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Ch. 22 – Nuclear Chemistry
Patterns of Stability
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Like charges repel.
Why does the nucleus stay together?
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“Strong Nuclear Force”
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The neutrons play a role.
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Patterns of Stability
Stable nuclei tend to have “stable ratios of protons to neutrons.
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We see this in the “belt of stability.”
Patterns of Stability
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Zr – 90
Patterns of Stability
40 / 50 1.25 : 1.00
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Sn – 120
50/70
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Hg – 200
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These ratios are quite stable.
80/120
1.4 : 1.0
1.5 : 1.0
Patterns of Stability
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Belt of stability ends at #83
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All elements #84 and above are radioactive.
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All isotopes of U are radioactive.
Patterns of Stability
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3 situations can now occur.
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1) Nuclei above the belt of stabilitycan lower their atomic number via Beta emission.
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2) Nuclei below can increase at.no. vie positron emission of electron capture.
Patterns of Stability
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3) Nuclei with at. No. ≥ 84 :
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Undergo alpha emission. Decrease both at. No and at. Mass to move toward the belt
of stability.
Patterns of Stability
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Magic Numbers.
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2, 8, 20, 28, 50, 82 protons
2, 8, 20, 28, 50, 126 neutrons
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Are more stable than nuclei which do not contain these numbers.
Patterns of Stability
• Even Numbers
• Nuclei with even number of protons and neutrons are more stable than those with odd
numbers.
Sample 21.4 as to how this works.
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Radioactivity
The nucleus undergoes a spontaneous decay emitting a ray or particle.
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Nuclear Chemistry is the study of nuclear reactions and their uses with chemistry.
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Many misunderstandings
Radioactivity
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Nucleons – protons and neutrons.
Mass Numbers
Isotopes –
A
AT.Mass
ZX.
At No.X
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U-233
233
92U
U-235
235
92U
U-238
238
92U
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Radioactivity
Nuclide refers to a specific Nucleus.
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Radionuclide refers to radioactive nuclei.
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Radioisotopes refer to atoms containing the above.
Nuclear Equations
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U-238 undergoes alpha decay
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238
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Radioactive Decay
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238
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Mass Must balance
238 =
234
+ 4
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Charge must balance
92 =
90
+ 2
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Alpha Decay –
4
2He
or
4
2á
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Beta Decay -
0
-1e
or
0
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Gamma Ray -
0
92U
Æ 23490Th
+
4
+
4
2He
Nuclear Equations
92U
Æ 23490Th
2He
Types of Decay
-1â
0ã
Types of Decay
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I-131 undergoes Beta Decay
131
131
0
53I Æ
54Xe + -1â
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Beta is equivalent to conversion of a neutron to a proton.
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1
0n
Æ 11p +
0
-1
â
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Types of Decay
Gamma Radiation consists of High Energy Photons.
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0
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See table 22-6
0ã
Other Types of Decay
Positron Emission
11
6C
Æ 115B
+
0
1e
Equivalent to converting a proton to a neutron.
1
1p
Æ 10n +
0
1e
Other Types of Decay
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Electron Capture
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81
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Equivalent to converting a proton to a neutron.
1
0
1
1p +
-1â Æ
0n
37Rb
+ 0 -1â Æ
81
36Kr
Chem.03.04
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Radioactive Series
Some nuclei cannot gain stability by a single emission. This leads to a series of
emissions.
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U-238 Æ Th-234 Æ Pa-234 Æ eventually to Pb-208
Radioactive Series
Radioactive Series
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Radioactive Series or Nuclear Disintegration Series
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Radioactive Daughters.
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Transmutations
Induced nuclear changes involve a nucleus that is struck by a particle and then
undergoes changes
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Nuclear Transmutations.
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14
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Here N-14 is changed to O-17.
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The Alchemist’s Dream?
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Transmutations
Commonly list the transmutation in order:
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Target nucleus: Bombarding particle: ejected particle: product nucleus
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Example 21.5
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Transmutations
Sometimes these particles must go very fast to induce radioactivity.
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In this case we use particle accelerators.
Transmutations
7N
+
4
2HE
Æ
17
8O
+
1
1H
Fermilab
Fermilab
Fermilab
Fermilab
Fermilab
S.L.A.C.
S.L.A.C.
Using Neutrons
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58
26Fe
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•
+
59
59
27Co
+
1
0n
26Fe
1
0n
Æ
59
26Fe
Æ 5927Co +
Æ
60
27C0
0
-1
e
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238
92U
•
239
93Np
Æ
•
239
94Pu
+
1
+ 0n Æ
239
4
239
92U Æ
239
0
e
94Pu
+
Æ
242
2He
-1
Transuranium elements
0
93Np + -1 e
96Cm
+
1
0n
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Half-Life
Radioactivity follows classic first order reaction kinetics.
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There is only a single reactant and it reacts spontaneously.
• It will show a half –life
t 1/2
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Half-Life
In one t ½ the material diminishes by 50%.
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In the next t ½ the remaining material diminishes by 50%
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Etc, etc, etc, ……………….
Half-Life – Sr-90
Half-Life
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Check the numbers for half-life
Time
Counts
0
1000
1
500
2
250
3
125
4
62
5
32
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Time
6
7
8
9
10
11
Counts
16
8
4
2
1
50/50 chance
% Decay
0
50
75
87.5
94.0
96.9
Half-Life
% Decay
98.4
99.2
99.6
99.8
99.9
Half-Life
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ALL half-lifes are the same.
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What are the differences?
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Length of half-life / Intensity of decay / Type of decay
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Dating
We can use Radioisotopes for dating purposes.
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(which for some of you will be the only dating you’ll do this year……….)
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Dating
C14 undergoes neutron capture in the upper atmosphere.
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14
•
14
7
N +
6
C Æ
1
0
n Æ
14
7
14
N Æ
6
C +
0
-1
1
1
p
e
Dating
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C-14 has a half-life of 5715 Years.
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Based on our 10 half-life rule of thumb, that makes about 50,000 yrs worth of dating.
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After Death C-12 remains constant while the C-14 levels drop due to Decay.
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Dating
We can use a variety of isotopes to bridge and leap back in time as the dating process
goes on.
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See Table 22-2 – for various Half-lives.
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Nuclear Rxns and Energy
2
E= mc
E = energy; m= mass; c=speed of light
(3.0x108 m/s)
In nuclear changes it is possible for a tiny amount of mass to convert to energy, which
of course is substantial.
Mass Loss
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238
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238
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233.9942 + 4.0015 – 238.0003 = -.0046
92U
Æ
234
90Th
+
4
2He
U = 238.0003amu
Th= 233.9942 amu
4
He = 4.0015amu
234
Mass Loss
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If we do this on a gram scale……
Mass loss is = .0046g
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E = (-.0046g) x (3.00x108m/s)2
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E = -4.1 x 1011 Kg-m/s2
• E = -4.1 x 1011 J
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Binding Energies
But what really holds the protons and neutrons together in the nucleus?
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Let’s look at the numbers involved.
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He-4
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Two protons
2 x 1.00728 amu
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Two neutrons
2 x 1.00866 amu
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Should = 4.03188 amu
He - 4
He-4
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Actual mass of He-4 = 4.00150 amu
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Calculated =
Actual
=
“MASS DEFECT”
4.03188 amu
4.00150 amu
---------------0.03038 amu
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Mass Defect
Mass difference between a nucleus and its constituent nucleons.
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E +
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U E = c2 Um
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Mass Defect
UE = (2.9979x10 m/s) x (.03038amu)
x (1g/6.022 x1023) (1kg / 1000g) =
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4.543 x 10 -12 J
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Binding Energy per nucleon.
4
2He
Æ 2 11 P + 2 10 n
8
2
Binding Energy
Finally
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The End
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Chem. ’03 – ‘04