Transcript Chapter 22 - Nuclear Chemistry

NUCLEAR CHEMISTRY
Section 22-1: The Nucleus
Objectives
1. Explain what nucleons are.
2. Explain what a nuclide is, and describe
the different ways it can be written.
3. Define nuclear binding energy.
4. Explain the relationship between nucleon
number and stability of nuclei.
The Nucleus
 The nucleus is composed of nucleons
 Protons
 Neutrons
 A nucleus is characterized by two
numbers
 mass number (A; total # of nucleons)
 atomic number (Z; number of protons)
A
Z
E
27
13
Al
 total number of nucleons is 27
 total number of protons is 13
 the number of neutrons is 14
 in nuclear chemistry, an atom is referred
to as a nuclide.
Subatomic Particles
one atomic mass unit (u) is defined as 1/12th the
mass of a carbon-12 atom
Particle
electron
proton
neutron
mass in kg
9.11 x 10
-31
mass in u
-4
kg
5.485 x 10 u
1.673 x 10
-27
kg
1.0073 u
1.675 x 10
-27
kg
1.0087 u
Mass of
4
2
He
The actual mass is 4.00260 amu.
Why the difference in mass? mass defect
Einstein’s Equation
•
•
•
•
Energy and mass can be interconverted
E = mc2
E- energy, m-mass, c-speed of light
When protons & neutrons are packed
together to form a nucleus, some of the
mass is converted to energy and
released.
• Mass is lost
Nuclear Binding Energy
• Nuclear Binding Energy – the energy
released when a nucleus is formed from
protons and neutrons.
• Can also be thought of as the amount of
energy required to break apart the nucleus.
of an existing atom.
• The higher the binding energy, the more
tightly the nucleus is held together.
Binding Energy Curve
• graph peaks at A=56
• the higher the BE, the
more stable the
nucleus
• mass number of 56 is
maximum possible
stability
How Many Neutrons?
 Stable nuclides have certain characteristics
 The number of neutrons in a nucleus can
vary as we have seen
 Range limited by the degree of instability
created by:


having too many neutrons
too few neutrons
 Stable nuclei do not decay spontaneously
 Unstable nuclei have a certain probability to
decay
Nuclear Stability Facts
•
•
•
•
265 stable nuclides
For light elements (Z  20), Z:N ratio is ~1
Example: helium-4 (2 neutrons, 2 protons)
Z:N ratio increases toward 1.5 for heavy
elements
• Example: lead-206 (124 neutrons, 82 protons)
• For Z > 83 (bismuth), all isotopes are
radioactive
• This is due to repulsive forces of the protons
Nuclear Stability Facts
 Stable nuclei tend to have an even
number of nucleons.
 Out of 265 stable nuclides, 159 have even
numbers of both protons and neutrons.
 Only 4 nuclides have odd numbers of
both.
 The most stable nuclides are those having:
2, 8, 20, 28, 50, 82, 126
Protons, neutrons or total nucleons
Examples:
 Sn (Z=50) has 10 isotopes; In (Z=49) &
Sb (Z=51) have only 2 isotopes
 Pb-208 has a double magic number
(126n, 82p) & is very stable
 “Magic numbers” of protons or neutrons
which are unusually stable
Nuclear Reactions
 Nuclear Reaction – a reaction that affects
the nucleus of an atom.
 Unstable nuclei undergo spontaneous
changes that change their number of protons
and neutrons.
 They also give off a large amount of energy
and increase their stability in the process.
Nuclear Reactions
 In a nuclear reaction, the total of the
atomic numbers and the total of the mass
numbers must be equal on both sides of
the equation.
 Example:
9
4
Be +
4
2
He
12
6
C +
1
0
n
9
4
Be +
4
2
He
12
6
C +
1
0
n
 Note that when the atomic number
changes, the identity of the element
changes.
 Transmutation – a change in the identity
of a nucleus (element) as a result of a
change in the number of its protons.
Sample Problem
Identify the product that balances the following
nuclear reaction:
212
84
Po
?
+
4
2
He
Sample Problem
Identify the product that balances the following
nuclear reaction:
212
84
Po
208
82
Pb
+
4
2
He
Classwork
Problems 1-3, page 704
Homework
Page 724
Problems: 31, 33 and 40
Section 22-2
Radioactive Decay
Radioactivity
Objectives
1. Define the terms radioactive decay and
nuclear radiation.
2. Describe the different types of radioactive
decay.
3. Define the term half-life, and how it relates
to stability.
Radioactivity
 The spontaneous decay of an unstable
nucleus into a more stable nucleus.
 Energy is released.
 Certain isotopes are just not stable and
will spontaneously decay.
Types of Radioactive Decay
 All elements with 84 or more protons
(Polonium) are unstable, and will undergo
radioactive decay.
 Naturally occurring radioactive isotopes
decay in four primary ways:
 Alpha particle emission
 Beta particle emission
 Gamma radiation emission
 Positron emission
Alpha Emission
Alpha particle (a) – is two protons and two
neutrons bound together and emitted from the
nucleus.
They are helium nuclei with a charge of +2.
It has no electrons.
Represented by the symbol: 2 He or 2 a
Restricted to heavy elements: Ex. uranium
4
4
Alpha Emission
Process which is effective to lose a lot of
mass form the element.
Example:
210
84
Po
206
82
Pb
+
4
2
a
The atomic number decreases by two (a new
element) and the mass number decreases
by four.
Alpha Emission

+
(or
 Quick way for a large atom to lose a lot of nucleons
235
92 U
231
90 Th
4
2 a
4
2 He)
Beta Emission
Beta particle – is essentially an electron that’s
0
emitted from the nucleus: -1 b
In the nucleus, a neutron is converted (decayed)
1
1
0
into a proton and an electron. 0 n
1 p + -1 b
The electron is emitted as a beta particle.
Represented by the symbol:
e or -1 b
0
-1
0
A good way to decrease the number of neutrons.
Beta Emission
By decreasing the number of neutrons you
improve the neutron/proton ratio.
Example:
131
53
I
131
54
Xe
+
0
-1
b
The mass number stays the same in going
from I-131 to Xe-131 but the atomic number
increases by 1. Loss of neutron!
Beta Emission
• 19 K
20 Ca + -1 b
• Identity of atom changes
40
40
0
Gamma Emission
Gamma rays – are high energy
electromagnetic waves emitted from the
nucleus.
There is no mass change with gamma
emission, only radiation.
Usually occurs immediately following other
types of decay.
Not shown in a balanced nuclear reaction.
Gamma Emission
• An example is cobalt-60 (Co-60) which gives
off a large amount of gamma radiation.
• Co-60 used in the radiation treatment of
cancer.
Electromagnetic Radiation
• Electromagnetic radiation is a form of
energy that can pass through empty
space
• It is not just a particle, and it is not just a
wave. It may be both.
Electromagnetic Radiation
• Gamma rays are similar to x-rays – high
energy, short wavelength radiation.
Positron Emission
Positron particle – is essentially an electron
that has a positive charge.
A proton can be converted into a neutron by
1
1
0
emitting a positron 1 p
0 n + +1 b
A useful way to decrease the number of
protons.
Represented by the symbol:
e or +1 b
0
+1
0
Positron Emission
Example:
38
19
K
38
18
Ar
+
0
+1
b
Notice that the atomic number decreases
by one but the mass number stays the
same.
Can be viewed as the opposite of beta
emission.
Half-Life
Half-life - the amount of time it takes for onehalf of a radioactive sample to decay is called
the half-life of the isotope
It is given the symbol: t1/2
No two radioactive isotopes decay at the
same rate
Half-Life
Useful application of half-life is radioactive
dating. Used to determine the age of things.
Carbon-14 (t1/2 – 5715 years) dating can be
used to determine the age of something that
was once alive.
Examples include animal and plant species.
Cannot be used to determine the age of
rocks.
Half-Life
Radium-226 has a half-life of 1599 years.
Half of a given amount of radium-226
decays in 1599 years.
In another 1599 years, half of the
remaining radium-226 will decay.
This will continue until there is a negligible
amount of radium-226 remaining.
Half-Life
Decay of radium-226
Half-Life
• Each radioactive element has its own halflife.
• More stable elements decay slowly and
have longer half-lives.
• Less stable elements decay quicker and
have shorter half-lives
Radioactive Decay Rates
Half-Life
The time required for half of a sample to decay
Half-Life
Number of
Half-Lives
Fraction of
Initial Amount
Remaining
Amount
Remaining (mg)
0
1
2
3
4
5
1
1/2
1/4
1/8
1/16
1/32
20.00 (initial)
10.00
5.00
2.50
1.25
0.625
Half-Life
Problem:
Phosphorus-32 has a half-life of 14.3 days.
How many milligrams of phosphorus-32
remain after 57.2 days if you start with 4.0 mg
of the isotope.
First determine the number of half-lives that
have elapsed!
Half-Life
Answer:
First determine the number of half-lives that
have elapsed!
57.2 days / 14.3 days
4.0 mg
2.0
1
=
4 half lives
1.0
2
0.5
3
0.25 mg
4
Classwork
Page 709
Problems 1, 3, 4 and 6
Review HW
Page 724
Problems: 31, 33 and 40
Decay Series
• Some nuclides (particularly those Z>83)
cannot attain a stable, nonradioactive
nucleus by a single emission.
• The product of such an emission is itself
radioactive and will undergo a further
decay process.
• Heavy nuclei may undergo a whole decay
series of nuclear disintegrations before
reaching a nonradioactive product.
Trying To Reach Nuclear
Stability
Decay Series – a series of radioactive
nuclides produced by successive radioactive
decay until a stable nuclide is reached.
The heaviest nuclide of each series is called
the parent nuclide.
The nuclides produced are called daughter
nuclides.
The Four Known Decay
Series
Parent
Radioisotope
# of Decay
Steps
Final Product
of Series
Uranium-238
14
Lead-206
Thorium-232
10
Lead-208
Uranium-235
11
Lead-207
Plutonium-241
13
Bismuth-209
Artificial Transmutations
 Transmutation – a change in the identity of a
nucleus (element) as a result of a change in
the number of its protons.
 Artificial radioactive nuclides are radioactive
nuclides not found naturally on Earth.
 Artificial Transmutation – bombardment of
stable nuclei with charged and uncharged
particles (protons, alpha particles and
neutrons)
Artificial Transmutations
 Great quantities of energy are required
to bombard nuclei with these particles.
 These reactions are usually done by
accelerating the particles in a high
magnetic or electrical field.
 The instrument used is a particle
accelerator.
Artificial Transmutations
Used to produce all the elements in the
periodic table with more than 92 protons in
their nucleus.
Also used to produce technetium (43 protons),
astatine (85), francium (87) and promethium
(61).
Examples of elements created by artificial
transmutation
Homework
Page 724
Problems 36 - 39
Section 22-4
Nuclear Fission
and
Nuclear Fusion
Objectives
1. Define the terms nuclear fission, chain
reaction and nuclear fusion.
2. Explain how a fission reaction is used to
generate power.
3. Discuss fusion reactions.
Nuclear Fission
Nuclear Fission – a very heavy nucleus
splits into more stable nuclei of
intermediate mass.
This process releases an enormous
amount of energy.
Can occur spontaneously or when nuclei
are bombarded with particles.
Nuclear Fission
Example: Uranium-235
When uranium-235 is bombarded with
neutrons, the uranium nuclei becomes
unstable.
The nucleus splits into medium-mass parts
with the emission of more neutrons.
The mass of the products is less than the
mass of the reactants. The missing mass is
converted to energy.
Example: Uranium-235
235
U
92
+
1
0 n
142
Ba
56
+
91
36 Kr
The product are barium, krypton and
3 more neutrons.
The reaction is balanced.
+ 3
1
0 n
Nuclear Fission
When fission of an atom bombarded by
neutrons produces more neutrons, a chain
reaction can occur.
Chain Reaction – a reaction in which material
that starts the reaction is also one of the
products and can start another reaction.
In the last slide neutrons are generated from
the initial reaction and can start a second
reaction.
The chain reaction continues until all of the
uranium-235 atoms have split or until the
neutrons fail to strike uranium-235 nuclei.
Critical Mass - the minimum amount of
nuclide that provides the number of neutrons
needed to sustain a chain reaction.
Atomic Bombs – uncontrolled chain reactions.
Nuclear Reactors – controlled chain reactions.
Nuclear Fission
Only two fissionable isotopes are used
during nuclear reactions.
Uranium-235 and plutonium-239
Uranium-238 (the more abundant isotope)
only produces 1 neutron. This will not
sustain a chain reaction.
238
92
U +
1
0
n
145
56
Ba +
93
36 Kr
1
+ 10 n
Atomic Bombs
In an atomic bomb, two pieces of a fissionable
isotope are kept apart. Each piece by itself is
not going to explode.
When time comes for the bomb to explode,
conventional explosives force the two pieces
together to cause a critical mass.
The resulting chain reaction is uncontrolled,
releasing a tremendous amount of energy.
Nuclear Power Plants
The secret to controlling a chain reaction is to
control the neutrons.
If the neutrons can be controlled, then the
energy produced can be controlled.
That is what takes place with nuclear power
plants.
Nuclear Power Plants
In many respects, a nuclear power plant is
similar to a conventional power plant (coal, oil,
natural gas).
In these power plants the fuel is burned and
the heat produced is used to boil water, which
in turn is used to make steam.
The steam is then used to turn a turbine that’s
attached to a generator that produces
electricity.
Nuclear Power Plants
In a nuclear power plant the heat is
produced through a fission reaction.
The fissionable isotope (uranium-235) is
contained in fuel rods in the reactor core.
Control rods made of boron or cadmium
(not fissionable) are also in the reactor
core.
Nuclear Power Plants
The control rods act like neutron sponges
to control the rate of radioactive decay of
the uranium-235.
Operators can stop a chain reaction by
pushing all the control rods into the reactor
core, where they absorb all the neutrons.
Operators can pull out the control rods
slowly to produce the desired amount of
heat.
Nuclear Power Plants
Water is circulated through the reactor core,
and the heat generated by the fission
reaction is absorbed
The water flows to a steam generator where
steam is produced.
The steam then is used to turn the turbine
that generates electricity.
The water is recycled in a closed system.
Nuclear Reactor
Nuclear Fusion
Nuclear Fusion – light-mass nuclei combine
to form a heavier, more stable nucleus.
Nuclear fusion is essentially the opposite of
nuclear fission.
Nuclear fusion releases more energy than
nuclear fission.
Nuclear Fusion
Nuclear fusion reactions are what power the
sun for light and heat.
1
41 H
4
2
He + 2 1 b
0
Nuclear Fusion
Uncontrolled fusion reactions of hydrogen
are the source of energy for the hydrogen
bomb.
A fission reaction is used to provide the heat
and pressure necessary for the fusion
reaction.
A hydrogen bomb is about 1,000 times more
powerful than an atomic bomb.
Nuclear Fusion
Nuclear fusion: the goal of scientists has
been to control a fusion reaction.
Fusion reactions can provide an unlimited
supply of energy that has no nuclear
waste.
The product from a fusion reaction is
helium, already found in our atmosphere.
Nuclear Fusion
Three major obstacles need to be solved:
1) Temperature - The estimated temperature
required to start the reaction is 40,000,000K.
2) Time – nuclei must be held together long
enough.
3) Containment – At the required temperature
all known material would vaporize.
Classwork
1) Distinguish between nuclear fission and
fusion
2) Define chain reaction
3) Explain how fusion can be a good
source of energy
Review Classwork
Page 724
Problems: 36 - 39
End of Chapter
Radiation Energetics
 Alpha Particles


relatively heavy and doubly charged
lose energy quickly in matter
 Beta Particles


much smaller and singly charged
interact more slowly with matter
 Gamma Rays & X-rays


high energy
more lengthy interaction with matter
Hazards of Radiation Types
§ Alpha Emissions


easily shielded
considered hazardous if alpha emitting material
is ingested or inhaled
§ Beta Emissions


shielded by thin layers of material
considered hazardous if a beta emitter is
ingested or inhaled
§ Gamma Emissions


need dense material for shielding
considered hazardous when external to the body
The Radon Story
Radon-222
§ Originates from U-238 which occurs
naturally in most types of granite
§ Radon-222 has a half-life of 3.825 days
§ It decays via alpha emissions
§ This isotope is a particular problem
because it is a gas which can leave the
surrounding rock and enter buildings with
the atmospheric air