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U13 – Nuclear Chemistry
Chapter 19
19.1
Nuclear Stability and Radioactive Decay
Review
• Atomic Number (Z) – number of protons
• Mass Number (A) – sum of protons and neutrons
A
Z
X
19.1
Nuclear Stability and Radioactive Decay
• Nucleus undergoes decomposition to form a
different nucleus.
• Nuclides with 84 or more protons are unstable.
• Light nuclides are stable when Z equals A – Z
(neutron/proton ratio is 1).
• For heavier elements the neutron/proton ratio
required for stability is greater than 1 and increases
with Z.
19.1
Nuclear Stability and Radioactive Decay
• Certain combinations of protons and neutrons seem
to confer special stability.
 Even numbers of protons and neutrons are more
often stable than those with odd numbers.
• Certain specific numbers of protons or neutrons
produce especially stable nuclides.
 2, 8, 20, 28, 50, 82, and 126
19.1
Nuclear Stability and Radioactive Decay
Zone of Stability
19.1
Nuclear Stability and Radioactive Decay
19.1
Nuclear Stability and Radioactive Decay
Types of Radioactive Decay
• Alpha production ( ):
• Beta production ( ):
A neutron is converted into a proton and electron
19.1
Nuclear Stability and Radioactive Decay
Types of Radioactive Decay
• Gamma ray production ( ) w/ alpha particle emission:
Gamma rays are high energy photons
• Positron production:
A proton is converted to a neutron and a positron (an electron w/ a positive charge)
19.1
Nuclear Stability and Radioactive Decay
Types of Radioactive Decay
• Electron capture:
Inner-orbital electron
The nucleus absorbs an electron changing a proton to a neutron
19.2
Kinetics of Nuclear Decay
Rate of Decay
Rate = kN
• The rate of decay is proportional to the number of
nuclides. This represents a first-order process.
Half Life
• Time required for the number of nuclides to reach
half the original value
• Amount = original amount x (.5)(# half lives)
• Or
ln  2  0.693
t1/ 2 =
=
k
k
19.3
Nuclear Transformations
The sum of the A values on either side of the arrow must be equal.
The same is true for the Z values.
27
13
249
98
Al + He 
4
2
30
15
1
0
P+ n
263
Cf + 188 O  106
Sg + 4 01 n
Figure out the type of radiation first, then determine A and Z
values, and then identify elements.
19.4
Detection and Uses
• This section is not directly tested on the AP
exam.
• They may ask how radiation is used in
industry, but that is all. Please read it.
– Medical uses
– Radioactive dating
– Geiger Counters
19.5
Thermodynamic Stability of the Nucleus
Energy and Mass
• When a system gains or loses energy it also
gains or loses a quantity of mass.
E = mc2
m = mass defect
E = change in energy
• If E is negative (exothermic), mass is lost from
the system.
19.5
Thermodynamic Stability of the Nucleus
Mass Defect (Δm)
• Calculating the mass defect for 42 He:

Since atomic masses include the masses of the electrons,
we must account for the electron mass.
4.0026 = mass of
1.0078 = mass of
•
4
2 He
1
1H
atom = mass of
atom = mass of
4
2 He
1
1H
nucleus + 2me
nucleus + me
4
2
He nucleus is “synthesized” from 2 protons and
two neutrons.
m =  4.0026  2me  
m =  0.0304 amu
2 1.0078  me  + 2 1.0087
19.5
Thermodynamic Stability of the Nucleus
• The energy required to decompose the nucleus into
its components.
• Iron-56 is the most stable nucleus and has a binding
energy of 8.97 MeV.
1 eV (electron-volt) is a unit of energy equal to 1.602 x 10-19 J. It is
related to the energy of an electron as it accelerates through a potential
difference of one volt.
So it would take (8.97 x 109) x 1.602 x 10-19 J to break apart one iron
atom. It would take (6.0221 x 1023) x (8.97 x 109) x (1.602 x 10-19) to
break apart 56g (1 mole) of iron. That would be 8.65 x 1013 J. This is
roughly equivalent to 1.15 x 107 kg or 12,600 tons of dynamite.
19.5
Thermodynamic Stability of the Nucleus
19.6
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.
1
0
n+
235
92
U
142
56
Ba +
91
36
1
0
Kr + 3 n
19.6
Nuclear Fission and Nuclear Fusion
• A self-sustaining fission process is called a chain
reaction.
Neutrons
Causing
Fission
Event
Event
subcritical
<1
critical
=1
supercritical
>1
Result
reaction stops
sustained reaction
violent explosion
19.6
Nuclear Fission and Nuclear Fusion
Nuclear
Power
Plant
Schematic
19.6
Nuclear Fission and Nuclear Fusion
Nuclear Core Schematic
19.7
Effects of Radiation
Depend on:
1. Energy of the radiation
2. Penetrating ability of the radiation
3. Ionizing ability of the radiation
4. Chemical properties of the radiation source
19.7
Effects of Radiation
• The energy dose of the radiation and its
effectiveness in causing biologic damage must be
taken into account.
Number of rems = (number of rads) × RBE
rads =
RBE =
radiation absorbed dose
relative effectiveness of the
radiation in causing biologic
damage