Transcript NUCLEAR CHEMISTRY - Watchung Hills Regional High School

NUCLEAR CHEMISTRY
By: Stephanie Chen
and
Stephanie Ng
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
23.1
Types of Radiation
• Alpha (ά) – a positively
charged (+2) helium isotope we usually ignore the charge because it involves
electrons, not protons and neutrons
•Beta (β) – an electron
•Gamma (γ) – pure energy;
called a ray rather than a
particle
4
2
He
0
1
e

0
0
Other Nuclear Particles
• Neutron
• Positron – a positive
electron
•Proton – usually referred to
as hydrogen-1
•Any other elemental isotope
1
0
n
0
1
1
1
e
H
Penetrating Ability
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
23.1
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
23.1
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
23.1
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
23.2
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
Decrease # of neutrons by 2
Decrease # of protons by 2
Spontaneous fission
252Cf
98
1n
2125
In
+
2
49
0
23.2
Learning Check
What radioactive isotope is produced in
the following bombardment of boron?
10B
5
+ 4He
2
13N
7
+
0
1n
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
 reactions because a  ray is
usually emitted.
Radioisotopes used in medicine are often
made by  reactions.
Artificial Nuclear Reactions
Example of a 
reaction is
production of radioactive 31P for use
in studies of P uptake in the body.
31 P
15
+
1 n
0
--->
32 P
15
+ 
Transuranium Elements
Elements beyond 92 (transuranium)
made starting with an  reaction
238 U
92
+
239 U
92
239 Np
93
1 n
0
--->
239 U
92
+ 
--->
239 Np
93
+ 0-1b
--->
239 Pu
94
+
0 b
-1
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
23.2
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
with 50 grams of Ra-234
AfterIf3.6you
daysstart
> 25 grams
After 7.2 days > 12.5 grams
After 10.8 days > 6.25 grams
Kinetics of Radioactive Decay
N
daughter
DN
rate = Dt
rate = lN
DN
= lN
Dt
N = N0e(-lt)
lnN = lnN0 - lt
N = the number of atoms at time t
N0 = the number of atoms at time t = 0
l is the decay constant (sometimes called k)
Ln 2
l =
t½
k=
23.3
Kinetics of Radioactive Decay
ln[N] = ln[N]0 - lt
ln [N]
[N]
[N] = [N]0exp(-lt)
23.3
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
23.3
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?
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
23.5
Representation of a fission process.
Mass Defect
• Some of the mass can be converted
into energy
• Shown by a very famous equation!
2
E=mc
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
23.2
Nuclear binding energy per nucleon vs Mass number
nuclear binding energy
nucleon
nuclear stability
23.2
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
23.5
Nuclear Fusion
Fusion
small nuclei combine
2H
1
+
3H
1
4He
2
+ 1n +
0
Occurs in the sun and other stars
Energy
Nuclear Fusion
Fusion
• Excessive heat can not be
contained
• Attempts at “cold” fusion have
FAILED.
• “Hot” fusion is difficult to
contain