B.E. is 800 billion kJ/kg.
Boiling water has a heat of vaporization of 2260 kJ/kg.
Burning gasoline 4.7 x 104 kJ/kg.
Binding Energy per Nucleon-the most important graph in science
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Binding energy per nucleon – For a given
nucleus, the binding energy per nucleon is
found by dividing the total binding energy of
the nucleus by the number of nucleons
(protons and neutrons) it contains.
The binding energy for deuterium is:
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2.2MeV / 2=1.1MeV/nucleon
Binding Energy
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The greater the binding energy per
nucleon, the more stable the
nucleus.
The graph has its maximum of 8.8
MeV/nucleon when the number of
nucleons is 56.
That nucleus corresponds to an iron
isotope
56
26
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Fe
This is the most stable nucleus of
all, since the most energy is needed
to pull a nucleon away from it.
Larger and smaller nuclei are less
stable.
Binding Energy Per Nucleon
This figure is the key to energy production in the universe.
The Most Important Graph in Science
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Two conclusions can be made from figure 7-14.
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If a heavy nucleus is split into 2 medium size ones, each of the
new nuclei will have more binding energy per nucleon (and
hence less mass per nucleon) than the original nucleus did.
The extra energy will be given off.
Problem:
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If 235-U is broken into 2 smaller nuclei, the difference in b.e. per
nucleon is about 0.8 MeV. 235-U contains 235 nucleons. The
total energy given off can be calculated as:
(0.8 MeV/nucleon)x(235 nucleons) = 188
MeV
This is nuclear fission and involves a
hundred million time more energy per
atom than burning coal or oil.
The Most Important Graph in Science
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Two conclusions can be made from figure 7-14.
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The other notable conclusion from Fig. 7-14 is that joining
two light nuclei together into a single nucleus of medium
size also means more binding energy per nucleon in the
new nucleus.
If 2 deuterium nuclei combine to form a helium
nucleus, over 23 MeV is released!
This is nuclear fusion and is a very effective way to
obtain energy.
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Nuclear fusion is the main energy source of the sun and
other stars.
More About Fission and Fusion
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Nuclear Fission – Splitting of a nucleus
Splitting a nucleus is not easy.
When splitting a heavy nucleus you must NOT use
more energy to split than you get back from the
process.
In 1939, a discovery was made: a nucleus of 235-U
undergoes fission when struck by a NEUTRON.
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What actually occurs is the neutron is absorbed by the 235U nucleus and becomes 236-U.
236-U is so unstable that it almost immediately splits into 2
pieces.
Nuclear Fission
Nuclear Fission and Chain Reactions
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When a nucleus breaks apart, 2 or 3 neutrons are
set free.
A chain reaction can occur when at least one
neutron produced by each fission, on the average,
leads to another fission and does not either escape
or become absorbed without producing a fission.
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If exactly one neutron per fission causes another fission,
energy is released at a steady rate.
This is what occurs in nuclear reactors.
Chain reactions were first demonstrated by Enrico Fermi in
1942.
Chain Reaction (Nuclear Reactor)
Nuclear Fission and Chain Reactions
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If more than one neutron from each fission
causes another fission, the chain reaction
speeds up and an explosion occurs.
An atomic bomb makes use of this effect.
How Nuclear Reactors Works
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Reactors need 235U for the
fissionable isotope in the
production of electricity.
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Each fission of a 235U liberates
an average of 2.5 neutrons.
However, natural uranium
contains only 0.7% 235U. The
rest is 238U, an isotope that
captures the rapidly moving
neutrons emitted during 235U
fission but usually does not
undergo fission afterward.
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The neutrons absorbed by 238U
are wasted.
Did you know for every gram of
uranium that undergoes fission in a
reactor, 2.6 tons of coal must
be burned?
How Nuclear Reactors Work
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Since the neutrons absorbed by 238U are wasted,
and since 99.3% of natural U is 238U, too many
disappear for a chain reaction to occur in a solid
lump of natural U.
Slow neutrons are more likely to induce fission in
235U than fast ones.
To get around this problem of fast moving neutrons,
they are slowed down to induce fission in 235U.
Nuclear Reactors
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To slow down fission neutrons, the uranium
fuel in a reactor is mixed with a moderator, a
substance whose nuclei absorb energy from
fast neutrons that collide with them.
Hydrogen is widely used as a moderator in
the form of water.
Also, enriched uranium is used in nuclear
reactors where 235U content has been
increased to about 3%.
Nuclear Fusion
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Fusion – joining of 2 nuclei
Fusion is the sun and stars main energy
source.
The fusion of small nuclei to form larger ones
can give out even more energy per kg of
starting material than fission.
It is possible that fusion will become the
ultimate source of energy.
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Safe, almost nonpolluting,
supplying limitless fuel.
and
the
oceans
Nuclear Fusion
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Three conditions must
successful fusion reactor:
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by
a
100 million °C or more so that the nuclei are moving fast
enough to collide despite the repulsion of their positive
electric charges.
High concentration of nuclei
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met
High temperature
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be
To ensure collisions are frequent
Reacting nuclei must remain together long
enough to give off more energy than the reactor’s
operation uses.
Nuclear Fusion
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The fusion reaction that is the basis of current
research is:
2
1
H  H  He  n  17.6MeV
deuterium
3
1
tritium
4
2
1
0
helium
neutron
energy
Most of the energy given off is carried by the neutron that
is emitted.
About 0.015 percent of the waters of the world is deuteriumno scarcity.
A gallon of seawater has the potential for fusion energy
equivalent to the chemical energy in 600 gallons of gasoline.
Elementary Particles
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The electrons, protons, and neutrons of which
atoms are composed are considered elementary
particles because they cannot be broken down into
anything else.
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A great many other elementary particles besides electrons,
protons, and neutrons exist.
Although protons and neutrons consist of still
smaller particles called quarks, the quarks in a
nucleon stick together too tightly to permit the
nucleon from splitting apart, so nucleons are
regarded as elementary.
Elementary Particles
Antiparticles
The same but different….
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Nearly all elementary particles have
antiparticles.
The antiparticle of a given particle has the
same mass as the particle and behaves
similarly, but its electric charge is opposite in
sign.
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Antiproton p- is the antiparticle of the proton p+
Positron e+ is the antiparticle of the electron eNeutrons also have antiparticles that have
properties other than charge that are different.
Antiparticles
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Antiparticles are hard to find because when a
particle and its antiparticle come together they
destroy each other in a process called
annihilation.
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The loss of mass reappears as energy in the form of
gamma rays when electrons and positrons are
annihilated.
Unstable particles of other types may be produced
instead of gamma rays when protons and antiprotons
(or neutrons and antineutrons) are annihilated.
Annihilation
The Reverse of Annihilation
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Energy becomes matter and electric charge
being created where none existed before in
the process of Pair Production.
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A particle and its anti-particle materialize when a
high-energy gamma ray passes near an atomic
nucleus.
The antiparticles formed in pair production exist
for only a short time before they meet their particle
counterparts in ordinary matter and are
annihilated.
Fundamental Interactions
Only 4 Give Rise to All Physical Processes

Elementary particles interact with each other
in only four ways. In order of decreasing
strength, these interactions are:
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Strong
Electromagnetic
Weak
Gravitational
Fundamental Interactions
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Strong interaction- which holds protons and
neutrons together to form atomic nuclei
despite the mutual repulsion of the protons.
Electromagnetic interaction– which gives
rise to electric and magnetic forces between
charged particles.
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Electromagnetic forces act on electrons, unlike
the strong forces.
Fundamental Interactions
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Weak interaction – which affects all particles
and, by causing beta decay, helps determine
the compositions of atomic nuclei.
Gravitational interaction – which is
responsible for the attractive force one mass
exerts on another.
4 Fundamental Interactions
Unifying the Interactions
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In the 1960’s, Weinberg and Salam indicated
that the weak and electromagnetic
interactions are really different aspects of the
same basic phenomenon.
A more recent development links the
electroweak and strong interactions.
Although a grand unified theory is still far
from its final form, it seems to be on the right
track.
A GOAL OF PHYSICS:
Developing a Single Theory of the Universe
Leptons and Hadrons

All elementary particles fall into two broad
categories that depend on their response to the
strong interaction:

Leptons- are not affected by this interaction and seem to
be point particles with no size or internal structure.

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The electron and neutrino (p.279) are leptons.
Hadrons- are subject to the strong interaction and have
definite sizes and internal structures.
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The proton and neutron are hadrons.
Several hundred other particles are also included in the
hadron family, most with very short lifetimes (less than a
billionth of a second for some).
Quarks
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The particles that make up hadrons are quarks.
Only six kinds of quarks are needed to account
for all known hadrons.
A quark’s electric charge is less than  e
Quarks do not seem to be able to exist outside of
hadrons.
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No quark has ever been found by itself.
The End