Transcript Nuclear Stability

Chapter 18
The Nucleus: A Chemist’s View
• All nuclides with 84 or more protons are unstable
with respect to radio active decay.
• Light nuclides are stable when neutron/proton = 1.
For heavier elements the neutron /proton ratio
required for stability is greater than 1 and increases
with Z.
• Nuclides with even numbers of protons and
neutrons are more stable.
• Specific numbers of protons or neutrons (magic
numbers) such as 2, 8, 20, 28, 50, 82, and 126
produce stable nuclides.
Figure 18.1 The Zone of Stability
Types of Radioactive Decay
A nucleus will undergo decomposition to form a
different nucleus which is known as radioactive
decay.
4
Alpha production (): helium nucleus, 2 He
238
4
92 U  2 He

234
90Th
0
Beta production (): 1 e (mass
number remains
constant). Net effect is to change a neutron to a
proton.
234
234
0
90Th 
(thorium)
91Pa

1e
(protactinium)
Types of Radioactive Decay
Gamma ray production (): (high energy
photon)
238
4
92 U  2 He

234
90Th
0
 20 
Positron production: (particle with same
mass as the electron but net effect is to
change a proton to a neutron.
22
0
11 Na  1e

22
10 Ne
Electron capture: (inner-orbital electron is
captured by the nucleus)
201
0
201
Hg

e

80
1
79 Au
 00 
Decay Series
Sometimes a radioactive nucleus
cannot reach a stable state
through a single decay process.
A radioactive nucleus reaches a
stable state by a series of steps.
232 series of decays 208
Th




Pb
90
82
Figure 18.2 A Decay Series
Rate of Decay
Rate of decay is the negative of the change in the
number of nuclides per unit time,
rate = -(N/t)N
Rate = -N/ t = kN (k = proportionality constant)
The rate of decay is proportional to the number of
nuclides. This represents a first-order process.
Integrated first-order rate law is:
ln(N/No) = -kt
where, No = original number of nuclides (at t = 0)
and N = number remaining at time t.
Half-Life
. . . the time required for the number of nuclides to
reach half the original value (N0/2).
When, t = t1/2, N = No/2
ln(N/No) = -kt  ln[(No/2)/No] = -kt1/2
t1/ 2
ln(2) 0.693


k
k
(half life is constant)
If the half-life of a radioactive nuclide is known, the
rate constant can be calculated.
Figure 18.3 The Decay of a 10.0g Sample of Strontium-90 Over Time
Nuclear Transformation
The change of one element into
another.
27
4
30
1
13 Al  2 He  15 P  0 n
249
Cf
98

18
263
1
O

X

4
n
8
0
106
Figure 18.5 A Schematic Diagram of a Cyclotron
Figure 18.6 A Schematic Diagram of a Linear Accelerator
Detection of Radioactivity
Geiger-Muler Counter: High energy particle from
radioactive decay processes produce ions when
they travel through matter. The probe of the Geiger
counter is filled with Ar gas which can be ionized
by a rapidly moving particle.
high energy
Ar(g)
Ar+(g) + eparticle
Electric device detect the current flow and the
number of events can be counted. Thus the decay
rate of the radioactive sample can be determined.
Figure 18.7 A Schematic Representation of a Geiger-Müller Counter
Detection of Radioactivity
Scintillation Counter: Takes the advantage
of the fact that certain substances, such as
zinc sulfide, gives off light when they are
struck by high energy radiation. A photocell
senses the flashes of light that occur as the
radiation strikes and thus measures the
number of decay events per unit of time.
Energy and Mass
When a system gains or loses energy it
also gains or loses a quantity of mass.
E = mc2
E
2  m
c
m = mass defect
E = change in energy
If E =  (exothermic), mass is lost from
the system.
Binding Energy
. . .is the energy required to decompose
the nucleus into its components.
56
Iron-56 26 Fe is the most stable nucleus,
which has a binding energy
per nucleon of 8.79 MeV.
Figure 18.9 The Binding Energy Per Nucleon as a Function of Mass Number
Nuclear Fission and Fusion
Fusion: Combining two light nuclei to form
a heavier, more stable nucleus.
3
1
4
0
2 He  1H  2 He  1e
Fission: Splitting a heavy nucleus into two
nuclei with smaller mass numbers.
1
235
142
91
1
0 n  92 U  56 Ba  36 Kr  30 n
Figure 21.10 Both Fission and Fusion Produce More Stable Nuclides
Figure 18.11 Fission
Figure 18.12 Fission Produces a Chain Reaction
Fission Processes
A self-sustaining fission process is called
a chain reaction.
Neutrons
Causing
Event
Fission
subcritical
<1
critical
=1
supercritical > 1
Result
reaction stops
sustained reaction
violent explosion
Figure 18.13 Fission Produces Two Neutrons
Key Parts of a Fission Reactor
Because of tremendous energies involved, the
fission process can be used as an energy source to
produce electricity. Reactors were designed in
which controlled fission can occur. The resulting
energy is used to heat water to produce steam to
run turbine generators.
Reactor Core: 3%
rods.
Coolant
Containment Shell
235
92
U
+ moderator and control
Figure 18.14 A Schematic Diagram of a Nuclear Power Plant
Figure 18.15 A Schematic Diagram of a Reactor Core
Breeder Reactors
Fissionable fuel is produced while the reactor
runs ( 235
92 U is split, giving neutrons for the
creation of 239
):
94 Pu
1
0n

238
239
92 U  92 U
239
239
92 U  93 Np

239
239
93 Np  94 Pu

0
1 e
0
1e
Biological Effects of Radiation
• Somatic damage: Damage to the organism
itself
• Genetic damage: Damage to the genetic
machinery.
• Biological effects depend on:
1. Energy of the radiation
2. Penetration ability of the radiation
3. Ionizing ability of the radiation
4. Chemical properties of the radiation
source