Download Notes: Nuclear Chemistry

Notes: Nuclear Chemistry
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*Note:
Nuclear reaction = any reaction in which there is a change to the atomic nucleus.
a. These reactions cannot be turned off.
a. These reactions are not affected by change in temperature or pressure.
a. In these reactions atoms of one element are usually converted into atoms of different elements.
a. In nuclear reactions matter and energy are inter-converted (E = mc2).
a. One gram of matter produces 9.00 x1010 kJ of energy. ΔE = (9.00 x1010 kJ/g)(Δm)
a. Nucleons are conserved during nuclear reactions.
Transmutation = the induced conversion of an atom of one element to an atom of another element.
Natural transmutations are termed decay.
a. Artificial transmutation occurs when high-energy particles (protons, neutrons or alpha particles)
are bombarded at very high speeds against the nucleus of an atom.
a. The first artificial transmutation was performed by Rutherford in 1919; this led to the discovery of
the proton. 147N + 42He  178O + 11H (proton)
a. The discovery of the neutron in 1932 by Chadwick was also the result of artificial transmutation.
a. The trans-uranium elements (those with atomic numbers above 92) are synthesized using artificial
transmutation and are all radioactive elements.
a. Neptunium and plutonium, #93 & #94, respectively, were synthesized in 1940. Since, many more
have been synthesized.
Radioisotopes = isotopes of an element which have unstable nuclei.
a. Stability of the nucleus depends on the ratio between the # of protons and the # of neutrons (too
many or too few neutrons).
a. For light elements (1-20), proton to neutron ratio should be 1:1 for stable nuclei (non-radioactive).
a. For heavier elements (above 20), proton to neutron ratio increases gradually to 1:1.5 for stable
nuclei.
Radioactivity = the spontaneous emission of penetrating rays/particles (radiation) by a radioactive source.
a. Emissions can be of particle nature or consist of high-energy electromagnetic waves (photons).
a. Radioactive isotopes gain stability by making changes in their nuclei through spontaneous
disintegration. This is natural radioactive decay.
a. Artificial bombardment of the nucleus with an energetic particle that changes the composition of
the nucleus. This is artificial transmutation.
a. Discovered by scientists Becquerel and his students, Marie & Pierre Curie in 1896.
a. They won the Nobel Prize for their work in 1903.
Radioactive decay = natural spontaneous process that does not require the input of energy and that
produces changes in the nuclei of unstable isotopes; breaking down of an unstable nucleus which loses
energy by the emission of radiation, creating a more stable nucleus.
a. These changes are accompanied by the emission of large amounts of radiation in the form of
i. Alpha radiation (α)
ii. Beta radiation (β-)
iii. Gamma radiation (γ)
iv. Positron radiation (β+)
v. X-ray radiation
Half-life (t1/2) = the time required for one-half of the atoms of a radioisotope to emit radiation and to decay
to other products.
a. The stability of a radioisotope is determined by its half-life; the longer the half-life, the more
stable the isotope.
a. Half-lives vary from fractions of seconds to millions of years.
a. Short half-life (highly unstable) isotopes are useful in medicine because they are not long-term
biological radiation hazards.
a nuclear reaction transforms an isotope of one element (nuclide) to an isotope of another element.
Types of radiation:
1. Emission of alpha particles = the equivalent of a helium nucleus containing two protons and two neutrons.
The symbol for alpha radiations is α or 42He. It has the lowest penetrating power and a +2 charge. Skin or
a sheet of paper can stop alpha particles. In magnetic or electric fields, they are deflected toward the
negative end.
© T. M. Chipi, chemistry instructor
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Notes: Nuclear Chemistry
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Emission of beta particles = the equivalent of an electron caused by the breaking down of a neutron into a
proton and a fast-moving electron. The symbol for beta radiation is β- or 0-1e. It has moderate penetrating
power and a -1 charge. Beta particles need thin sheets of metal (>3mm) to stop their penetration. They are
deflected toward the positive end in a magnetic or electric field.
Emission of gamma rays = the equivalent to high-energy electromagnetic radiation (visible light is lowenergy electromagnetic radiation). It is emitted by the nuclei of disintegrating radioactive atoms along with
alpha, beta, or positron radiation. The symbol for gamma radiation is γ or 00γ. Gamma rays have no mass
and no electrical charge. Except for where they originate, gamma rays (nucleus) and x-rays (electron
cloud) contain essentially the same amount of electromagnetic energy. They both have the highest
penetrating power (very dangerous). Thick lead shielding and/or thick walls of high-density concrete can
stop gamma rays. Gamma rays are unaffected by magnetic or electric fields.
Emission of positrons = the equivalent of a positive electron or a positron (particle of antimatter) caused by
the breakdown of a proton into a positron and a neutron. The symbol for positron radiation is β+ or 0+1e. It
has a limited penetrating power and a +1 charge. Positrons have limited penetration into matter because
when a positron comes into contact with an electron, they annihilate each other producing two γ-rays.
Positrons are deflected towards the negative end in a magnetic or electric field.
Annihilation: the destruction of a particle and its antiparticle due to a collision. 0+1e + 0-1e  2 00γ (energy)
Emission of x-rays = the equivalent to high-energy electromagnetic radiation similar to gamma rays. X-rays
originate in the electron cloud due to the fall down of an outer electron to occupy the space left by a firstenergy level electron (K-level) captured by the nucleus. In the nucleus, the electron combines with a proton
to form a neutron. High energy (x-ray) is released in the electron cloud by the movement of an electron
from a higher energy level to the lowest energy level. It has a high penetrating power and it is very
dangerous. Thick lead shielding and/or thick walls of high-density concrete can stop X-rays. X-rays are
unaffected by magnetic or electric fields.
Nuclear Reactions:
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Nuclear Binding Energies:
ΔE = Δmc
2
ΔE = Δm(3.00x10 8
m 2
)
s
2
m2
J
m
m2
1
ΔE = Δm(9.00x10 2 ) 1J = 1kg 2
2 =1
kg so
s but
s and s
16 J
10 kJ
ΔE = 9.00x10
(Δm) ΔE = 9.00x10
(Δm)
kg
g
or
where 1g = 6.022 x 1023amu
16
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€
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The mass of a nuclide
€ (nucleus) is less€than the mass of its nucleons (protons and neutrons).
Mass defect is the difference in mass between the nucleus and the masses of its nucleons.
Nuclear binding energy is the energy required to separate a nuclide into its nucleons. It follows that
this binding energy
€ is released when a nucleus forms from its nucleons (a lower energy state is a more
stable state; a nucleus is more stable than its separate particles).
d. Since E = mc2, the binding energy is related to the mass defect.
e. The larger the binding energy, the less likely a nucleus is to decompose.
f. Average binding energies increase to a maximum at a mass number between 50 and 60; Iron-56 has
the largest binding energy per nucleon (1.41 x 10-12 J) and is considered the most stable isotope.
g. Heavy nuclei gain stability by splitting into smaller nuclei. They give off energy if fragmented into two
mid-sized nuclei (near that of Fe-56). This reaction is called fission.
h. Very light nuclei are combined or fused together to form more massive nuclei. Energy is released
from this nuclear fusion.
Fission = the splitting of the nucleus of an atom into two or more nuclei.
a. During a nuclear fission reaction, enormous amounts of energy are produced.
b. Nuclear fission occurs as a chain reaction (critical mass needed) and must be controlled.
c. If a fission reaction is left uncontrolled, then it results in a nuclear explosion.
d. There are only two naturally occurring fissionable isotopes: U-235 (0.7% abundant) and U-233 (less)
e. The control of nuclear fission involves:
© T. M. Chipi, chemistry instructor
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Notes: Nuclear Chemistry
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Neutron moderation = uses a moderator (water or carbon) to slow down the neutrons
being bombarded so they can be captured by the reactor fuel (usually 3% enriched
uranium-235 or Pu-239) to continue the chain reaction.
ii.
Neutron absorption = uses control rods (usually cadmium) to decrease the number of
slow neutrons by absorbing them and preventing them from hitting fissionable atoms,
thus slowing down the chain reaction.
iii.
All nuclear reactors have 5 basic components:
1. Fuel; enriched (3%) U-235 or Pu-239, which are fissionable.
2. Moderator; carbon or water (may be deuterium oxide, D2O; also called heavy water)
3. Control rods; cadmium
4. Coolant; liquid sodium or water (may be deuterium oxide/heavy water)
5. Shielding; high-density concrete walls
Fusion = the combining of two atomic nuclei to produce one nucleus of heavier mass.
a. Fusion reactions release more energy than fission reactions.
b. Fusing nuclei must be brought to a distance less than 10-13cm in order to overcome the strong repulsive
forces between two neighboring positively charged nuclei.
c. Fusion takes place at temperatures that exceed 40,000,000 K (Kinetic Energy).
d. At these high temperatures, matter exists in the form of plasma (atomic nuclei with loose electrons).
e. Containing plasma is a nearly impossible task in fusion reactions. Hence, fusion reactions are difficult
to control, or virtually uncontrollable on Earth.
f. The Tokomak has been used to contain small amounts of plasma resulting from fusion reactions. The
plasma is suspended in a magnetic field.
g. The Tokomak fusion reactor at Princeton was shut down in 1997 when government funding stopped.
h. The energy released from the sun is a result of nuclear fusion, or thermonuclear reactions.
i. In solar fusion, hydrogen nuclei, with the addition of two beta particles, are fused to make helium
nuclei releasing the energy that has warmed the earth for billions of years:
411H + 2 0-1e  42He + energy
Comparing and contrasting fission and fusion:
Fission
Splitting of one heavy nucleus into two lighter ones
Releases enormous amounts of energy
Uses fissionable U-235 as fuel (0.7% of uranium sample)
Produces radioactive waste products
Uncontrolled results in explosion (i.e. Nuclear bomb)
Controlled is used in nuclear power plants to generate
thermal energy that runs generators producing electricity
Radioactive waste is a disposal hazard that has not yet
been resolved posing a bigger threat for future generations
Radiation Shielding:
© T. M. Chipi, chemistry instructor
Fusion
Combining of two light nuclei into one larger nucleus
Releases more energy than fission
Uses light nuclei as fuel; Hydrogen-2 from seawater
Produces NO radioactive waste products
Requires over 40,000,000K
Hydrogen bombs use fission to produce the energy
needed for the fusion reaction to cause the explosion
Require <10-13cm to overcome strong nuclear repulsion
Conducted in small scale in a Tokomak reactor
Attraction to a Magnetic Field:
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Notes: Nuclear Chemistry
When light nuclei fuse together, the combination actually has less mass than the sum of the parts. Thus, when four
protons are combined into a helium nucleus, the helium nucleus has less mass than the four protons, despite the fact
that they both have the same number of particles. The difference in mass is converted into energy, according to
Einstein's equation E = mc2. That is where the energy comes from in fusion.
In the fission process, too, where heavy elements are split into lighter ones, the heavy nucleus is actually heavier that
the sum of the products it splits into, and the difference in energy is released in the reaction. If we make a plot of
mass per nucleon for all the different types of atoms, we find that there is a minimum mass per nucleon, which
coincides with the Fe (iron) nucleus.
This is a plot of binding energy per nucleon, which is opposite to the mass per nucleon. The peak in binding energy
at iron (Fe) corresponds to a minimum in the mass per nucleon. Both fission and fusion release energy as the
products come closer to iron, but once iron is reached, this is the minimum energy state and no more energy is
available to extract from the nucleus. To either split or fuse iron takes energy rather than releasing it.
That means that if the Fe nucleus splits into smaller nuclei, or fuses into larger nuclei, in either case its mass per
nucleon increases. This extra mass has to come from somewhere, and the only way is to use energy to do it. So iron
sucks up energy rather than releasing it!
Natural Decay Series: there are three natural decay series: U-235, U-238, and Th-232.
Nuclear Stability and Magic Numbers: there are 264 stable nuclei with the following proton and neutron
combinations:
Stable Isotopes
Protons
Neutrons
157
Even
Even
55
Even
Odd
50
Odd
Even
5
Odd
Odd
© T. M. Chipi, chemistry instructor
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