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Lecture 63 Nuclear Chemistry II

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when an organism dies, exchange of 14C with the biosphere stops ... non-radioactive nuclide. There are three such series: A = 238U, A = 232Th, A = 235U ... – PowerPoint PPT presentation

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Title: Lecture 63 Nuclear Chemistry II


1
Lecture 63 Nuclear Chemistry II
2
Review
  • Decay DZ DA Dn Mode
  • a -2 -4 -2
  • b- 1 0 -1
  • b -1 0 1
  • EC -1 0 1
  • g 0 0 0

3
Radioisotope Dating
  • ratio of 14C to total C in the
  • biosphere is a constant
  • when an organism dies, exchange of 14C with the
    biosphere stops
  • 14C in dead organism decreases

4
Radiocarbon Dating
e.g. the C14/C ratio in an artifact is 0.10 of
the ratio in living stuff. How old is
the artifact? t1/2 (14C) 5730 y
k 0.693/t1/2 1.21 x 10-4 y-1
ln (N/No) - kt
ln (0.10 / 1.0) -1.21 x 10-4 y-1 (t)
solving, t 19030 y
5
Nuclear Stability e.g. Pt (Z78)
N123
b- (n/p )
non-radioactive
b, EC (n/p )
a (n/p )
N90
6
Nuclear Stability
140
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neutrons needed for stability
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neutrons (N)
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N/Z1
Island of Stability
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0
protons (Z)
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90
7
Nuclear Stability
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b- decay
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neutrons needed for stability
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neutrons (N)
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Island of Stability
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a decay
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b, EC decay
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0
protons (Z)
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8
The Magic Numbers
  • Very stable nuclei (large t1/2) if n or p
  • 2, 8, 20, 28, 50, 82, 126
  • (recall electron shells 2, 8, 18,)
  • ? Shell Model of Nucleus

9
Decay Series
A ? B ? C ? D ? ... ? non-radioactive nuclide
There are three such series
A 238U, A 232Th, A 235U
and
10
238U Decay Series
a
b-
b-
a
238U ? 234Th ? 234Pa ? 234U ? 230Th

a
a
a
a
226Ra
222Rn(g)
218Po
214Pb
b-

b-
a
b-
214Bi ? 214Po ? 210Pb ? 210Bi
b-

a
210Po
206Pb
non-radioactive
11
Nuclear Reactions
  • A B ? C

mass (A) mass (B) ¹ mass (C)
e.g.
find mass before and after reaction
12
Nuclear Reactions
mass before 26 mp 30 mn 26(1.00728 amu)
30(1.00866 amu) 56.44908 amu mass after mass
56Fe atom - 26 me 55.9349 amu - 26(.0005486
amu) 55.92070 amu
mass defect 0.52838 amu
13
Nuclear Reactions
The mass is converted to energy!
14
"It followed from the special theory of
relativity that mass and energy are both but
different manifestations of the same thing -- a
somewhat unfamiliar conception for the average
mind. Furthermore, the equation E is equal to m
c-squared, in which energy is put
equal to mass, multiplied by the square of the
velocity of light, showed that very small amounts
of mass may be converted into a very large amount
of energy and vice versa. The mass and energy
were in fact equivalent, according to the formula
mentioned before. This was demonstrated by
Cockcroft and Walton in 1932, experimentally."
15
Nuclear Reactions
The mass is converted to energy!
DE Dmc2
Dm 0.52838 amu / 56Fe nucleus 0.52838
g/mol 0.52838 x 10-3 kg/mol
c 3.00 x 108 m/s
16
Nuclear Reactions
DE Dmc2
0.52838 x 10-3 kg/mol (3.00 x 108 m/s)2
4.75 x 1013 (kg m2 s-2) / mol
4.75 x 1013 J/mol
4.75 x 1010 kJ/mol
the binding energy of 56Fe
17
Binding Energy
  • is a maximum at 56Fe

heavier elements become more stable upon fission
lighter elements become more stable upon fusion
18
Energy of Nuclear Decay
e.g. 241Am 4He 237Np
241.0567 amu
237.0480 amu
4.0026 amu
Dm (4.0026 237.0480 - 241.0567)
-0.0061 amu
DE Dmc2 9.1 x 10-13 J / alpha particle
5.5 x 1011 J/mol
(very energetic particles)
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