Transcript Chapter 37

Chapter 37
Nuclear Chemistry
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Copyright (c) 2011 by Michael A. Janusa, PhD. All rights reserved.
37.1 Radioactivity
Radioactive decay is the process in which a
nucleus spontaneously disintegrates, giving off
radiation.
Radiation are the particles or rays emitted.
Radiation comes from the nucleus as a result of
an alteration in nuclear composition or structure.
This occurs in a nucleus that is unstable and
hence radioactive.
Nuclear symbols are used to designate the nucleus and
consists of
A
- Atomic symbol (element symbol)
Z
- Atomic number (Z : #protons)
- Mass number (A : #protons + #neutrons)
E
2
mass number A number of
protons and neutrons
5 protons , 6 neutrons
11
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B
atomic symbol
atomic number Z number of
protons
• This symbol is the same as writing boron-11 and
defines an isotope of boron.
• In nuclear chemistry this is often called a nuclide.
• This is not the only isotope (nuclide) of boron.
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• Some isotopes are stable
• The unstable isotopes are the ones that
produce radioactivity (emit particles); the
actual process of radioactive decay.
• Radioactive decay is the process in which
nucleus spontaneously disintegrates,
giving off radiation.
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Radioactivity
• Radioactivity was discovered by Antoine
Henri Becquerel in 1896.
– The work involved uranium salts which lead to the
conclusion that the minerals gave off some sort of
radiation.
– This radiation was later shown to be separable by
electric (and magnetic) fields into three types;
alpha (a), beta (b), and gamma (g) rays.
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Radioactivity
– Alpha rays (a) bend away from a positive plate
indicating they are positively charged.
– They are known to consist of helium-4 nuclei
(nuclei with two protons and two neutrons).
– Slow moving (relatively large mass compared to
other nuclear particles; therefore, moves slow 10% speed of light)
– Stopped by small barriers as thin as few pages of
paper.
– Symbolized in the following ways:
2p, 2n
4
2
He
2
4
2
He
α
4
2
α
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Radioactivity
– Beta rays (b) bend in the opposite direction
indicating they have a negative charge.
• They are known to consist of high speed electrons
(90% speed of light).
• Emitted from the nucleus by conversion of neutron
into a proton.
• Higher speed particles; therefore, more penetrating
than alpha particles (stopped by only more dense
materials such as wood, metal, or several layers of
clothing).
• The symbol is basically equivalent to electron
0
0
pure electron
1
-1
e
β
β
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Radioactivity
– Gamma rays (g) are unaffected by electric and
magnetic fields.
– They have been shown to be a form of
electromagnetic radiation (pure energy) similar
to x rays, but higher in energy and shorter in
wavelength.
– Alpha and beta radiation are matter; contains p, n,
or e while gamma is pure energy (no p, n, e).
– Highly energetic, the most penetrating form of
radiation (barriers of lead, concrete, or more often,
a combination is required for protection).
– Symbol is
g
or
g
0
0
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37. 2 Nuclear Equations
• A nuclear equation is a symbolic
representation of a nuclear reaction using
nuclide symbols.
– For example, the nuclide symbol for
uranium-238 is
238
92 U
92 p, 146 n
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Nuclear Equations
– Reactant and product nuclei are represented
in nuclear equations by their nuclide symbol.
– The radioactive decay of 238
92 U by alpha-particle
emission (loss of a 42 He nucleus) is written
238
234
4
92 U  90Th  2 He
lost 2p & 2n
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Nuclear Equations
– Other particles are given the following symbols.
Proton
1
1H
or
1
1p
Neutron
1
0n
Electron
0
1 b
or
0
1 e
Positron
0
1b
or
0
1e
Gamma photon
0
0g
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Nuclear Equations
• In a nuclear equation, you do not balance the
elements, instead...
– the total mass on each side of the reaction arrow
must be identical (this means that the sum of the
superscripts for the products must equal the
sum of the superscripts for the reactants).
– the sum of the atomic numbers on each side of the
reaction arrow must be identical (this means that the
sum of the subscripts for the products must
equal the sum of the subscripts for the
reactants).
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Alpha Decay
238
92
U
238
=
Th  He
234
90
234
mass number
=
90 +
92
4
2
+
4
2
atomic number
Ex. Plutonium 239 emits an alpha particle when it decays, write the
balanced nuclear equation.
mass A:
239
94
239
94
Pu  E?  He
Pu 
A
Z
235
92
4
2
U  He
4
2
239 = 4 + A
A = 239 – 4 = 235
Z:
94 = 2 + Z
Z = 94 – 2 = 92
U
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Beta Decay
16
7
N O  e
16
8
0
-1
np
Ex. Protactinium 234 undergoes beta decay. Write the balanced
nuclear equation.
234
91
Pa  E?  e
mass A:
234 = 0 + A
A = 234 – 0 = 234
234
91
Pa 
Z:
91 = -1 + Z
Z = 91 + 1 = 92  U
A
Z
234
92
0
-1
U e
0
-1
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A Problem To Consider
• Technetium-99 is a long-lived radioactive
isotope of technetium. Each nucleus decays
by emitting one beta particle. What is the
product nucleus?
– The nuclear equation is
99
A
0
43Tc Z X  1 b
mass A:
99 = 0 + A
A = 99 – 0 = 99
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A Problem To Consider
• Technetium-99 is a long-lived radioactive
isotope of technetium. Each nucleus decays
by emitting one beta particle. What is the
product nucleus?
– The nuclear equation is
99
A
0
43Tc Z X  1 b
Z:
43 = -1 + Z
Z = 43 + 1 = 44  Ru
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A Problem To Consider
• Technetium-99 is a long-lived radioactive
isotope of technetium. Each nucleus decays
by emitting one beta particle. What is the
product nucleus?
– The nuclear equation is
99
A
0
43Tc Z X  1 b
– Hence A = 99 and Z = 44 (Ruthenium), so the
product is
99
44 Ru
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37.4 Nuclear Structure and Stability
• Binding Energy - the energy that holds the
protons, neutrons, and other particles
together in the nucleus.
• Binding energy is very large for unstable
isotopes.
• When isotopes decay (forming more stable
isotopes,) binding energy is released (go to
lower E state; more stable arrangement).
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• Important factors for stable isotopesnuclear stability correlates with:
– Ratio of neutrons to protons in the isotope.
– Nuclei with large number of protons (84 or
more) tend to be unstable.
– The “magic numbers” of 2, 8, 20, 50, 82, or 126
help determine stability. These numbers of
protons or neutrons are stable. These numbers,
called magic numbers, are the numbers of
nuclear particles in a completed shell of protons
or neutrons.
– Even numbers of protons or neutrons are
generally more stable than those with odd
numbers.
– All isotopes (except 1H) with more protons than
neutrons are unstable.
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Nuclear Stability
• Several factors appear to contribute the
stability of a nucleus.
– when you plot each stable nuclide on a graph of
neutrons vs. protons, these stable nuclei fall in a
certain region, or band.
– The band of stability is the region in which stable
nuclides lie in a plot of number of neutrons against
number of protons.
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Figure :
Band of stability.
np
more neutrons than
needed for stability
pn
more protons than
needed for stability
21
Ebbing, D. D.; Gammon, S. D. General Chemistry,
8th ed., Houghton Mifflin, New York, NY, 2005.
Predicting the Type of Radioactive Decay
• Nuclides outside the band of stability are
generally radioactive.
– Nuclides to the left of the band have more
neutrons than that needed for a stable nucleus.
– These nuclides tend to decay by beta emission
because it reduces the neutron-to-proton ratio.
n  p emit electron in process
more neutrons than needed for stability
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Predicting the Type of Radioactive Decay
• Nuclides outside the band of stability are
generally radioactive.
– In contrast, nuclides to the right of the band of
stability have a neutron-to-proton ratio smaller
than that needed for a stable nucleus.
– These nuclides tend to decay by positron
emission or electron capture because it
increases the neutron to proton ratio.
pn
more protons than needed for stability
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Predicting the Type of Radioactive Decay
• Nuclides outside the band of stability are
generally radioactive.
– In the very heavy elements, especially those with
Z greater than 83, radioactive decay is often by
alpha emission.
Lose 2p and 2n - emit alpha particle
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Ebbing, D. D.; Gammon, S. D. General Chemistry,
8th ed., Houghton Mifflin, New York, NY, 2005.
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37.3 Types of Radioactive Decay
• There are six common types of radioactive
decay.
– Alpha emission (abbreviated a): emission of
a 4 He nucleus, or alpha particle, from an
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unstable nucleus.
– An example is the radioactive decay of radium-226.
226
222
4
88 Ra 86 Rn  2 He
Lost 2p & 2n
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Types of Radioactive Decay
• There are six common types of radioactive
decay.
– Beta emission (abbreviated b or b-): emission
of a high speed electron from a unstable
nucleus.
– This is equivalent to the conversion of a neutron to a
proton.
1
1
0
0 n 1 p  1 e
– An example is the radioactive decay of carbon-14.
14
14
0
6 C 7 N  1 b
n p
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Types of Radioactive Decay
• There are six common types of radioactive
decay.
– Positron emission (abbreviated b+): emission of
a positron from an unstable nucleus.
– This is equivalent to the conversion of a proton to a
neutron.
1
1
0
1 p0 n  1 e
– The radioactive decay of technetium-95 is an
example of positron emission.
p n
95
95
0
43Tc 42 Mo  1 e
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Types of Radioactive Decay
• There are six common types of radioactive decay.
– Electron capture (abbreviated EC): the decay of
an unstable nucleus by capturing, or picking up,
an electron from an inner orbital of an atom.
– In effect, a proton is changed to a neutron, as in
positron emission.
1
0
1
1 p  1 e 0 n
– An example is the radioactive decay of
potassium-40.
40
0
40
19 K  1 e 18 Ar
p n
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Types of Radioactive Decay
• There are six common types of radioactive decay.
– Gamma emission (abbreviated g): emission from
an excited nucleus of a gamma photon,
corresponding to radiation with a wavelength of
about 10-12 m.
– In many cases, radioactive decay produces a
product nuclide in a metastable excited state.
– The excited state is unstable and emits a gamma
photon and goes to a lower energy state (more
stable). The atomic mass and number do not
change.
– An example is metastable technetium-99.
99 m
99
0
43Tc 43Tc  0 g
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Types of Radioactive Decay
• There are six common types of radioactive
decay.
– Spontaneous fission: the spontaneous decay of
an unstable nucleus in which a heavy nucleus of
mass number greater than 89 splits into lighter
nuclei and energy is released.
– For example, uranium-236 undergoes spontaneous
fission.
236
96
136
1
92 U  39Y  53 I  40 n
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37.5 Rate of Radioactive Decay
• The rate of radioactive decay, that is the
number of disintegrations per unit time, is
proportional to the number of radioactive
nuclei in the sample.
– You can express this rate mathematically as
Rate  kN t
where Nt is the number of radioactive nuclei at time t,
and k is the radioactive decay constant.
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Rate of Radioactive Decay
– All radioactive decay follows first order kinetics
as outlined in kinetics chapter.
– Therefore, the half-life of a radioactive sample is
related only to the radioactive decay constant.
– The half-life, t½ ,of a radioactive nucleus is the time
required for one-half of the nuclei in a sample to decay.
– The first-order relationship between t½ and the
decay constant k is
0.693
t 
k
1
2
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Rate of Radioactive Decay
• Once you know the decay constant, you can
calculate the fraction of radioactive nuclei
remaining (Nt/No) after a given period of time.
– Recall the first-order time-concentration equation
is
Nt
ln
 kt
No
– Or if we don’t know k we can substitute k = 0.693/t½
and get
N t  0.693 t
ln

No
t
1
2
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A Problem To Consider
• Phosphorus-32 has a half-life of 14.3 days. What
fraction of a sample of phosphorus-32 would remain
after 5.5 days?
or
N t  0.693 t
ln

No
t
1
2
N t  0.693 (5.5d)
ln

 0.267
No
(14.3 d)
1
Fraction nuclei remaining  n
2
t
n
t1/ 2
Nt
Fraction nuclei remaining 
 e 0.267  0.77
No
or 77% remaining
code: second
HW 51 35
37.4.1 Nuclear Fission and Nuclear Fusion
• Nuclear fission is a nuclear reaction in which
a heavy nucleus splits into lighter nuclei and
energy is released.
– For example, one of the possible mechanisms for
the decay of californium-252 is
252
142
106
1
Cf

Ba

Mo

4
98
56
42
0n
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Nuclear Fission and Nuclear Fusion
– In some cases a nucleus can be induced to
undergo fission by bombardment with neutrons.
1
0
n
235
92
U
236
92
U  Kr 
92
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Ba  3 n  energy
141
56
1
0
– When uranium-235 undergoes fission, more
neutrons are released creating the possibility of
a chain reaction.
– A chain reaction is a self-sustaining series of nuclear
fissions caused by the absorption of neutrons
released from previous nuclear fissions.
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Ebbing, D. D.; Gammon, S. D. General Chemistry,
8th ed., Houghton Mifflin, New York, NY, 2005.
• Chain reaction - the reaction sustains
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itself by producing more neutrons
Nuclear Fission and Nuclear Fusion
• Nuclear fusion is a nuclear reaction in which
light nuclei combine to give a stable heavy
nucleus plus possibly several neutrons, and
energy is released.
– Such fusion reactions have been observed in the
laboratory using particle accelerators.
– Sustainable fusion reactions require
temperatures of about 100 million oC.
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Nuclear Fission and Nuclear Fusion
• Fusion (to join together) - combination of
two small nuclei to form a larger nucleus.
• Large amounts of energy is released.
• Best example is the sun.
• An Example:
2
1
H  31H  42 He  01n  energy
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