Transcript 25.2 Nuclear Transformations

25.2 Nuclear Transformations >
Chapter 25
Nuclear Chemistry
25.1 Nuclear Radiation
25.2 Nuclear Transformations
25.3 Fission and Fusion
25.4 Radiation in Your Life
1
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25.2 Nuclear Transformations >
CHEMISTRY
& YOU
What is the source of radon in homes?
Radon may
accumulate in a
basement that is
not well ventilated.
Test kits are
available to
measure the
levels of radon in a building.
2
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
Nuclear Stability and Decay
What determines the type of decay a
radioisotope undergoes?
3
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
The nuclear force is an attractive force
that acts between all nuclear particles that
are extremely close together, such as
protons and neutrons in a nucleus.
4
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
The nuclear force is an attractive force
that acts between all nuclear particles that
are extremely close together, such as
protons and neutrons in a nucleus.
• At these short distances, the nuclear force
dominates over electromagnetic repulsions
and holds the nucleus together.
5
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25.2 Nuclear Transformations >
Interpret Data
The stability of a nucleus depends on the
ratio of neutrons to protons.
• This graph
shows the
number of
neutrons vs. the
number of
protons for all
known stable
nuclei.
6
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25.2 Nuclear Transformations >
Interpret Data
The stability of a nucleus depends on the
ratio of neutrons to protons.
• The region of the
graph in which
these points are
located is called
the band of
stability.
7
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25.2 Nuclear Transformations >
Interpret Data
The stability of a nucleus depends on the
ratio of neutrons to protons.
• For elements of low
atomic number
(below about 20),
this ratio is about 1.
• Above atomic
number 20, stable
nuclei have more
neutrons than
protons.
8
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
A nucleus may be unstable and undergo
spontaneous decay for different reasons.
The neutron-to-proton ratio in a
radioisotope determines the type of decay
that occurs.
9
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
Some nuclei are unstable because they
have too many neutrons relative to the
number of protons.
• When one of these nuclei decays, a neutron
emits a beta particle (fast-moving electron) from
the nucleus.
10
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
Some nuclei are unstable because they
have too many neutrons relative to the
number of protons.
• When one of these nuclei decays, a neutron
emits a beta particle (fast-moving electron) from
the nucleus.
– A neutron that emits an electron becomes a proton.
1
0
11
n
1
1
p
+
0
–1
e
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
Some nuclei are unstable because they
have too many neutrons relative to the
number of protons.
• When one of these nuclei decays, a neutron
emits a beta particle (fast-moving electron) from
the nucleus.
– A neutron that emits an electron becomes a proton.
1
0
n
1
1
p
+
0
–1
e
– This process is known as beta emission.
– It increases the number of protons while
decreasing the number of neutrons.
12
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
Radioisotopes that undergo beta emission
include the following.
66
29
13
Cu
66
30
14
6
14
7
C
Zn +
N +
0
–1
0
–1
e
e
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
Other nuclei are unstable because they
have too few neutrons relative to the
number of protons.
• These nuclei increase their stability by
converting a proton to a neutron.
– An electron is captured by the nucleus during this
process, which is called electron capture.
59
28
37
18
14
Ni +
0
–1
Ar +
0
–1
e
59
27
Co
e
37
17
Cl
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
A positron is a particle with the mass of an
electron but a positive charge.
• Its symbol is
0
+1
e.
• During positron emission, a proton changes to
a neutron, just as in electron capture.
8
5
15
8
B
8
4
O
15
7
Be +
N +
0
+1
0
+1
e
e
– When a proton is converted to a neutron, the
atomic number decreases by 1 and the number of
neutrons increases by 1.
15
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
Nuclei that have an atomic number greater
than 83 are radioactive.
• These nuclei have both too many neutrons and
too many protons to be stable.
– Therefore, they undergo radioactive decay.
• Most of them emit alpha particles.
– Alpha emission increases the neutron-to-proton
ratio, which tends to increase the stability of the
nucleus.
16
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
In alpha emission, the mass number
decreases by four and the atomic number
decreases by two.
226
88
232
90
17
Ra
222
86
Th
228
88
Rn +
4
2
He
Ra +
4
2
He
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Nuclear Stability and
25.2 Nuclear Transformations > Decay
Recall that conservation of mass is an
important property of chemical reactions.
• In contrast, mass is not conserved during
nuclear reactions.
• An extremely small quantity of mass is
converted into energy released during
radioactive decay.
18
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25.2 Nuclear Transformations >
During nuclear decay, if the atomic
number decreases by one but the mass
number is unchanged, the radiation
emitted is
A. a positron.
B. an alpha particle.
C. a beta particle.
D. a proton.
19
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25.2 Nuclear Transformations >
During nuclear decay, if the atomic
number decreases by one but the mass
number is unchanged, the radiation
emitted is
A. a positron.
B. an alpha particle.
C. a beta particle.
D. a proton.
20
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25.2 Nuclear Transformations > Half-Life
Half-Life
How much of a radioactive sample
remains after each half-life?
21
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25.2 Nuclear Transformations >
Interpret Graphs
A half-life (t 1) is the time required for one2
half of the nuclei in a radioisotope sample
to decay to products.
After each halflife, half of the
original
radioactive
atoms have
decayed into
atoms of a new
element.
22
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25.2 Nuclear Transformations > Half-Life
Comparing Half-Lives
Half-lives can be as short as a second or as
long as billions of years.
Half-Lives of Some Naturally Occurring Radioisotopes
Isotope
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Half-life
Radiation emitted
Carbon-14
5.73 × 103 years
b
Potassium-40
1.25 × 109 years
b, g
Radon-222
3.8 days
a
Radium-226
1.6 × 103 years
a, g
Thorium-234
24.1 days
b, g
Uranium-235
7.0 × 108 years
a, g
Uranium-238
4.5 × 109 years
a
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25.2 Nuclear Transformations > Half-Life
Comparing Half-Lives
• Scientists use half-lives of some long-term
radioisotopes to determine the age of ancient
objects.
• Many artificially produced radioisotopes have
short half-lives, which makes them useful in
nuclear medicine.
– Short-lived isotopes are not a long-term
radiation hazard for patients.
24
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25.2 Nuclear Transformations > Half-Life
Comparing Half-Lives
Uranium-238 decays through a complex series of
unstable isotopes to the stable isotope lead-206.
• The age of uraniumcontaining minerals can be
estimated by measuring
the ratio of uranium-238 to
lead-206.
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• Because the half-life of
uranium-238 is 4.5 × 109
years, it is possible to use
its half-life to date rocks as
old as the solar system.
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25.2 Nuclear Transformations >
CHEMISTRY
& YOU
Uranium compounds are found in
rocks and in soils that form from these
rocks. How can these uranium
compounds lead to a buildup of radon
in homes and other buildings?
26
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25.2 Nuclear Transformations >
CHEMISTRY
& YOU
Uranium compounds are found in
rocks and in soils that form from these
rocks. How can these uranium
compounds lead to a buildup of radon
in homes and other buildings?
Radon gas is a product of the decay of
uranium. As the uranium compounds in
the soil beneath homes and buildings
decay, radon is produced and seeps into
the structure.
27
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25.2 Nuclear Transformations > Half-Life
Radiocarbon Dating
Plants use carbon dioxide to produce
carbon compounds, such as glucose.
• The ratio of carbon-14 to other carbon isotopes
is constant during an organism’s life.
• When an organism dies, it stops exchanging
carbon with the environment and its radioactive
14
6 C atoms decay without being replaced.
• Archaeologists can use this data to estimate
when an organism died.
28
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25.2 Nuclear Transformations > Half-Life
Exponential Decay Function
You can use the following equation to
calculate how much of an isotope will
remain after a given number of half-lives.
A = A0 
1
2
n
• A stands for the amount remaining.
• A0 stands for the initial amount.
• n stands for the number of half-lives.
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25.2 Nuclear Transformations > Half-Life
Exponential Decay Function
A = A0 
1
2
n
• The exponent n indicates how many times A0
must be multiplied by 12 to determine A.
30
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25.2 Nuclear Transformations > Half-Life
Exponential Decay Function
This table shows examples in which
n = 1 and n = 2.
Decay of Initial Amount (A0) of
Radioisotope
Half-Life Amount Remaining
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0
1
A0 × (12 )0 = A0
A0 × (12 )1 = A0 ×12
2
A0 × (12 )2 = A0 ×12
×
1
2
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25.2 Nuclear Transformations >
Sample Problem 25.1
Using Half-Lives in Calculations
Carbon-14 emits beta radiation and decays with a
half-life (t 12 ) of 5730 years. Assume that you start
with a mass of 2.00 × 10–12 g of carbon-14.
a. How long is three half-lives?
b. How many grams of the
isotope remain at the end of
three half-lives?
32
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25.2 Nuclear Transformations >
Sample Problem 25.1
1 Analyze List the knowns and the unknowns.
• To calculate the length of three half-lives,
multiply the half-life by three.
• To find the mass of the radioisotope
1
remaining, multiply the original mass by 2 for
each half-life that has elapsed.
KNOWNS
UNKNOWNS
t12 = 5730 years
3 half-lives = ? years
initial mass (A0) = 2.00 × 10–12
g
mass remaining = ? g
number of half-lives (n) = 3
33
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25.2 Nuclear Transformations >
Sample Problem 25.1
2 Calculate Solve for the unknowns.
a. Multiply the half-life of carbon-14 by
the total number of half-lives.
t 12 × n = 5730 years × 3 = 17,190 years
34
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25.2 Nuclear Transformations >
Sample Problem 25.1
2 Calculate Solve for the unknowns.
b. The initial mass of carbon-14 is
reduced by one-half for each half-life.
So, multiply by 12 three times.
Remaining mass = 2.00 × 10–12 g ×12
×
= 0.250 × 10–12 g
1
×
2
1
2
= 2.50 × 10–13 g
35
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25.2 Nuclear Transformations >
Sample Problem 25.1
2 Calculate Solve for the unknowns.
b. You can get the same answer by
using the equation for an exponential
decay function.
3
1 n
1
–12 g)
=
(2.00
×
10
2
2
()
A = A0
= (2.00 ×
10–12
()
g)( )
1
8
= 0.250 × 10–12 g
= 2.50 × 10–13 g
36
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25.2 Nuclear Transformations >
Sample Problem 25.1
3 Evaluate Do the results make sense?
• The mass of carbon-14 after three half-lives
should be one-eighth of the original mass.
• If you divide 2.5 × 10–13 g by 2.00 × 10–12 g,
you will get 12.5%, or 18 .
37
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25.2 Nuclear Transformations >
The half-life of phosphorus-32 is
14.3 days. How many milligrams of
phosphorus-32 remain after 100.1
days if you begin with 2.5 mg of the
radioisotope?
38
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25.2 Nuclear Transformations >
The half-life of phosphorus-32 is
14.3 days. How many milligrams of
phosphorus-32 remain after 100.1
days if you begin with 2.5 mg of the
radioisotope?
1 half-life
n = 100.1 days ×
= 7 half-lives
14.3 days
1 n
1 7
2 = (2.5 mg) 2
1
= (2.5 mg) 128
()
A = A0
39
()
( ) = 2.0 × 10
–2
mg
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25.2 Nuclear Transformations > Transmutation Reactions
Transmutation Reactions
What are two ways in which
transmutation can occur?
40
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25.2 Nuclear Transformations > Transmutation Reactions
For thousands of years, alchemists tried
to change lead into gold.
• What they wanted to achieve is
transmutation, or the conversion of an atom
of one element into an atom of another
element.
41
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25.2 Nuclear Transformations > Transmutation Reactions
Transmutation can occur by radioactive
decay, or when particles bombard the
nucleus of an atom.
• The particles may be protons, neutrons,
alpha particles, or small atoms.
42
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25.2 Nuclear Transformations > Transmutation Reactions
Ernst Rutherford performed the earliest
artificial transmutation in 1919.
• He bombarded nitrogen gas with alpha
particles.
14
4
18
7N +
2 He
9F
Nitrogen-14
Alpha
particle
Fluorine-18
• The unstable fluorine atoms quickly decay to
form a stable isotope of oxygen and a proton.
18
9
F
Fluorine-18
43
17
8
O +
Oxygen-17
1
1
p
Proton
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25.2 Nuclear Transformations > Transmutation Reactions
Rutherford’s experiment eventually led to
the discovery of the proton.
P
1
1
Proton
4
2
He
Alpha
particle
14
7
N
Nitrogen
atom
18
9
F
Unstable
fluorine atom
17
8
O
Oxygen
• He and other scientists noticed a pattern as they did
different transmutation experiments. Hydrogen nuclei
were emitted.
• Scientists realized that these hydrogen nuclei (protons)
must have a fundamental role in atomic structure.
44
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25.2 Nuclear Transformations > Transmutation Reactions
James Chadwick’s discovery of the neutron
in 1932 also involved a transmutation
experiment.
• Neutrons were produced when beryllium-9
was bombarded with alpha particles.
9
4
Be +
Beryllium-9
45
4
2
He
Alpha
particle
12
6
C +
1
0
n
Carbon-12 Neutron
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25.2 Nuclear Transformations > Transmutation Reactions
Elements with atomic numbers above 92,
the atomic number of uranium, are called
transuranium elements.
• None of these elements occurs in nature.
• All of them are radioactive.
• All transuranium elements undergo
transmutation.
46
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25.2 Nuclear Transformations > Transmutation Reactions
Transuranium elements are synthesized in nuclear
reactors and nuclear accelerators.
• Reactors produce beams of lowenergy particles.
• Accelerators are used to increase
the speed of bombarding
particles to very high speeds.
– The European Organization for
Nuclear Research, CERN, has
a number of accelerators. The
figure at right shows CERN’s
largest accelerator.
47
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25.2 Nuclear Transformations > Transmutation Reactions
• When uranium-238 is bombarded with the relatively
slow neutrons from a nuclear reactor, some uranium
nuclei capture these neutrons. The product is uranium239.
238
1
239
92
U +
0
n
92
U
• Uranium-239 is radioactive and emits a beta particle.
The other product is an isotope of the artificial
radioactive element neptunium (atomic number 93).
239
92
U
239
93
Np +
0
–1
e
• Neptunium is unstable and decays, emitting a beta
particle and a second artificial element, plutonium
(atomic number 94).
239
93
48
Np
239
94
Pu +
0
–1
e
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25.2 Nuclear Transformations > Transmutation Reactions
Scientists in Berkeley, California,
synthesized the first two artificial elements
in 1940.
• Since that time, more than 20 additional
transuranium elements have been produced
artificially.
49
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25.2 Nuclear Transformations >
Which of the following always changes
when transmutation occurs?
A. The number of electrons
B. The mass number
C. The atomic number
D. The number of neutrons
50
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25.2 Nuclear Transformations >
Which of the following always changes
when transmutation occurs?
A. The number of electrons
B. The mass number
C. The atomic number
D. The number of neutrons
51
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25.2 Nuclear Transformations > Key Concepts
The neutron-to-proton ratio in a
radioisotope determines the type of decay
that occurs.
After each half-life, half of the original
radioactive atoms have decayed into
atoms of a new element.
Transmutation can occur by radioactive
decay, or when particles bombard the
nucleus of an atom.
52
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25.2 Nuclear Transformations > Key Equation
A = A0 
53
1
2
n
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25.2 Nuclear Transformations > Glossary Terms
• nuclear force: an attractive force that acts
between all nuclear particles that are
extremely close together, like protons and
neutrons in a nucleus
• band of stability: the location of stable nuclei
on a neutron-vs.-proton plot
• positron: a particle with the mass of an
electron but a positive charge
54
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25.2 Nuclear Transformations > Glossary Terms
• half-life: the time required for one-half of the
nuclei of a radioisotope sample to decay to
products
• transmutation: the conversion of an atom of
one element to an atom of another element
• transuranium elements: any elements in the
periodic table with atomic number above 92,
the atomic number of uranium
55
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25.2 Nuclear Transformations >
BIG IDEA
Electrons and the Structure of Atoms
• Unstable atomic nuclei decay by emitting
alpha or beta particles.
• Often gamma rays are emitted.
56
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25.2 Nuclear Transformations >
END OF 25.2
57
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