Radioactivity and Nuclear Chemistry
Nuclear Chemistry
 Nuclei that are unstable and spontaneously
decompose are said to be radioactive.
 Nuclear chemistry is the study of nuclear reactions
and their uses in chemistry.
 Nuclear energy is produced from the radioactive nuclei
and accounts for 20% of our electrical generation in
the US.
Radioactivity
 Atomic number = number of protons in the nucleus
 Mass number = number of protons and neutrons in
the nucleus
 Protons and Neutrons collectively called nucleons.
Radioactivity
 Isotopes = atoms with the same atomic number, but
have different mass numbers.
 Each element can have one or more isotopes that occur
naturally.
 Example) Uranium has three isotopes: U-234, U-235, U-
238
 Each isotope has an abundance.
 U-238 = 99.3%, U-235 = 0.7%, and U-234 = trace
Radioactivity
 Most isotopes are stable and only a few are unstable.
 Unstable isotopes are called radioisotopes.
 A nuclear equation is used to describe a decay process.
238
92
U   
Parent Nuclei
4
2
234
90
Th
Daughter Nuclei
Types of Decay

There are a few common types of particles found in
nuclear reactions.
1.
2.
3.
Alpha = a helium nuclei (He-4). This is a massive
particle, but relatively low energy.
Beta = an electron. The electron comes from the
neutron changing it into a proton. Light mass, but
higher energy.
Gamma = release of a photon of energy. Very light
mass with very high energy.
Types of Decay
4.
5.
6.
Positron = a positively charged electron. Converts a
proton into a neutron. Like the beta particle except
charge is plus one.
Neutrons can be generated in some reactions. These
have a moderate mass and energy.
Electron capture (EC) = the capture of an inner shell
electron by the nucleus. Converts a proton into a
neutron.
Balancing Nuclear Decays
 The sum of all the mass numbers must be equal.
 The sum of all the atomic numbers must be equal.
 The atomic number identifies the nuclide or particle.
 LEP #1
Fundamental Forces
Force
Range
Strength Mediators
Strong Nuclear
10-15 m
1038
gluons
Weak
10-18 m
1025
Electromagnetic infinite
1036
W and Z
bosons
photons
Gravitation
1
???
infinite
Patterns of Nuclear Stability
 Stability of nuclei depends on many factors and no
one factor allows us to predict stability.
 The Strong Force holds the nucleons together.
 So powerful that it can hold together like charges
(protons).
 The Weak Force describes the nuclear decay
processes.
Neutron / Proton Numbers
 For Atomic number of 20 or less, a 1:1 ratio is preferred.
Odd number elements may have exactly one more
neutron than protons.
 For Atomic numbers greater than 20, more and more
neutrons are required. Hg-200 has a 1.5:1 ratio.
 For Atomic numbers greater than 83, all are
radioactive.
Band of Stability
 Shows stable isotopes.
 To the left and less than 83
protons – will decay via Beta
particle.
 To the right and less than 83
protons – will either decay via
positron or do an EC.
 Greater than 83 will decay via
an alpha particle.
Very Heavy Isotopes
 Many very large nuclei will decay via a series of alpha and beta
emissions – this is called a decay series.
Magic Numbers
 Certain numbers of protons and neutrons have a
special stability.
 These are referred to as “Magic” numbers.
 protons = 2, 8, 20, 28, 50, or 82
 neutrons = same as above, but also 126
 When a nucleus has a magic number of both protons
and neutrons, then nucleus is particularly stable.
Even, Odd
 Evens are favored over odds.
 Even protons and neutrons = 157 stable isotopes.
 Even, Odd = 53.
 Odd, Even = 50
 Odd, Odd = only 5!
 obvious one is N-14
 LEP #2
Nuclear Transmutations
 A nuclear reaction can be induced by colliding two
nuclei together.
 Rutherford was first to do this in 1919.
N  He  O  H
14
4
17
1
7
2
8
1
 Used to produce the very largest man-made isotopes.
 LEP #3
 LEP #4
Rates of Decay
 Radioactive decay follows first order kinetics.
 Same as Ch. 14
Nt
0.693
n
 kt and t1/2 
No
k
Rates of Decay
Decay of I-131 (half-life = 8 days)
Amount (grams)
1000
800
600
400
200
0
0
10
20
30
Time (Days)
40
50
60
Carbon Dating
 Carbon-14 is produced in the upper atmosphere from
the nuclear reaction of N-14 with a solar neutron.
 The C-14 is radioactive and undergoes decay with a
half-life of 5730 years.
 Assumption is made that C-14 levels have been stable
for the past 50,000 years.
Carbon Dating
 Thus, all living things – both plants and animals –
have a steady state amount of C-14 until death.
 After death, the C-14 slowly decays and can be
compared to levels in living things.
 Can provide ages for between 100 – 50,000 years old.
 LEP #5
Other Dating Techniques
 Rocks can be aged by comparison of their U-238 to
Pb-206 masses.
 Rock must contain U-238 at formation ,but be free of
any lead.
 In time, U-238 decays back to Pb-206 (see decay series
earlier).
 Calculations using these techniques.
 LEP #6
Detection
 Photographic material – incorporated into a badge
that a worker wears.
 Geiger tube – contains Argon gas inside of a tube that
has a positively charged wire.
2
E=mc
 The energy associated with a nuclear reaction is due to
the loss of mass which is converted to energy.
 Even a very tiny loss of mass can produce a huge
quantity of energy.
 Say, for example, 1 x 10-6 kg is lost. Then,
E = (1 x 10-6kg)(3.00 x 108 m/s)2 = 9 x 1010 J
E=
2
mc
 The energy produced is many orders of magnitude
larger than ordinary exothermic reactions.
 Example: The decay of one mole of U-238 produces
50,000 times more energy than the combustion of one
mole of CH4.
 This is why nuclear energy is so attractive!
 LEP #7
Nuclear Binding Energies
 Scientists in the 1930’s discovered that the mass of
every nuclei after hydrogen is always LESS than the
sum of the individual masses of the protons and
neutrons that make them up.
 Example: Mass of He-4 = 4.0015amu
Mass of 2p + 2n = 4.03188amu
Nuclear Binding Energies
 Missing mass is called the mass defect.
For He-4 = 4.03188amu – 4.00150amu = 0.03038amu
 This is then converted to an energy per nucleon.
(0.0000308kg) x (3 x 108 m/s)2 = 2.772 x 1012 J
2.772 x 1012 J / 6.02 x 1023 atoms/mole = 4.60 x 10-12 J
4.06 x 10-12 J / 4 nucleons = 1.15 x 10-12 J/nucleon
LEP #8
Nuclear Binding Energies
Nuclear Fission
 Nuclear Fission – the process of splitting larger nuclei
into smaller ones.
 U-235, U-233, and Pu-239 will undergo fission when
the nucleus is struck by a slow moving neutron.
 The heavier nuclei does not split the same way – rather
a whole variety of nuclear reactions result.
Nuclear Fission
 All fission reactions produce two smaller nuclei and
several neutrons.
 One possible reaction for U-235 is:
91
1
U  01n  142
Ba

Kr

3
56
36
0n
 Note that the 3 neutrons produced can strike another
U-235 nuclei and split it as well.
235
92
Chain Reactions
 Because each U-235 that splits generates two or
more neutrons, the possibility of a chain reaction
occurs.
 Critical Mass – the minimum amount of U-235
necessary to maintain the chain reaction. This
means that exactly one neutron will continue the
reaction each time.
 Supercritical Mass – exceeds the critical mass.
Results in an uncontrolled chain reaction.
Splitting of U-235
Critical Mass
Nuclear Weapons
 A nuclear weapon (bomb) can be constructed if
you have two or more sub-critical masses of U-235,
which when combined would produce a critical
mass.
 The critical mass of U-235 is about 1 kilogram.
 Problem: Naturally occurring Uranium contains
only 0.7% U-235.
 Solution: Must separate the U-235 from the other
isotopes.
Enrichment
 Enrichment is the process by which the quantity of U-
235 present in a sample is increased by removing the
other undesirable isotopes.
 This is NOT easy to do!
 U.S. used gaseous diffusion of UF6 back in the early
1940’s to obtain enough U-235.
 Current methods involving using centrifuges.
Weapon Design
 Problem: sub-critical masses need to be kept separate
until the weapon is deployed. Then, they must be
combined to produce the super-critical mass.
 Solution: Implosion of sub-critical masses forces them
together.
 First design was relatively simple.
Basic Weapon Design
Nuclear Reactors
 The energy of a nuclear reaction can be captured in a
nuclear reactor.
 Uranium ore is enriched to about 3% U-235 and
converted to UO2. The UO2 pellets are then encased
in either Zr or stainless steel tubes and referred to as
fuel rods.
 Rods composed of Cd or B, which are good absorbers
of neutrons are also constructed and are referred to as
control rods.
Simple Reactor Design
Simple Reactor Design
Fast Breeder Reactor
 A proven, yet unused method to make more nuclear fuel than
it consumes.
 Uses “fast” neutrons and heats a liquid metal like sodium.
Nuclear Wastes
 Fission products accumulate as the reactor operates.
 Fuel rods must be replaced or reprocessed periodically.
 Every year about 1/3 of the fuel rods are replaced or
repacked.
 When replaced, the spent fuel rods are still highly
radioactive and are stored on site in large water pools.
Nuclear Wastes
 One of the side products is Pu-239 – another
fissionable isotope.
 Pu-239 can be separated from the other wastes and is
easily weaponized.
 US foreign policy.
Fusion
 The Sun and other stars use a different type of nuclear
reaction called fusion.
 Fusion occurs when two or more smaller nuclei are
squeezed together to make a larger isotope.
 The net reaction on the Sun is:

4 H  He + 2 e+
 This requires very high temperatures and pressures –
of the type found only in stars.
Fusion
 Fusion on this planet can be achieved in a special
reactor.
 The lowest energy reaction for fusion is:
2
1
H  31H  42 He  01n
 There is enough deuterium (H-2) and tritium (H-3)
present in the world’s oceans to supply us with fusion
energy forever.
 Why is this not feasible?
 LEP #9
Fusion
 A novel process for fusion has been proposed by Dr. Robert
Bussard (deceased).
 Uses a Boron-11 and H-1 collision to generate three alpha
particles.
 Research efforts can be followed at: http://focusfusion.org/
Biological Effects of Radiation
 We all receive some radiation whether we want it or not.
 Background radiation comes from many sources including:
 Food – K-40
 Air – Rn-222
 Ground – U-238
 Also are exposed to man-made sources like X-rays, nuclear
medicine, air travel, and cigarettes.
 Total background average is about 360mrem.
Ionizing Radiation
 When molecules absorb radiation it can lose an
electron.
 For example, when radiation strikes a water
molecule:
H2O + radiation  H2O+ + 1e That ion then reacts with a second water molecule:
H2O+ + H2O  H3O+ + OH
Free Radical
 The OH has an odd number of electrons and is called
a free radical.
 Any free radical is highly reactive and can cause
biomolecules to form free radicals.
 Free radicals can also interfere with electron transfer
reactions.
Radiation Doses
 Two factors are combined:
 rad = radiation absorbed dose = 1 x 10-2 J/kg of body
tissue
 RBE = multiplier that depends on the particle

RBE = 1 for beta and gamma, RBE = 10 for alpha
 rem = rad x RBE
Damages By Particle
 Alpha = least penetrating (skin is enough protection),
but have highest RBE.
 Can cause great damage internally in soft tissues like
the lungs.
 Beta = can penetrate the upper layers of the skin (thick
clothing provides protection).
 Gamma = can penetrate completely.
Total Effects
 Damage depends on the activity, source, and whether
it is internal or external exposure.
 Tissues most affected are those that reproduce rapidly
like the skin, marrow, and intestinal linings.
 Tissues least affected are those that undergo little or
no cell division like the brain, muscles, and nerves.
Radiation Levels
 A whole body exposure for an adult of ___rem is called
an LD50 level.
 A ___rem exposure or more will kill all white blood
cells in the body.
 Exposure of less than 25rem has no noticeable effect,
but long term health effects are unknown.
 OSHA limit for workers is ___rem/year.
Medical Applications

Radioisotopes are used in two unique ways.
1.
2.
Diagnostic Use – low doses with short half-lives are
injected into the body. These help to illuminate a
targeted organ or region in the body.
Therapeutic Use – high doses with short half-lives
target tumor cells.