Transcript Photo chapter opener 21 Subatomic particle tracks in a bubble

The Nucleus:
A Chemist’s View
Nuclear Stability and Radioactive
• Thermodynamic stability－ potential
energy
• Kinetic stability－radioactive decay
1
Nuclide
• A nuclide is a type of atom characterized by its
proton number, neutron number and its energy
condition.
• Nuclides with identical proton number but
differing neutron number are called isotopes.
• Conditions with a life of less than 10-10s are
called excited conditions of a nuclide.
• At present, more than 2,770 different nuclides
are known, distributed over the 113 currently
known elements, only 279 are stable with
respect to radioactive decay.
2
A
Z
X
A : the sum of the neutrons and protons
Z : atomic numbers
3
Known nuclides
• All nuclides with 84 or
more protons are
unstable with respect
to radioactive decay.
• Light nuclides are
stable when (A-Z)/Z
ratio is 1.
• For heavier elements
for stability, (A-Z)/Z
ratio is greater than 1
and increases with Z.
4
• Magic numbers: 2, 8, 20, 28, 50, 82, 126
• Specific numbers of protons or neutrons
produce especially stable nuclides.
5
Types of Radioactive Decay
a-particle production
The common modes of decay
238
92
U He(a ) Th
4
2
234
90
Th  He(a ) Ra
230
90
4
2
226
88
Spontaneous fission
(a)The splitting of a heavy nuclide into two
lighter nuclides with similar mass numbers.
(b)Slow rate for most nuclides
6
Types of Radioactive Decay
b-particle production
The common modes of decay
Th  Pa  e(b )
234
90
131
53
234
91
0
1
I Xe  e(b )
131
54
0
1
(a)The net effect of b-particle production is to
change a neutron to a proton.
(b)The nuclides lie above the zone of stability.
(c)The ratios of neutron/proton are too high.
7
Types of Radioactive Decay
gray production
238
92
U He 
4
2
Th  2 γ
234
90
0
0
(a) high-energy photon
(b) g-ray production accompanies unclear
decays and particle reaction.
(c)The emission of g rays is one way a nucleus
with excess energy can relax to its ground state.
8
Types of Radioactive Decay
positron production
22
11
Na  Ne e
22
10
0
1
(a)The net effect of this process is to change a e a
proton to a neutron.
(b)Higher neutron/proton ratio
(c)Nuclides lie below the zone of stability.
9
Antiparticle
0
1
e  e 2 γ
0
1
0
0
Matter-antimatter collisions, is called annihilation.
10
11
Decay series
• Often a radioactive
nucleus cannot reach a
stable state through a
single decay process.
12
The Kinetic of Radioactive
Decay
• First order reaction
dN
N
 kt
Rate  
 kN  ln(
)  kt  N  N 0 e
dt
N0
t1/ 2
0.693

k
13
The decay of a 10 g sample
of Sr-90
14
Change in the amount of Mo
with time
15
16
Nuclear Transformations
• The conversion of one element into another
Observatio n from Rutherford
14
7
N  He  O H
4
2
17
8
1
1
Observatio n from Irene Curie
27
13
Al  He  P n
4
2
30
15
1
0
17
Experiments for Nuclear
Transformations cyclotron
Particle accelerators
18
Diagram of a linear accelerator
19
20
Detection of Radioactivity
Geiger-Muller counter

Ar(g) 
 Ar (g)  e
high energy particle

21
Carbon-14 Dating
14
6
C decay : C e N
14
6
C production : N n  C H
14
6
0
1
14
7
14
7
1
0
14
6
1
1
(a) Plants lives- 146 C/126 C ratio remains
the same as that in atmosphere .
(b) As a tree is cut,
14
6
C/126 C ratio begins to
decrease because of the ratioactiv e decay
of 146 C.
22
Drawbacks of Radiocarbon
Dating
• Errors up to 3000 years may have
occurred. (Measurements of U/Th ratio
development)
• Large piece of the object must be burned.
(Use mass spectrometry)
23
24
Figure 21.8: Consumption of
Na131I
Source: Visuals Unlimited
25
Thermodynamic Stability of the
Nucleus
E  mc 2 1905 Albert Einstein
801 n 811 H168 O
Mass of (801 n 811 H)  2.67804 10-23 g
Mass of 168 O  2.65535 10-23 g
Δm  -2.26910-25 g
The formation of 1 mole of 168 O
ΔE  Δmc 2  (-2.269 10-25 g)(6.02 10 23 )  (3 108 m/s) 2
ΔE  1.23 1013 J/mol
26
Energy for per Nucleon
13
1.23

10
J/mol
16
ΔE per 8 O nucleus 
6.022 10 23 nuclei/mol
 2.04 10 11 J/nucleus  -1.2810 2 Mev (1 Mev  1.6 10 13 J)
2
1.28

10
Mev
16
ΔE per nucleon for 8 O 
 8 Mev/nucleo n
16 nucleons/n ucleus
nucleon  neutron  proton
• The energy required to decompose this
nucleus into its components has same
numeric value but is positive.
• This is called the binding energy per
nucleon.
27
Binding energy per nucleon
as a function of mass number.
28
Nuclear Fission and Nuclear
Fusion
• Fusion: Combining two light nuclei to form
a heavier, more stable nucleus.
• Fission: Splitting a heavy nucleus into two
nuclei with smaller mass numbers.
29
Nuclear Fission and Nuclear
Fusion
30
Nuclear Fission
1
0
n
235
92
U Ba  Kr 3 n
141
56
92
36
1
0
31
Chain Reaction
• Neutrons are also
produced in the
fission reactions.
• Chain reaction－
This makes it
possible to
produce a selfsustaining fission
process.
32
Subcritical
• For the fission
reaction, at least one
neutron from each
fission event must go
on to split another
nucleus.
• Subcritical－If, on
the average, less
than one neutron
causes another
event, the process
dies out.
33
Critical
• If exactly one neutron from each fission
event causes another fission event, the
process sustains itself at the same level.
34
Supercritical
• If more than one
neutron from each
fission event
causes another
fission event, the
process rapidly
escalates and the
heat buildup
causes a violent
explosion.
35
Schematic diagram of a
nuclear power plant
36
Schematic of a reactor core
37
Nuclear Fusion
1
1
H H H e
1
1
H  H  He
1
1
2
1
2
1
0
1
3
2
3
2
He  He  He  2 H
3
2
He  H  He  e
3
2
1
1
4
2
4
2
1
1
0
1
• Bind two protons
together.
• A temperature of
4×107 K is required.
38
A schematic
diagram of
the tentative
plan for deep
underground
isolation of
nuclear
waste.
39
40
41