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Nuclear Chemistry
Nuclear Chemistry
With all the topics that we have discussed and with all the skills that you have hopefully
gained in this course, the chapter on nuclear chemistry should be manageable. In these
lectures, only the highlights of this topic will be covered. These are the concepts and
characteristics that are unique to nuclear chemistry.
Example of a nuclear reaction
The superscript for each symbol is the atomic mass (number of protons plus number of
neutrons).
The subscript is the charge. The electron is given the special symbol -10e.
Major differences between nuclear and chemical reactions
(1) Nuclear reactions involve a change in an atom's nucleus, usually producing a different
element. Chemical reactions, on the other hand, involve only a rearrangement of
electrons and do not involve changes in the nuclei.
(2) Different isotopes of an element normally behave similarly in chemical reactions.
The nuclear chemistry of different isotopes vary greatly from each other.
(3) Rates of chemical reactions are influenced by temperature and catalysts. Rates of
nuclear reactions are unaffected by such factors.
(4) Nuclear reactions are independent of the chemical form of the element.
(5) Energy changes accompanying nuclear reactions are much larger. This energy comes
from destruction of mass.
(6) In a nuclear reaction, mass is not strictly conserved. Some of the mass is converted
into energy, E = mc2.
Binding energy
The loss in mass that occurs when protons and neutrons combine to form a nucleus is
called the mass defect. This mass defect is converted into energy. It is the binding
energy that holds the nucleons (protons and neutrons) together.
In order to compare stabilities of different nuclides, binding energies can be expressed on
a per-nucleon basis using mega-electron volts as the energy unit. A mega-electron volt is
equal to 1.60 x 10-13 J. For example, the binding energy for an  particle (He nucleus) is
equal to 2.73 x 109 kJ/mol. We divide this number by Avogadro's number and by 4 (the
number of nucleons in the He nucleus, 2 protons plus 2 neutrons). We then obtain the
energy per nucleon, 7.08 MeV/nucleon.
Across the periodic table, the binding energy per nucleon reaches a maximum value, 8.79
MeV/nucleon, at 56Fe. Hence, nuclei with atomic numbers larger than 26 tend to split
into lighter nuclei while those with atomic numbers less than 26 tend to combine to form
heavier nuclei. The splitting reaction is called fission. The combination reaction is called
fusion.
Spontaneous nuclear reactions
(1)  radiation - emission of an alpha particle (a He nucleus), resulting in a decrease in
both mass and atomic number.
The above is an example of a balanced nuclear reaction. The sum of the superscripts are
the same on both sides. The same is true for the subscripts.
(2)  radiation - emission of a beta particle (an electron from the nucleus), resulting in
an increase in atomic number.
This is different from an oxidation reaction since the ejected electron is coming from the
nucleus (A neutron has turned into a proton, thereby ejecting an electron)
(3)  radiation - This is the photon that carries the energy that is emitted. The
wavelength is in the order of 10-11 to 10-14 m (higher energy than xrays).
(4) positron emission - emission of a positively charged electron (positron) from the
nucleus, resulting in a decrease in the atomic number. A positron has the same mass as
an electron, but opposite in charge. In other words, inside the nucleus, a proton is being
converted into a neutron.
(5) electron capture - This happens in heavy atoms in which an inner shell (1s) electron
is captured by the nucleus, resulting in a decrease in atomic number.
Summary
Reason behind spontaneous radioactive decay
The neutron/proton ratio plays a major role. Neutrons function like a nuclear "glue"
which holds nucleons together by overcoming the enormous repulsive interactions
between protons. The more protons, the more neutrons are needed.
Belt of stability
The above is a plot of the number of neutrons against the number of protons in stable
nuclei.
(1) As the number of protons increase, the ideal neutron/proton ratio increases.
(2) Nuclei that lie above the belt of stability undergo  emission.
(3) Nuclei that lie below this belt undergo positron emission or electron capture.
(4) Nuclei with atomic number greater than 84 undergo  emission.
Nuclei with 2, 8, 20, 28, 50 and 82 protons are especially stable (analogous to inert
gases) indicating that nucleons are described by shells as well.
Radioactive decays follow first-order kinetics. These rates are normally given as
half-lives.
Nuclear Chemistry
Following is a series of terms and concepts that are related to the
topic of Nuclear Chemistry.
 Alpha Emission
 Beta Emission

Binding Energy

Binding Energy Curve

Fission

Fusion

Gamma Emission

Mass Defect

Mass Number

Metastable

Neutron-to-Proton Ratio

Nuclear Glue

Nucleons

Nuclide

Radioactivity
Alpha Emission:



Alpha particles are nuclear decay particles.
They consist of two protons and two neutrons.
In essence, they are equivalent to a helium nucleus.
The particles are expelled from a nucleus at a fairly
low speed, approximately one-tenth the speed of light.


They are a minimal health risk to people unless
ingested or inhaled.
The large mass nuclei tend to use alpha emission
because it is a quick way for a large mass atom to lose
a lot of nucleons.
Beta Emission:
Beta Emission is a nuclear decay process. It is the process that ejects a high speed
electron from an unstable nucleus. The electron is formed on the spot within the
nucleus by the breakdown of a neutron into a proton and electron. The electron is
released from the system. The proton that was formed remains behind in the
nucleus. As a result of the addition of the proton, the atomic number of an element
increases during beta emission. Beta emission can be a significant health risk.
Binding Energy:
Binding Energy is the energy that a nucleus releases in the process of trying to
stabilize itself. The nucleus converts some of its own mass into energy. That energy is
ejected from the nucleus. The process of the loss of energy will then move the system
further down an energy level diagram. Thus, the system becomes more stable. This
process is necessary to relieve the instability associated with having a large mass of
positively charged protons so close together.
Binding Energy Curve:
The Binding Energy Curve helps to understand the ideas behind fission and fusion.
It is a graph that plots the Binding Energy per Nucleon as a vertical coordinate and
the Mass Number of the elements as the horizontal coordinate.
The graph peaks at a mass number of 56. The more binding energy that is released
per nucleon, the more stable a nucleus is. Since 56 is the high point of the graph, it
means that any nucleus with a mass number of 56 will achieve the maximum
stability possible. In theory, all nuclei will try to become larger or smaller, as
necessary, so that they will eventually have a total of 56 nucleons in their structures.
Elements to the right of 56 would like to become smaller. They do so with the process
known as fission. Elements to the left of 56 would like to become larger. They do so
with the process known as fusion.
Fission:
Fission is the process known as "splitting the atom." During fission, a large mass
nucleus is split into two or more smaller mass nuclei. Hopefully during fission, the
resulting new nuclei will have mass numbers that are closer to 56. During the process
large quantities of energy are released as the products move up the Binding Energy
Curve. Fission is the currently used process for the production of nuclear energy.
Fusion:
Fusion is the process that unites small mass nuclei into a larger mass nucleus.
During the fusion process, the newly formed nucleus will have a mass number that is
closer to 56. During fusion extremely large quantities of energy are released as the
nuclei move up the Binding Energy Curve. This is a much more efficient process
than fission. It produces considerably more energy that fission. Unfortunately, it is
very difficult to accomplish and is not being utilized as a source of energy by society.
Gamma Emission:
Gamma Emission occurs primarily after the emission of a decay particle. Gamma is
a form of high energy electromagnetic radiation. After a particle is ejected from a
nucleus the system may have some slight excess of energy, or exist in a metastable
state. This slight excess of energy is released as gamma. Gamma emission will not
change the isotope or the element. The wavelength of the emitted gamma radiation
will be be unique to each isotope. Gamma emission is a significant health risk.
Mass Defect:
Mass Defect is the mass in a nucleus that is converted into energy. This energy is
then ejected from the nucleus to stabilize the system. The mass defect will be the
difference between the theoretical mass, calculated as the sum of the parts of the
nucleus, and the experimental mass. This difference will be the mass that was lost in
the production of energy.
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Nuclear Reactions - Nuclear Decay
Chemical reactions all involve the exchange or sharing of electrons, they never
have an influence on the nucleus of the atom. Nuclear reactions involve a
change in the nucleus. There are forces in the nucleus that oppose each other,
the "Strong" force holding Protons and Neutrons to each other and the
electrostatic force of protons repelling other protons. Under certain
arrangements of protons and neutrons the electrostatic force can cause
instability in the nucleus causing it to decay. It will continue to decay until it
reaches a stable combination.
This graph shows the stable nuclei
in red. There are several things to
notice:



There are no stable nuclei
with an atomic number
higher than 83 or a neutron
number higher than 126.
The more protons in the
nuclei, the more neutrons are
needed for stability. Notice
how the stability band pulls
away from the P=N line.
Stability is favored by even
numbers of protons and even
numbers of neutrons. 168 of
the stable nuclei are eveneven while only 4 of the
stable nuclei are odd-odd.
(This can't be seen from this graph due
to its small size and lack of detail.)
Unstable nuclei, called radioactive isotopes, will undergo nuclear decay as it
becomes more stable. There are only certain types of nuclear decay which
means that most isotopes can't jump directly from being unstable to being
stable. It often takes several decays to eventually become a stable nuclei.
Types of Radioactive Decay
When unstable nuclei decay, the reactions generally involve the emission
of a particle and or energy. Below is a table describing the types of
nuclear decay. Notice that for each type of decay, the equation is
balanced with regard to atomic number and atomic mass. In other words,
the total atomic number before and after the reaction are equal. And the
total atomic mass before and after the reaction are also equal.
Particle
Name
relative
What is
penetrating
it?
power
alpha
helium
particles nuclei
or
Example
1
stopped by
the skin but
very
damaging
due to
Happens to nuclei with Z>83
ionization The 2 p+ 2n loss brings the atom down and to the left
toward the belt of stable nuclei.
beta
particles
high
100
speed penetrates
human
electron
tissue to
~1cm
Happens to nuclei with high neutron:proton ratio
A neutron becomes a proton causing a shift down and
to the right on the stability graph
or
gamma
Rays
high
energy
photon
10000
highly
penetrating
but not very
ionizing
Generally accompanies other radioactive radiation
because it is the energy lost from settling within the
nucleus after a change. Since gamma rays do not affect
the atomic number or mass number, it is generally not
shown in the nuclear equation.
positron positron
emission
100
Happens to nuclei with a low neutron:proton ratio
A proton becomes a neutron causing a shift up and to
the left
electron
capture
no release
inner
of
energy or
shell
particle
electron
Happens to nuclei with a low neutron:proton ratio
A proton becomes a neutron causing a shift up and to
the left. Always results in gamma radiation.
This graph shows all the trends of decay and the band of stable nuclei. There
are some exceptions to the trends but generally a nuclei will decay following
the trends (in multiple steps) until it becomes stable. For example 92U238 will go
through 8 alpha emissions and 6 beta emissions (not all in order) before
becoming 82Pb206
The steps a nuclei follows in becoming stable is called a radioactive series.
The series for 92U238 is shown below as an example.
Z > 83 -- alpha
unpredicted Beta
unpredicted Beta
Z > 83 -- alpha
Z > 83 -- alpha
Z > 83 -- alpha
Z > 83 -- alpha
Z > 83 -- alpha
Beta
Beta
Z > 83 -- alpha
Beta
Beta
Z > 83 -- alpha
stable
92U
238
=> 90Th234 + 2He4
234
90Th
=>
91Pa
234
=>
234
92U
91Pa
234
92U
234
+ -1eo
+ -1eo
=> 90Th230 + 2He4
230
90Th
=> 88Ra226 + 2He4
226
88Ra
=> 86Rn222 + 2He4
222
86Rn
=> 84Po218 + 2He4
218
=> 82Pb214 + 2He4
214
=> 83Bi214 + -1eo
84Po
82Pb
214
83Bi
=> 84Po214 + -1eo
214
=> 82Pb210 + 2He4
210
=> 83Bi210 + -1eo
210
=> 84Po210 + -1eo
210
=> 82Pb206 + 2He4
84Po
82Pb
83Po
84Po
82Pb
206
.
Take this Quiz to see what you learned...and learn more.
http://www.bcpl.net/~kdrews/nuclearchem/nuclear.html#Curve
Nuclear Changes
Another type of change the atom undergoes is a nuclear change.
This is when the nucleus of an element changes into a new and
different element. Since these reactions involve the nucleus of the
atom they are known as nuclear reactions. When nuclear reactions
occur radioactivity accompanies the reaction. Radioactivity is the
property of certain radioactive isotopes that spontaneously emit from
their nuclei certain radiation, with result in the formation of atoms of a
different element or atoms of an isotope of the original element.
Stability of the Nucleus
The changes in the nucleus of the atom depends on the stability of
the nucleus. Of the approximately 2000 known isotopes, there are
only 270 stable nuclei with respect to radioactive decay. Tin has the
largest number of stable isotopes--10. The stability of the nucleus
depends on the number of neutrons and protons. Figure 1
represents the plot of the stable nuclei as a function of the number of
proton (Z) and the number of neutrons (A-Z). Within the zone of
stability lies the stable nuclei.
Some observations concerning radioactive decay.
* all nuclides with 84 or more protons are unstable with respect to
radioactive decay
* Light nuclides are stable when Z equals A-Z, that is, when the
neutron/proton ratio is equal to 1. However, for heavier elements the
neutron/proton ratio required for stability is greater than 1 and
increases with Z.
* Certain combinations of protons and neutrons seem to confer
special stability. For example, nuclides with even numbers of protons
and neutrons are often stable than those with odd numbers, as
shown in Table 1.
* There are also certain specific numbers of proton or neutrons that
produce especially stable nuclides. They are 2, 8, 20, 28, 50, 82, and
126. It appears that this behavior parallels certain numbers for
electronic stability.
Nuclear Stability and Radioactive Decay
With a discussion of radioisotopes comes the topic of nuclear
stability. The nucleus of a radioisotope is unstable. In an attempt to
reach a more stable arrangement of its protons and neutrons, the
nucleus will spontaneously decompose to form a different nucleus. If
the number of neutrons changes in the process, a different isotopes
is formed. If the number of protons changes in the process, then an
atom of a different element is formed. This decomposition of the
nucleus is referred to as radioactive decay. During radioactive decay
an unstable nucleus spontaneosly decomposes to form a different
nucleus, giving off radiation in the form of atomic partices or high
energy rays. This decay occurs at a constant, predictable rate that is
referred to as half-life. A stable nucleus will not undergo this kind of
decay and is thus non-radioactive.
CHEM WINDOW - Half-life
Why are the nuclei of radioisotopes unstable? In order to answer this
question we must examine how the number of protons and neutrons
in a nucleus are related to its stability and how this relates to
radioactive decay.
The figure below shows a plot in which stable nuclei are positioned
according to the number of protons (Z) and the number of neutrons
(A-Z) that they contain. The stable (non-radioactive) nuclides are
shown to reside in the zone of stability. Nuclei of atoms that do not
contain a number of protons and neutrons that allows then to be
plotted in this region are unstable and they will spontaneously decay
until a nucleus is formed that does not reside in this stable zone.
Radioactive nuclei can undergo decomposition in a variety of ways.
The spontaneous decay process can produce particles as in the case
of alpha, beta, or positron emission. The alternate form of emission is
that of electromagnetic radiation such as x-rays or gamma-rays.
CHEM WINDOW - Electomagnetic Spectrum
When alpha, beta, or positrons are emitted from the nuclei of a
radioactive atom, it changes into a nucleus of another element.
Scientists refer to this as transformation. Emission of gamma rays
results only in a release of energy, not in transformation.
Alpha particles
An alpha particle is simply a helium nuclei (He) which is ejected with
high energy from an unstable nucleus. This particle, which consists of
two protons and two neutrons, has a net positive charge. Although
emitted with high energy, alpha particles lose energy quickly as they
pass through matter of air and therefore, do not travel long distances.
They can even be stopped by a piece of paper or the outer layers of
human skin. These slow moving particles are generally the product of
heavier elements.
Example : 23892U ----> 42He + 23490Th
What would the radioactive decay of 22688Ra look like?
Beta particles
Beta particles are identical to electrons and thus have a charge of (1). This type of decay process leaves the mass number of the nuclei
unchanged. A beta particle is minute in comparison to that of an
alpha particle and has about one hundred times the penetrating
ability. Where an alpha particle can be stopped by a piece of paper a
beta particle can pass right through. It takes aluminum foil or even
wood to stop a beta particle. The electron that is released was not
present before the decay occured, but was actually created in the
decay process itself.
Example : 3215P ----> 0-1e + 3216S
Note that the mass number is unchanged and a new element is
formed. So what was the effect of this Beta particle production? It
actually changed a neutron into a proton. Notice that this new
element will be down and to the right on the zone of stability plot.
Positron
This type of particle production is just the opposite of Beta particle
decay.
Example : Na ----> 0 1e + Ne
Notice that is still has the same zero mass as an electron but an
opposite charge. This is what is known as an antiparticle of the
electron.
What happens when a positron collides with an electron?
Annihilation!!
This can be shown by the following reaction:
Example : 0-1e + 01e ----> 2
Gamma Rays
As the name implies, these are not particles but high energy
photons and can be found on the electromagnetic spectrum. They are
very similar to x-rays but have a shorter wavelength and therefore
more energy. The penetrating ability of gamma rays is much greater
than that of alpha or beta particles. They can only be stopped by
several centimeters of lead or more than a meter of concrete. In fact,
gamma rays can pass right through the human body. Gamma rays
often accompany other processes of decay such as alpha or beta. An
example of this was our previous representation of an alpha particle
process.
23892U ----> 23490Th + 200 + 42He
A ramification of alpha or beta particle production is that the newly
formed nucleus is left in a state of excess energy. A way for the
nucleus to release this excess energy is by emitting gamma rays.
Since gamma rays have no mass, and are waves rather than
particles, the elements atomic number does not change after
emission.
Fill in the blanks :
12553I ----> 125Xe + 0-1e + 200
22688Ra ----> + 42He + 200