Transcript Nuclear Chemistry

Nuclear Chemistry
Chapter 18
Radioactivity
• One of the pieces of evidence for the
fact that atoms are made of smaller
particles came from the work of
Marie Curie (1876-1934).
• She discovered radioactivity,
the spontaneous disintegration of
some elements into smaller pieces.
Nuclear Reactions vs.
Normal Chemical Changes
• Nuclear reactions involve the nucleus
• The nucleus opens, and protons and
neutrons are rearranged
• The opening of the nucleus releases a
tremendous amount of energy that holds
the nucleus together – called binding
energy
• “Normal” Chemical Reactions involve
electrons, not protons and neutrons
Types of Radiation
• Alpha (a) – a positively
4
charged (+2) helium isotope - 2
He
we usually ignore the charge because it involves
electrons, not protons and neutrons
•Beta (b) – an electron
•Gamma (g) – pure energy;
called a ray rather than a
particle
0
1
0
0
e
g
Other Nuclear Particles
• Neutron
1
0
n
• Positron – a positive
electron
0
1
e
•Proton – usually referred to
as hydrogen-1
•Any other elemental isotope
1
1
H
Penetrating Ability
Geiger Counter
• Used to detect radioactive substances
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Mass Number
Atomic Number
A
ZX
Element Symbol
proton
1p
1H
or
1
1
neutron
1n
0
electron
0b
0e
or
-1
-1
positron
0b
0e
or
+1
+1
a particle
4He
4a
or
2
2
A
1
1
0
0
4
Z
1
0
-1
+1
2
Balancing Nuclear Equations
1. Conserve mass number (A).
The sum of protons plus neutrons in the products must equal
the sum of protons plus neutrons in the reactants.
235
92 U
+ 10n
138
55 Cs
+
96
37 Rb
+ 2 10n
235 + 1 = 138 + 96 + 2x1
2. Conserve atomic number (Z) or nuclear charge.
The sum of nuclear charges in the products must equal the
sum of nuclear charges in the reactants.
235
92 U
+ 10n
138
55 Cs
+
96
37 Rb
92 + 0 = 55 + 37 + 2x0
+ 2 10n
212Po
decays by alpha emission. Write the balanced
nuclear equation for the decay of 212Po.
4
alpha particle - 42He or 2a
212Po
84
4He
2
+ AZX
212 = 4 + A
A = 208
84 = 2 + Z
Z = 82
212Po
84
4He
2
+ 208
82Pb
Nuclear Stability and Radioactive Decay
Beta decay
+-10b + n
14C
6
14N
7
40K
19
40Ca
20
Decrease # of neutrons by 1
+ -10b + n
1n
0
Increase # of protons by 1
1p
1
+ -10b + n
Positron decay
++10b + n
Increase # of neutrons by 1
++10b + n
Decrease # of protons by 1
11C
6
11B
5
38
19K
38Ar
18
1p
1
1n
0
++10b + n
n and n have A = 0 and Z = 0
Nuclear Stability and Radioactive Decay
Electron capture decay
+n
37Ar
18
+ -10e
37Cl
17
55Fe
26
+ -10e
55Mn
25
1p
1
Increase # of neutrons by 1
+n
Decrease # of protons by 1
+ -10e
1n
0
+n
Alpha decay
212Po
84
4He
2
+ 208
82Pb
Spontaneous fission
252Cf
98
1n
2125
In
+
2
49
0
Decrease # of neutrons by 2
Decrease # of protons by 2
Learning Check
What radioactive isotope is produced in the
following bombardment of boron?
10B
5
+ 4He
2
? +
1n
0
Learning Check
What radioactive isotope is produced in the
following bombardment of boron?
10B
5
+ 4He
2
13N
7
+
1n
0
Write Nuclear Equations!
Write the nuclear equation for the beta
emitter Co-60.
60Co
27
0e
-1
+
60Ni
28
Artificial Nuclear Reactions
New elements or new isotopes of known elements
are produced by bombarding an atom with a
subatomic particle such as a proton or neutron
-- or even a much heavier particle such as 4He
and 11B.
Reactions using neutrons are called
g reactions because a g ray is usually
emitted.
Radioisotopes used in medicine are often made by
g reactions.
Artificial Nuclear Reactions
Example of a g
reaction is production
of radioactive 31P for use in studies of P
uptake in the body.
31 P
15
+
1
0n --->
32
15P
+ g
Transuranium Elements
Elements beyond 92 (transuranium) made
starting with an g reaction
1 n
0
92U +
239
92U
--->
239
0 b
Np
+
93
-1
239 Np
93
--->
239
94Pu +
--->
239
+ g
238
92U
0
-1b
Nuclear Stability
•
Certain numbers of neutrons and protons are extra stable
•
n or p = 2, 8, 20, 50, 82 and 126
•
Like extra stable numbers of electrons in noble gases
(e- = 2, 10, 18, 36, 54 and 86)
•
Nuclei with even numbers of both protons and neutrons
are more stable than those with odd numbers of neutron
and protons
•
All isotopes of the elements with atomic numbers higher
than 83 are radioactive
•
All isotopes of Tc and Pm are radioactive
Band of Stability
and Radioactive
Decay
Stability
of Nuclei
• Out of > 300 stable isotopes:
N
Even
Odd
Even
157
52
Odd
50
5
Z
19
9F
31 P
15
2
1
H, 63Li, 105B, 147N, 18073Ta
Half-Life
• HALF-LIFE is the time that it takes for
1/2 a sample to decompose.
• The rate of a nuclear transformation
depends only on the “reactant”
concentration.
Half-Life
Decay of 20.0 mg of 15O. What remains after 3 half-lives?
After 5 half-lives?
Kinetics of Radioactive Decay
For each duration (half-life), one half of the
substance decomposes.
For example: Ra-234 has a half-life of 3.6 days
If you start with 50 grams of Ra-234
After 3.6 days  25 grams
After 7.2 days  12.5 grams
After 10.8 days 6.25 grams
Kinetics of Radioactive Decay
X
daughter
DX
rate = Dt
X = X0
(-kt)
e
lnX = lnX0 - kt
X = the amount of atoms at time t
X0 = the amount of atoms at time t = 0
k is the decay constant (sometimes called l)
t½ = ln 2
k
0.693
t½ =
k
Radiocarbon Dating
14N
7
+ 01n
14C
6
14C
6
14N
7
+ 11H
+ -10b + n
t½ = 5730 years
Uranium-238 Dating
238U
92
206Pb
82
+ 8 24a + 6-10b
t½ = 4.51 x 109 years
Learning Check!
The half life of I-123 is 13 hr. How much of
a 64 mg sample of I-123 is left after 31
hours?
Biological Effects of Radiation
Radiation absorbed dose (rad)
1 rad = 1 x 10-5 J/g of material
Roentgen equivalent for man (rem)
1 rem = 1 rad x Q
Quality Factor
g-ray = 1
b=1
a = 20
Effects of Radiation
Nuclear Fission
Fission is the splitting of atoms
These are usually very large, so that they are not as stable
Fission chain has three general steps:
1. Initiation. Reaction of a single atom starts the
chain (e.g., 235U + neutron)
2. Propagation.
236U
fission releases neutrons that
initiate other fissions
3. Termination.
Nuclear Fission
Nuclear Fission
235U
92
+ 01n
90Sr
38
1n + Energy
+ 143
Xe
+
3
0
54
Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2
Energy = 3.3 x 10-11J per 235U
= 2.0 x 1013 J per mole 235U
Combustion of 1 ton of coal = 5 x 107 J
Representation of a fission process.
Mass Defect
• Some of the mass can be converted into
energy
• Shown by a very famous equation!
E=mc2
Energy
Mass
Speed of light
Nuclear binding energy (BE) is the energy required to break
up a nucleus into its component protons and neutrons.
BE + 199F
911p + 1010n
E = mc2
BE = 9 x (p mass) + 10 x (n mass) – 19F mass
BE (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984
BE = 0.1587 amu
1 amu = 1.49 x 10-10 J
BE = 2.37 x 10-11J
binding energy
binding energy per nucleon =
number of nucleons
2.37 x 10-11 J
= 1.25 x 10-12 J
=
19 nucleons
Nuclear binding energy per nucleon vs. Mass number
nuclear binding energy
nucleon
nuclear stability
Nuclear Fission
Nuclear chain reaction is a self-sustaining sequence of
nuclear fission reactions.
The minimum mass of fissionable material required to
generate a self-sustaining nuclear chain reaction is the
critical mass.
Non-critical
Critical
Nuclear Fission & POWER
• Currently about 103
nuclear power plants in
the U.S. and about 435
worldwide.
• 17% of the world’s
energy comes from
nuclear.
Diagram of a nuclear power plant
Nuclear Fission
Annual Waste Production
35,000 tons SO2
4.5 x 106 tons CO2
3.5 x 106
ft3 ash
1,000 MW coal-fired
power plant
70 ft3
vitrified
waste
1,000 MW nuclear
power plant
Nuclear Fission
Hazards of the
radioactivity in spent
fuel compared to
uranium ore
From “Science, Society and America’s Nuclear Waste,” DOE/RW-0361 TG
Chemistry In Action: Nature’s Own Fission Reactor
Natural Uranium
0.7202 % U-235 99.2798% U-238
Measured at Oklo
0.7171 % U-235
Nuclear Fusion
Fusion
small nuclei combine
2H
1
+
3H
4He
1
2
+ 1n +
0
Occurs in the sun and other stars
Energy
Nuclear Fusion
Fusion Reaction
2
2
3
1
1 H + 1H
1 H + 1H
2H
1
+ 13H
6Li
3
+ 12H
4He
2
2
+ 10n
4He
2
Energy Released
6.3 x 10-13 J
2.8 x 10-12 J
3.6 x 10-12 J
Tokamak magnetic
plasma
confinement
23.6
Nuclear Fusion
Fusion
• Excessive heat can not be contained
• Attempts at “cold” fusion have
FAILED.
• “Hot” fusion is difficult to contain
Radioisotopes in Medicine
•
1 out of every 3 hospital patients will undergo a nuclear
medicine procedure
•
24Na,
•
131I,
t½ = 14.8 hr, b emitter, thyroid gland activity
•
123I,
t½ = 13.3 hr, gray emitter, brain imaging
•
18F,
t½ = 1.8 hr, b emitter, positron emission tomography
•
99mTc,
t½ = 14.8 hr, b emitter, blood-flow tracer
t½ = 6 hr, gray emitter, imaging agent
Brain images
with 123I-labeled
compound
23.7
Chemistry In Action: Food Irradiation
Dosage
Effect
Up to 100 kilorad
Inhibits sprouting of potatoes, onions, garlics.
Inactivates trichinae in pork. Kills or prevents insects
from reproducing in grains, fruits, and vegetables.
100 – 1000 kilorads
Delays spoilage of meat poultry and fish. Reduces
salmonella. Extends shelf life of some fruit.
1000 to 10,000 kilorads
Sterilizes meat, poultry and fish. Kills insects and
microorganisms in spices and seasoning.