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NUCLEAR CHEMISTRY
The discovery of radiation
In 1896 Henri Becquerel made an important discovery.
He accidentally had placed a piece of uranium ore on
top of an unexposed photographic plate. Later, when
the plate was developed, the image of the rock was
found on the plate. Based on further experiments, he
concluded that the plate had been exposed by rays
given off by the uranium.
Madame Curie discovers Radium and
Polonium
Following
Becquerel’s
discovery,
Marie
Sklodowska Curie and her husband, Pierre Curie,
attempted to isolate the “radioactive” material
from the uranium ore.
In doing so they discovered two new elements,
Radium and Polonium, both of which were more
radioactive than the original ore.
Ernest Rutherford
Rutherford investigated this new property of
matter and discovered that, in the process of
emitting radiation, atoms of one element became
atoms of another element.
Today, we describe the process of an atom of one
element becoming an atom of a different element
as transmutation.
Rutherford was the first to identify and name two
different types of radiation given off when an
atom of one element underwent transmutation
and became an atom of another element. The
two types of radiation he found were:
•
The alpha particle (α)
•
The beta particle (β)
A third type of radiation that was discovered later is called:
•
Gamma radiation (γ)
The Nucleus
The nucleus is composed of nucleons
–
protons
–
neutrons
A nucleus is characterized by two numbers
–
atomic mass number(A; total number of nucleons)
–
atomic number (Z; number of protons)
A nuclide that has 26 protons and 33 neutrons is
used to study blood chemistry. Write its nuclide
symbol in the form of
Because this nuclide has 26 protons and 33
neutrons, so atomic number, Z, is 26 and
nucleon number A is 59 (26 protons +33
neutrons).
Isotopes
An isotope is an Atoms with the same number of
protons, but differing numbers of neutrons.
Isotopes are different forms of a single element.
Hydrogen Isotopes:
H protium most abundant isotope, nucleus consists of
a single proton
1
H deuterium one neutron and one proton often given
the symbol “D”. forms the hydrogen component of
heavy water (D2O).
2
H tritium one proton and 2 neutrons radioactive
isotope: half-life of 12.3 year not found in nature.
3
Isotope effects
Isotope effects
Properties that depend on mass will be different for different
isotopes of the same element:
•  Deuterium and hydrogen exhibit isotopic differences in their
properties.
Eg. boiling points of heavy water and conventional water are
slightly different allowing them to be separated by fractional
distillation.
•  Isotopes can be used to “label” compounds.
Why does the atom break up?
Remember that the nucleus of the
atom is held together by the strong
nuclear force.
This force is
normally strong enough to hold the
protons and neutrons together.
However, sometimes the force of
repulsion due to the protons having
the same charge overcomes the
strong nuclear force and the atom
breaks apart.
RADIOACTIVITY
Most naturally occurring isotopes of elements up to atomic
number 19 have stable nuclei. Elements with higher atomic
number (20-83) consist of a mixture isotopes, some of which may
have unstable nuclei. When the nucleus of an isotope is unstable,
it is radioactive, which means that it will spontaneously emit
energy to become more stable. This energy, called radiation, may
take form of particles such as alpha (α) particles or beta(β)
particles or pure energy such as gamma (γ) rays. Elements with
atomic numbers of 84 and higher consist only of radioactive
isotopes. So many protons and neutrons are crowded together in
their nuclei that the strong repulsions between the protons makes
those nuclei unstable.
TYPES OF RADIATION
Alpha Decay:
Alpha particles are helium nuclei, containing two
protons and two neutrons. They are deflected
slightly in an electric of magnetic field. Their
penetrating power is very low, being stoppable
by a thin sheet of aluminum or paper.
Beta Decay:
Beta particles are electrons capable of travelling at speeds
approaching the speed of light. Their low mass allows them to be
deflected greatly in an electric or magnetic field, in the opposite
direction as the deflection of alpha particles. Their high speed
gives them greater penetrating power than alpha particles. Some
beta particles can penetrate several centimetres of aluminum.
Beta particle emissions change the composition of the nucleus.
n
p + e-
Positron Emission:
A radioactive nucleus that undergoes positron emission
has a proton in its nucleus convert into a neutron and
an positron, then it ejects the positron. A positron is an
anti-electron: it has the same mass, but has a +1
change instead of a -1 charge. The remaining nucleus
has one less proton and one more neutron: the atomic
number decreases by one and the mass number stays
the same. n p + e+
Electron Capture
Electron capture is a process in which a protonrich nuclide absorbs an inner atomic electron,
thereby changing a nuclear proton to a neutron
and simultaneously causing the emission of an
electron neutrino. Various photon emissions
follow, as the energy of the atom to falls to the
ground state of the new nuclide. e-+p →n
Gamma Emission:
Gamma rays are high energy electromagnetic
radiation with short wavelengths. Gamma rays,
unlike alpha and beta particles, do not change
the composition of the nuclide. They have the
highest penetrating power, being able to
penetrate at least 30 centimetres of lead.
Penetrating Power of Radiation
•
Alpha radiation is least penetrating and can penetrate the
outer layer of skin. Alpha radiation is stopped by a sheet of
paper.
•
Beta radiation can penetrate through a few cm of skin and
tissue. Beta radiation is stopped by a sheet of aluminum foil.
•
Gamma radiation will pass right through a body. Gamma
radiation requires several cm of lead to stop.
Penetrating Power of Radiation
Mode of radioactive decay:
Type of Radiation
Alpha particle
Beta particle
Gamma ray
Charge
+2
-1
0
Speed
slow
fast
Very fast
Ionising ability
high
medium
0
Penetrating power
low
medium
high
paper
aluminium
lead
Stopped by:
Nuclear Equations:
Nuclear Changes:
Write nuclear equations for (a) alpha emission by
polonium-210, used in radiation therapy, (b) beta
emission by gold-198, used to assess kidney activity,
(c) positron emission by nitrogen-13, used in making
brain, heart, and liver images, and (d) electron
capture by gallium-67, used to do whole body scans
for tumors.
Solution:
A
B
C
D
Rates of Radioactive Decay
Because the different radioactive nuclides have
different stabilities, the rates at which they decay
differ as well. These rates are described in terms of
a nuclide’s half-life,
The half-life is defined as the time that it takes for one half of a sample
of a radioactive element to decay into another element.
2
For example, radioactive carbon-14, which decays to form
nitrogen-14 by emitting a beta particle, has a half-life of 5730
years. After 5730 years, one-half of a sample remains, and onehalf has become nitrogen-14. After 11,460 years (two half-lives),
half of that remainder will have decayed to form nitrogen-14,
bringing the sample down to one-fourth of its original amount.
After 17,190 years (three half-lives), half of what remained after
11,460 years will have decayed to form nitrogen-14, so one-eighth
of the original sample will remain. This continues, with one-half of
the sample decaying each half-life.
Here's a look at the effect of different doses of
radiation on the human body after acute, wholebody exposure. RAD - radiation absorbed dose is the amount of radiation that bombards a body.
Over 2,000 RAD: Death is a certainty.
At doses above 5,000 RAD, the central nervous system (brain and muscles) can no longer
control the body functions, including breathing and blood circulation. Everything happens very
quickly. Death occurs within days or hours. Nothing can be done, and medical care is for
comfort only.
1,000 to 2,000 RAD:
Probability of death increases to 100%within one to two weeks. The initial symptoms appear
immediately. Within a few days later the body breaks down very quickly since the
gastrointestinal system is destroyed. Once the GI system ceases to function, nothing can be
done, and medical care is for comfort only.
150 to 1,100 RAD:
Severe blood changes will be noted and symptoms appear immediately. Approximately two
weeks later, some exposed casualties may die. At 300-500 RAD, up to one half of the people
exposed will die within 30 days without intensive medical attention. Death is due to the
destruction of the blood forming organs. Without white blood cells, infection is likely. At the
lower end of the dose range, isolation, antibiotics, and transfusions may provide the bone
marrow with time to generate new blood cells and full recovery is possible. At the upper end of
the dose range, a bone marrow transplant may be required to produce new blood cells.
50 to 150 RAD:
Slight blood changes including temporary drop in production of new blood cells will be noted
and likely symptoms of nausea, fatigue and vomiting for one or two days.
5 to 50 RAD:
Slight blood changes may be detected by medical evaluation.
Less than 5 RAD: No immediate observable effects.
Medical Uses
Ionizing radiation has two very different uses in
medicine — for diagnosis and therapy. Both are
intended to benefit patients and, as with any use
of radiation, the benefit must outweigh the risk.
•
Radiation Therapy: Nuclear radiation can
be used to kill cancerous cells. Radiation is
most lethal to fastest growing cells. Radiation
is aimed at the cancerous tissue. Patients
undergoing radiation therapy often experience
nausea and vomiting, which are early signs of
radiation sickness.
Diagnostic Uses of Radiation
•
Gamma Ray Imaging or Positron:
Technetium-99m emits gamma radiation. It can
be used to image the heart and other organs and
tissues.
•
Positron Emission Tomography (PET): A
patient inhales or is injected with positronemitting isotopes such as carbon-11 or oxygen15. When positrons encounter electrons, they
emit two gamma rays, which exit the body in
opposite directions. PET scans can be used to
image dynamic processes.
Nuclear Fission
In a typical nuclear fission process, a neutron collides
with a large atom, such as uranium-235, and forms a
much less stable nuclide that spontaneously decomposes
into two medium sized atoms and 2 or 3 neutrons.
neutron +large nuclide →unstable nuclide
unstable nuclide →2 medium sized nuclides +2 or 3
neutrons
The nuclides produced in the reaction pictured
above are only two of many possible fission
products of uranium-235. More than 200 different
nuclides form, representing 35 different
elements. Two possible reactions are:
Nuclear reactions such as these are used to
power electrical generating plants.
The reason the fission of uranium-235 can
generate a lot of energy in a short period of time
is that under the right circumstances, it can
initiate a chain reaction, a process in which one
of the products of a reaction initiates another
identical reaction. In the fission of uranium-235,
one or more of the neutrons formed in the
reaction can collide with another uranium-235
atom and cause it to fission too
Uranium-235 Chain Reaction
Nuclear power plant
The nuclear reactor in a nuclear power plant is
really just a big furnace whose job is to generate
heat and thus convert liquid water to steam in
order to turn a steam turbine generator that
produces electricity. The electricity-generating
portion of a nuclear power plant is typically no
different than the electricity-generating portion of
a plant that generates heat from burning fossil
fuels.
Generating Electricity from Nuclear Power
Nuclear power is a major source of energy for
electrical generation worldwide. In March 2012,
nuclear power plants were found in 30 countries
and generated about 13% of the world’s
electricity. France got about 77% of its electricity
from nuclear power, and the United States got
about 19%.
The Atomic Bomb
The atomic bomb is a fission bomb; it operates
on the principle of a very fast chain reaction that
release a tremendous amount of energy. At
atomic bomb and nuclear reactor both depend on
self sustaining nuclear fission chain reaction. The
essential difference is that in a bomb the fission
is ”wild”, or uncontrolled whereas in a nuclear
reactor the fission is moderated and carefully
controlled
A minimum critical mass of fissionable material is needed for
a bomb, otherwise a major explosion will not occur. When a
quantity smaller than the critical mass is used, to many
neutrons formed in the fission step escape without combining
with another nucleus, and a chain reaction does not occur.
Therefore the fissionable material of an atomic bomb must be
stored as two or more subcritical masses and brought
together to form the critical mass as the desired time of
explosion. The temperature developed in an atomic bomb is
believed to be about 10 million degrees Celsius.