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Nuclear Chemistry
Types of Radiation
There are four main types of ionizing radiation:
1.alpha rays:
Helium nuclei - 2 protons + 2 neutrons
2.positron rays: Positrons - the antimatter counterparts to electrons; same mass
but charge is +1
3.beta rays:
Electrons
4.gamma rays: High energy photons - ν ≥ 1020 Hz - higher energy than xrays
radiation
alpha
positron
beta
gamma
symbol
4
2  or
0
 or
1
0
 or
-1
0
0
4
2 He
0
1e
0
-1 e
charge
mass
(per particle)
penetrating
power
+2
6.65 × 10-24 g
lowest
+1
9.11 × 10-28 g
medium
-1
9.11 × 10-28 g
medium
0
0
highest
Nuclear Reactions
o
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Chemical reactions never alter the identity of the atoms involved
In nuclear reactions, the total numbers of nucleons – protons, neutrons and electrons –
remains constant, but chemical identity can change when the particles interchange
Nuclear reactions permit the transmutation of elements
Balancing Nuclear Reactions
(1) The sum of the mass numbers of reacting nuclei must equal the sum of the mass
numbers of the nuclei produced (conservation of mass).
(2) To maintain charge balance, the sum of the atomic numbers of the products must equal
the sum of the reactants
Example of Balancing a Nuclear Reaction
226
88
Mass number (protons+neutrons)
Atomic number (protons only)
particle
radium226
Ra 
226
88


4
2
radon222
He 
4
2
+
+
222
86
Rn
222
86
7 Classes of Nuclear Reaction: 1 - 2
1. Alpha emission
In this spontaneous reaction an alpha particle is emitted.
234
92
U  42 He 
230
90
Th
2. Beta emission
In this spontaneous reaction an beta particle (electron) is emitted.
235
92
U 
0
1
235
93
Np
This implies that the nucleons are transformed during the process, where one of the
neutrons is converted into a proton and a high-energy electron is ejected:
n  01  11p
1
0
Note that beta emission increases the atomic number by one.
7 Classes of Nuclear Reaction: 3 - 4
3. Positron emission
This is the opposite of a beta emission where a new particle, called the positron is emitted.
207
84
Po   
0
1
207
83
Bi
Essentially, a proton decomposes to a neutron and a positron which is emitted by the nucleus. This
transformation, at the nucleon level, implies the reaction occurs within the nucleus:
p  n
1
1
0
1
1
0
Positrons and electrons are antiparticles, meaning that If the two were to meet, they annihilate each
other releasing two high-energy gamma ray photons.
This reaction decreases the atomic number by 1.
4. Electron (K) capture
An electron from the atom is incorporated into the nucleus. The electron is from the 1s level also known
by the older name of K-level. This spontaneous process is the opposite of beta emission, where a proton
takes on an electron to form a neutron
7
4
Be 
0
1
e  37 Li
This reaction gives same net product as positron emission, decreasing the atomic number by 1.
7 Classes of Nuclear Reaction: 5
5. Nuclear fission
Nuclear fission is always an induced process, where a large nucleus splits into several
smaller nuclei. The actual fission process below is the decay of uranium-236. 236U is made
by neutron bombardment of 235U. :
235
92
U  01n 
236
92
U  141
56 Ba 
92
36
Kr  3 01n
Fission is accompanied by the release of a lot of energy, and this is the source of the power
both of the atomic bomb and nuclear power
7 Classes of Nuclear Reaction: 6 - 7
6. Nuclear fusion
Fusion is the combining of two light elements into a heavier nucleus. A lot of energy is
released. (hydrogen bomb - where the hydrogen refers to the heavier hydrogen isotopes
deuterium and tritium).
H + H  He + n
2
1
3
1
4
2
1
0
7. Nuclear transmutation
The combination of heavier nuclei to produce artificial elements. All the elements beyond
uranium are artificial, in the sense that none have stable isotopes.
10
5
B
252
98
Cf 
257
103
Lr  5 01n
This classification includes all the remaining artificial, i.e. induced, nuclear reactions
Practice
Question
1
Example
Problem
o
Complete or balance the following nuclear reaction equations by identifying the missing
components. Classify each reaction according to one of the 7 classes of reaction introduced.
U
a)
239
92
b)
2
1
c)
1
0
d)
38
19
e)
235
92
f)
44
22
g)
246
96
239
93
0
1
Np  e
 emission
H  11H  31He
Nuclear fusion
32
4
n  35
Cl

P

17
15
2
Induced α emission,
 nuclear transmutation
e
K  38
18 Ar  1
0
1
I
U  95
Y
+
+3
53
39
0n
0
Ti + -1e 
137
44
21
Sc
1
Cm  126 C  254
No

4
102
0n
Positron emission
Nuclear fission
Electron capture
Nuclear transmutation
Gamma rays (and neutrinos)
o
Gamma rays accompany most nuclear reactions.
o
When a nucleus emits an alpha particle, the resulting nucleus is usually in an unstable,
higher energy form - Excited nuclear state
It quickly decays to the nuclear ground state by releasing the excess energy as a gamma
ray.
The most dangerous part of a nuclear process, as gamma rays have high penetrating
power.
o
o
o
o
o
The energy of the gamma rays varies greatly, but is always characteristic of the type of
nuclear transformation.
Gamma spectroscopy analyzes the wavelengths of the emitted gamma rays, which can be
used to identify the source of the nuclear reaction.
Hence it is possible to detect the kind of radioactive process occurring when radioactivity is
detected
Why are some nuclei stable, others not?
o
o
o
o
o
o
o
o
Black dots – stable nuclides
Red dots - unstable nuclides
Pink region - all combinations of N (number of
neutrons) and Z (number of protons) that
cannot exist
Zone of black dots - belt of stabilityneutrons are needed to stabilize protons (+ve
repulsion)
At low Z, N ≈ Z; at high Z, N > Z
Unstable nuclides surround the belt of
stability
Z > N - Nuclides with more protons allowed
for stability, attain it, by changing the ratio of
N/Z by:
 Heavy nuclei - alpha (α) emission
 Light nuclei- positron emission or
electron capture
N > Z -Nuclides with more neutrons than
protons convert neutrons into protons
 This is achieved by beta () emission
N
1
Z
Binding energy
4
2
56
26
He
Fusion
zone
Region
of
greatest
stability
Fe the most stable nuclide in the Universe
Susceptible to
fission
All nuclei strive to
reactions
maximize stability
via nuclear reactions
Binding energy is the energy change that
occurs if a nucleus were formed directly
from its component protons and neutrons.
The source of the energy is a loss of mass
E  m  c
2
Calculating binding energy
o
o
We use the tabulated masses of free protons and neutrons, and compare them to that of
any given nuclide
The difference in mass is converted to energy using the Einstein equation from special
relativity
 The mass of one mole of “free” protons is 1.0072765 g/mol
 The mass of one mole of “free” neutrons is 1.0086649 g/mol
 The mass of one mole of “free” electrons is 0.0005486 g/mol
Example: Calculate the binding energy of the 16O isotope with mass of
15.994916 g/mol, and the binding energy per nucleon. Note that the isotope
mass includes the electrons
m  8  1.0072765  8  1.0086649  15.994916  8  0.0005468  0.136986
proton
neutron
isotope
0.136986 gmol
8
E  m c 

(2.9979246

10
1000 g kg
2
Ebinding
E 1.2312  1010


16
16
kJ
mol
electron
m
2
s) 
 7.70  108
kJ
1kJ
 1.2312  1010
1000J
mol
of nucleons
kJ
mol
g
mol
Example – Binding Energy
Example 1
Given the mass of 55Mn (z = 25) is 54.9380 amu. Calculate the binding energy
for a mole of nucleons
8.49*1011 J/mol
Range of binding energies
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In the sample calculation on the previous slide, we calculated the binding energy of the 16O
nuclide and also the binding energy per nucleon
A nucleon is either a proton or a neutron
It is found that the binding energies of all the known nuclides falls in the range from:
~ 0 – 9.0  108 kJ/mol of nucleon = 0 – 900 GJ/mol of nucleon
o
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o
The most stable nuclide is 56Fe
Sometime values are given in MeV, which is a measurement per individual nucleon
Chemists most often work out energies in units of kJ for mole quantities of matter, so that
we will always work out total and per-nucleon binding energies in kJ/mol
However, the MeV unit is widely used in the nuclear industry. The conversion factor is:
1 eV = 96.485342 kJ/mol
o
1 MeV = 9.6485342  107 kJ/mol ≈ 108 kJ/mol = 100 GJ/mol
The range of binding energies per nucleon in MeV is therefore:
~ 0 – 9.0 MeV
Rates of Nuclear Decay
Nuclear decay is measured as the number of nuclei that disintegrate in a given period of
time. This is termed the activity (A) of a sample and is directly proportional to the number
of radioactive atoms (N) in the sample:
N
A
 kN
t
where k is the rate constant (decay constant).
By integrating this equation, we get a first-order rate equation:
 A 
ln 
  k t
 Ao 
N
ln     k t
 No 
where A0 and N0 are the initial activity and number of radioactive atoms while A and N
are the activity and number of radioactive atoms after time ‘t’.
At t1/2, N = ½ N0 and A = ½ Ao. A relationship between k and t1/2 can be found as:
 Ao / 2 
 1
ln 
   k t1/ 2  ln    0.693
2
 Ao 
0.693
t1/ 2 
k
Examples – Radioactive Decay
Example 1
is a -emitter used to treat cancer. Calculate the fraction of 60Co left
after 20 years of preparation.
60Co
7.79 %
Example 2
A 2.00 mg sample of pure 32P was prepared was found to contain 0.40 mg of
32P after 33.3 days. Determine t1/2.
14.3 days
Examples – Dating
Example 3 - Uranium Dating.
A sample of ore contains 5.20 mg of 238U and 1.85 mg 206Pb. Calculate the
age of the rock. (t1/2 = 4.51*109 years)
t = 2.26*109 years
Example 4 - Radio Carbon Dating
A charcoal sample taken from Stonehenge has an activity of 9.65 events per
minute per gram of carbon. Determine the age of the sample given that
ambient activity of carbon is 15.3 min-1g-1 and t1/2 = 5730 years.
t = 3830 years
Nuclear Decay – The Uranium Series
All isotopes of
uranium are
unstable, but
238U decays
extremely
slowly.
In this region there are several
pathways, but all lead to the same
species, 206Pb
This is the 238U
decay series,
showing the
steps in the
decay of this
nucleus to
eventually
produce stable
206Pb
Nucleosynthesis by transmutation
o
o
All the elements beyond 92U are synthetic. They are made either as a by product of
operating a nuclear reaction (neutron source) or by deliberate transmutation
Latest to be recognized: 272
111Rg It was made by a nucleosynthesis reaction:
209
83
o
o
Bi 
o
o
Ni 
Rg  01n
272
111
Roentgenium is the most recent of the transuranium elements which start with 93Np,
complete the actinides and most of the 6d series of the PT
Early efforts at transmutation date back all the way to Rutherford. For example, he
performed the reaction:
4
2
o
64
28
He 
14
7
N  178 O  11H
Alpha-particles: high repulsive forces experienced when the 2+ alpha nucleus comes near a
strongly positive charged nucleus
Neutron: uncharged particle reaches target much easier, and efforts at transmutation
switched to neutron bombardment
235
The discovery of fission was a result of such studies: neutron bombardment of 92 U
Nucleosynthesis by transmutation
o
Successful neutron bombardment: an example is the “manufacture” of plutonium, which is a
two-stage process:
n
1
0
238
92
239
93
o
o
o
o
239
92
U
Np  -10 β +
239
93
239
94
Np 

0
1
Pu
This is the process that is used in “breeder” reactors to convert unusable 238U into
fissionable 239Pu
This method works well up to element 101, Mendelevium, but beyond this larger “bullets”
must be used, as for example in:
10
5
o
U
B
252
98
Cf 
257
103
Lr  5 01n
Such reactions suffer from very strong nuclei-nuclei repulsion, and require great effort and
very specialized apparatus
Many transuranium elements from the Actinides have been made in sufficient quantity to
have some of their basic chemical properties investigated
The “super-heavy” elements of the 6d block are usually only made a few atoms, or “events”
at a time and nothing is known of their chemistry. Roentgenium is “heavy” gold!
Energy ranges of the main types of nuclear
radiation and detection of radiation



o
 particles - energy range 3.5 - 10 MeV
 particles (and positrons) - energy range 0.18 - 3.6 MeV
 radiation - energy range 0.008 - 7.11 MeV
Radiation may be detected by a Geiger counter.
o
This device is suitable for any type of radiation capable of ionizing argon gas, causing a
current to flow and a needle or loudspeaker to sound.
This includes those we have talked about, , , and , as well as high-energy X-rays. As a
group, these are called "ionizing radiation".
o
Radiation Safety
o
The unit of radioactivity:
Curie Ci; 1 Ci = 3.70  1010 events per second (Becquerel, Bq)
The Curie measures the rate of nuclear decay - similar to half-life.
For a given isotope, with a known half-life, the Curie indicates much of that isotope is
present
o
The unit of radiation intensity:
rad = radiation absorbed dose = amount of radiation that results in absorption of 1  10-5 J
g-1 of absorbing material.
It varies with the type of radiation and its source. All sources of nuclear radiation are
distinguishable by type and energy.
o
The Röntgen equivalent for man: Unit of exposure – related to risk
rem ; 1 = 1 rad  1 RBE
This measures the relative effect of different types of radiation (, ,  or X) on humans,
which is expressed as the RBE, relative biological effectiveness. Multiplying the RBE by rad
results in a unit that tells just how much a certain radioactive source, with a certain intensity,
will harm you.
Nuclear Power -The fission process
o
Typical reaction is 235U fission is illustrated by the equation:
n
1
0
o
o
o
o
o
235
92
U
Ba 
141
56
92
36
Kr  3 n
1
0
E = –2.1  1010 kJ / mol
This is not the only possible reaction: a variety of daughter isotopes are produced (As, Br,
Sr, Zn, and Zr), some of which are stable, but most of which are radioactive themselves (e.g.
as -, + or  emitters).
These reaction can release 1, 2 or 3 neutrons, and on average 235U fission releases 2
neutrons for every one captured.
To be self-sustaining, a nuclear reactor needs to control the fast neutrons produced in
fission.
The right number of fast neutrons must be slowed down to a speed where they can be
captured by 235U nuclei.
This is accomplished by using a moderator. Properties of a moderator include:
a) must not absorb many neutrons, since these are required to sustain a nuclear chain
reaction
b) must be light in mass so that the neutrons are not slowed too much
c) must not react with neutrons to form radioactive species
d) must have a high probability of collision with a neutron
Nuclear chain reactions
o
o
The splitting of 235U requires first that it is hit by a neutron to make 236U, which then
spontaneously breaks into many pieces, including two lighter units and several neutrons
The substances which have been used as moderators are water, D2O (heavy water) and
graphite. Water and D2O are favoured because they are liquids, and can double as heat
transfer agents by circulation through the reactor and a heat exchanger.
Energy considerations in nuclear power
o
Nuclear reactors have all sorts of technical difficulties associated with handling radioactive
fuels and by-products. Why bother?
o
A small amount of nuclear fuel can release a large amount of energy. The origin of this
energy is the nuclear binding energy.
o
The nucleons in the daughter isotopes have much less total binding energy than the parent.
This excess binding energy is released and converted into heat.
o
o
o
For this fission reaction the total mass change is 0.19 u per 235U nucleus.
This converts to 7.3  1010 J or 73 GJ per gram of 235U.
In consumer terms, where 1 kW hr of power is equivalent to 3.6 MJ, each gram of 235U can
supply 20,000 kW hr of heat energy. (average household uses ~10 GJ per Month)
o
This is the maximum possible there are significant losses in electric power generation.
Environmental factors for nuclear power
FOR
1.
Small fuel quantities – less mining activity
s;
2. less mining
No greenhouse gasses (CO2) or acid rain (SO2)
1.
AGAINST
Risk of accidental explosion
2.
Leakage of radioactive material
3.
No fly ash
3.
Disposal of radioactive spent fuel
4.
No solid ash
4.
High capital cost
5.
no flooding of large areas (Hydro)
6.
No need for long transmission lines
o
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o
o
o
o
The major difficulty with non-nuclear power generation is simply the huge scale of these
operations.
A 2000 MW coal-fired power station releases 42,000 tons of CO2, 600 tons of SO2 (and
related acid gases) and 10 tons of fly ash per day.
Scrubbers can reduce the acid gases and fly ash considerably, but nothing can be done
about the CO2 greenhouse gas
Power in Alberta is generated near Edmonton from large coal thermal generating stations
It must be transmitted great distances, with considerable losses during transmission
Nuclear reactors are usually located close to residential areas: thus they overcome the large
losses from long-distance transmission
The CANDU reactor for electric power generation
o
Schematic diagram of a CANDU nuclear reactor as used in a typical electric generating station
o
o
o
This is a unique Canadian design that has been used in several countries including Canada
There are none in Western Canada, but several in Ontario, Quebec and New Brunswick
The name stands for CANada Deuterium Uranium. It is unique in its ability to burn normal
uranium dioxide instead of needing enriched fuels
Application of Nuclear Reaction in Medicine
Imaging methods
The pattern of radiation emitted from a nuclear reaction in the body can be
reconstructed to make images of parts of the body. An agent that undergoes a
nuclear reaction must be administered (invasive techniques)
Chemical Identification
An agent that undergoes a nuclear reaction can be attached to a drug molecule
that binds to a specific area in the body of interest. Ex) cancer cells, blood clots,
diseased cells, receptors specific to an organ.( in the brain). Imaging methods
are used to observe the phenomenon of interest.
Treatment
In radiation therapy radiation produced from a nuclear reaction can be used a
destroy diseased tissue. Sometime a drug is administered to make the tissue
more susceptible to radiation damage.
Imaging
A common positron
emitter is 18F,
18
9
F  01  188 O
PET – Positron
Emission
Tomography
It is incorporated into
[18F]-dopamine.
The dopamine accumulates in the putamen of a normal human
brain (right). A Parkinson’s disease patient shows much lower
accumulation in the putamen (left)
The emitted positron immediately encounters an electron, which
undergoes an annihilation event releasing two photons. The
detector senses the photons and where they come from which is
used to reconstruct an image.
A whole-body scan (photographic process) undertaken with a phosphate
complex labeled with 99mTc that accumulates in bones. 99mTc emits gamma
radiation with is detected.