CC2 Unit CC Notes: Nuclear Chemistry
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CC-1 Reading: Changes in the Nucleus and Nuclear Stability
We have previously discussed chemical reactions, which result from the rearrangement of
electrons of different atoms to form new compounds. The centers of atoms (nuclei) were not
involved in chemical reactions. This unit discusses reactions that involve the nucleus of the atom.
These are not chemical changes, but nuclear changes or reactions.
Scientists first learned about nuclear reactions when radioactivity was discovered. In 1895,
Wilhelm Roentgen discovered X-rays, a type of high energy electromagnetic wave radiation.
Other types of radiation have since been discovered by important researchers such as Henri
Becquerel (he discovered radiation in uranium ore), and Pierre and Marie Curie (they isolated the
radioactive elements radium and polonium).
The elements worked on by the Curies emitted very high radiation levels, so they were able to
determine the properties of the radiation:
 While passing through air, the gas molecules became ionized so that air conducted electricity.
 Radiation caused phosphorescent substances (remember ZnS—zinc sulfide?) to glow brightly.
 When bacteria and other small organisms were exposed to this radiation, they died.
 The temperature near the surface of the radium was elevated.
All of these characteristics indicated that the radium was releasing energy.
Scientists later found that this radiation was produced when the nuclei of atoms changed,
producing atoms of different elements. This change of one element into an entirely different
element (or elements) is called transmutation. The radium atoms (atomic # 88) used by the
Curies broke down to produce radon (atomic # 86) and helium (atomic # 2). Isotopes of the same
element that are identified by their number of protons and different number of neutrons (and
therefore mass number) are called nuclides. Particles that reside in the nucleus (like protons and
neutrons) are called nucleons. Most nuclides found in nature are stable, but some are not.
Nuclear Stability



One would think that the like charges of all the protons in an atomic nucleus would cause
enormous repulsive forces that would force the nucleus apart. This doesn’t happen because
the neutrons help to generate a strong nuclear attractive force that counteracts the repulsive
force existing between the protons. All elements with atomic number  2 have neutrons.
Stable nuclei up until atomic number 20 will have roughly equal numbers of protons and
neutrons. As the number of protons increases beyond this number, a greater number of
neutrons than protons are needed to maintain the nuclear force that keeps the nucleus stable.
Figure 6 on page 646 shows the so-called belt, or band, of stability. It describes the ratio of
neutrons to protons necessary to maintain a stable nucleus as the atomic number increases
from 2 through 83 (bismuth). Above this atomic number, all nuclei are radioactive.
There is also the concept of magic numbers. Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2,
8, 20, 28, 50, 82, or 126 neutrons are generally more stable than those that don’t have these
numbers of nucleons. These numbers correspond to complete filling of nucleon energy levels.
Scientists see this as analogous to the chemical stability imparted to elements with completely
filled electron energy levels (noble gases).
Additionally, nuclei with even numbers of both protons and neutrons tend to be more stable
than those with odd numbers of nucleons. This observation is analogous to the idea that a
completely filled electron orbital (2 electrons) is more stable than a half-filled orbital.
CC2 Unit CC Notes: Nuclear Chemistry
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Unstable nuclei undergo spontaneous changes that change their number of protons and or
neutrons, thus leading to the transmutation of one element into another. We can express the
changes that occur through the use of nuclear reaction equations. These are actually pretty easy
to deal with—all you have to remember is that the total of the atomic numbers (representing
charges of the particles involved) and the total of the mass numbers (representing the sum of the
number of protons and neutrons) must be equal on both sides of the equation:
9
4
Be  24He 126C  01n
Notice that all of the lower-left subscripts add up to 6 on each side, while all of the upper-left
superscripts add up to 13 on each side. The equality of the superscripts indicates the
conservation of mass, and the equality of the subscripts indicates the conservation of charge
that must occur in every balanced nuclear reaction equation.
Remember how we discussed in Unit C the Law of Conservation of Mass put forth by Antoine
Lavoisier? It is often stated as “Matter is neither created nor destroyed.” We amended it in the
modern atomic theory by stating that “Matter is neither created nor destroyed in a regular
chemical process.” I implied in our discussion that matter can be converted to energy during
nuclear processes, and Albert Einstein figured out the mathematical mass-energy relationship:
m
E = mc2, where m is the change in mass in kg, c is the speed of light (3.0  108 s ), and E is
energy in joules
This concept that mass can be gained or lost (indicated by m) during a nuclear reaction so that
the law of conservation of mass doesn’t hold is called mass defect. The change in mass for a
nuclear reaction is going to be extremely small, but you can see that the speed of light is very
large, and the square of that value is gigantic, so even though the m is teensy, the energy change
generated by a nuclear reaction can be substantial. The energy released or absorbed during a
nuclear process is called the nuclear binding energy, because it is related to the energy involved
with generating or breaking down the strong nuclear forces that hold the nucleus togetherp. This
is the basis for our nuclear energy industry as well as atomic bombs.
CC-2 Reading: Radioactivity and Types of Radiation
The emission of radiation due to the spontaneous disintegration of a nucleus to form other lighter
elements is called radioactivity. The process is called radioactive decay.
Natural radioactivity
The unstable, or radioactive, isotopes are called radioisotopes. The radioisotopes that are isolated
from samples in nature exhibit natural radioactivity. Most isotopes of the lighter elements are
stable, and therefore are not radioactive. Scientists are able to make radioactive isotopes of these
and some heavier elements by bombarding (hitting) the nuclei of stable isotopes with high-energy
particles. The decay of these resulting unstable isotopes produce induced radioactivity.
Artificial (Induced) Radioactivity:
We already talked about how Rutherford discovered the different types of emissions during his
study of naturally radioactive elements. Once he characterized alpha particles, he discovered that
he could make rare isotopes of elements by transmutating non-radioactive elements. He did this
by bombarding stable nitrogen-14 atoms with alpha particles to produce radioactive oxygen-17
and emit protons:
14
7
N  24He 178 O  11H
CC2 Unit CC Notes: Nuclear Chemistry
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You hopefully will recall that we mentioned that James Chadwick discovered the neutron in
1932. He did this using a transmutation experiment: He bombarded beryllium-9 with alpha
particles to produce carbon-12 and an emitted neutron:
9
4
Be  24He 126 C  01n
Both of the above transmutations produced stable elements, but it was only a matter of time
before scientists started creating unstable (radioactive) elements. Irene and Frederic Joliot-Curie
did this first by bombarding stable aluminum-27 with alpha particles to create radioactive
phosphorus-30 and emitting a neutron. This unstable isotope then degraded to silicon-30 by
positron emission:
27
13
30
Al  24He 15
P  01n
30
15
30
P 14
Si  10 e
Transuranium elements are elements with more than 92 protons in their nuclei, and they are all
radioactive. Many have been made through artificial transmutation.
Do you remember the Rutherford Gold Foil experiments that used alpha (α) particles given off
by a radioactive source? I mentioned that Rutherford had discovered them in his studies of the
radiation emitted by radioactive substances. In fact, he discovered three types of radiation and
characterized those using electrical plates similar to those used by J.J. Thomson when he
concluded cathode rays were negatively charged. Rutherford concluded that α rays were
positively charged because they were attracted toward a negatively-charged plate, that beta (β)
rays were negatively-charged because they were attracted toward a positively-charged plate.
The third type of radiation was gamma (γ) rays, and was not affected by charged plates.
Although α and β rays have properties of both waves and particles, they are now usually called
particles. Gamma rays are considered to be only electromagnetic radiation.
You can contrast and compare the types of radiation using the chart below:
Types of Radiation
Alpha rays or particles
Beta rays or particles
Gamma rays
Nature
Sometimes behave like particles,
sometimes like waves
Sometimes behave like particles;
sometimes like waves
Electromagnetic waves of
extremely short wavelength
Speed
About 1/10 the speed of light
Approaching light speed
Speed of light
Mass
4 amu
0.00055 amu
Penetrating power
Relatively weak (can be stopped by
a single sheet of paper)
Greater than alpha (can be
stopped by a thin sheet of
aluminum)
very penetrating (several
centimeters of lead needed to
stop them)
Ionizing ability
Ionizes gas molecules
Ionizes gas molecules
Ionizes the atoms in flesh, and
causes severe damage to cells.
Symbol
4
2
He or 24
0
1
e or
0
0
1

0
0

You will note that the symbols shown above have similar notation to those discussed in our
atomic structure unit. The left superscript is the mass number of the particle, and the left subscript
is the number of protons (in the case of alpha emission) or the charge of the emitted particle or
radiation (as in beta and gamma emission).
Now, let’s talk about when each type of emission occurs:
CC2 Unit CC Notes: Nuclear Chemistry
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Alpha particles () are high energy helium-4 nuclei ( 24 He 2 ) that are ejected by a very heavy
unstable nucleus (atomic number  84). They are sometimes represented as 24 . We can visualize
the radioactive decay of uranium-238 (U-238) by the spontaneous emission of an  particle with
a nuclear equation:
4
U  234
90Th  2 He
238
92
Sometimes this process is called alpha decay. It’s important to note that the sum of the atomic
numbers is the same on each side. Similarly, the sum of the mass numbers is also the same on
both sides. Remember that the radioactive properties of an element are independent of their
state of chemical combination. This means that radioactive nuclides of an element will
chemically react with other elements in a manner that is identical to that of their non-radioactive
counterparts of the same element. We exploit this property in medical diagnostic techniques and
dating of artifacts.
Practice Exercise
What element undergoes alpha decay to form lead-208?
4
? 208
82 Pb 2 He
212
84
4
Po 208
82 Pb 2 He
Explanation: You can see that the atomic number of the species on the left must be 82 + 2, or 84.
If we look up the element with the atomic number of 84 on the periodic table, we see that it is
polonium (Po). It must have a mass number of 208 + 4, or 212.
When a nuclide has a neutron : proton ratio that lies above the band of stability, high speed
electrons are emitted by that unstable nucleus, and they are called beta particles. They are
presented in nuclear equations as either 10 e OR 01 . The zero superscript means that the mass of
the electron is negligible compared to the mass of neutrons and protons. The 1 subscript
indicates the charge of the  particle, which is opposite that of a proton. See the sample emission reaction below:
131
53
0
I 131
54 Xe 1 e
The above reaction shows the atomic number increasing by 1, so a -emission can be viewed as
the conversion of a neutron into a proton and an electron, the latter of which is emitted:
1
0
n11 p  10 e
THIS OCCURS ONLY DURING NUCLEAR PROCESSES—electrons don’t reside in the
nucleus. You should also be able to see that the neutron : proton ratio has now been reduced, thus
moving the new element formed closer to, or into, the band of stability.
Gamma radiation (gamma rays) are high energy photons (short wavelength EM radiation). It
doesn’t change the atomic number or the mass number of a nucleus, so it is represented as  or
0
0 . It is present along with most other forms of radiation because it represents the energy lost
when the remaining nucleons reorganize into more stable configurations.
CC2 Unit CC Notes: Nuclear Chemistry
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A positron, 01 e, is a particle with the same mass as an electron, but an opposite charge. It doesn’t
last very long, as it is destroyed, or annihilated, when is collides with an electron: 01 e +
0
1 e

2 . Positrons are sometimes called “anti-electrons,” as they are the first type of “anti-matter”
that was found. Positron emission is demonstrated by the decay of carbon-11:
0
0
C 115B  10 e
11
6
You can see that the neutron : proton ratio has increased with the formation of boron-11, so this
form of decay often occurs in unstable nuclides with neutron : proton ratios that lie below the belt
of stability. Because the number of protons is reduced, the atomic number decreases by 1, so we
say that a proton has been converted into a positron and a neutron:
p  01 n  10 e
1
1
Electron capture (sometimes called “K-capture”) involves the trapping of an inner shell electron from the
electron cloud outside the nucleus:
81
37 Rb
+
0
1 e
(orbital electron) 
81
36 Kr
Notice that the mass number stays the same, but the atomic number decreases by 1, so this can be viewed
as the conversion of a proton into a neutron:
1
1p
+
0
1 e
1
 0n
It appears to be the reverse of the beta emission reaction, but it also acts to reduce the neutron : proton ratio
for unstable nuclides that are above the band of stability.
Radioactive decay and balancing nuclear equations:
When unstable isotopes decay by emitting (giving off) an alpha particle, scientists say that alpha
emission is occurring. Because alpha particles contain two protons and two neutrons, the atomic
number of the radioactive atom that decayed will transmutate into an isotope of a different
element (perhaps radioactive, perhaps not). The decay of uranium 238 by alpha emission can be
expressed as a word nuclear equation or one with symbols:
Uranium-238 → thorium-234 + alpha particle
OR
4
U  234
90Th  2 He
238
92
The production of a different element where it was not previously present due to radioactive
decay or emission is a nuclear reaction. It is important to note in the above equation that the
sums of the superscripts and subscripts on the right side are equal to the mass number and atomic
number (# of protons) of uranium-238 respectively to the left of the arrow.
The Uranium-238 Decay Series and Beta Emission
Sometimes after emitting an alpha particle, the nucleus of an isotope is still not stable. The
thorium-234 produced above is one of those types of nuclides. It continues to decay via beta
emissions and alpha emissions. If the next step in the decay is a beta emission, let’s figure out
how this would work:
Th  ? 10 e
234
90
In order to balance charge and mass number on each side of the equation, the missing item (?)
must have an atomic number of 91, and a mass number of 234:
CC2 Unit CC Notes: Nuclear Chemistry
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0
Th 234
91 ? 1 e
234
90
The element that has an atomic number of 91 is protactinium, Pa, so the final balanced nuclear
equation is:
0
Th 234
91 Pa  1 e
234
90
Don’t worry about memorizing a decay series—just realize that you can predict the final products
by knowing the starting nuclide, the end nuclide, and the number of each type of emission.
CC-3 Reading: Fission and Fusion
Nuclear Fission
Fission means splitting, so nuclear fission refers to the breaking apart of an atomic nucleus into 2
or more pieces of smaller mass that are more stable. As mentioned in the previous section,
breaking apart a nucleus liberates a great deal of energy. We use nuclear fission reactors to
carefully control the splitting reaction, and use the energy given off to do work. An example of a
nuclear fission reaction that scientists have studied extensively is the bombardment of uranium235 by slow neutrons to produce barium-140, krypton-93, more neutrons and a large quantity of
energy:
93
1
U  01n140
56 Ba  36 Kr  3_ 0 n  energy
235
92
A picture of this reaction appears in Figure 11 on page 654. The neutrons produced can collide
with other U-235 nuclei to split successively more and more uranium-235 atoms and release more
energy. It is easy to see that a chain reaction rapidly develops that perpetuates itself until too
little uranium is left to be split.
If the starting mass of U-235 is too small, the neutrons produced by the first fission reaction will
escape without striking other U-235 nuclei. A chain reaction therefore will not occur. As the size
of the U-235 sample is increased, a point is reached at which enough of the neutrons are captured
to keep the chain reaction going. This sample amount is called the critical mass. A very small
additional increase in the amount of U-235 beyond this point will lead to a rapid build-up in the
rate of fission, and to the generation of so much heat energy that an explosion will occur.
Explosions from the fission of U-235 and plutonium are used to make atomic bombs. In this case,
neutron bombardment is initiated when two or more portions of fissionable material (U-235 or
plutonium) are rapidly brought together. The mass of each separate sample is less than critical
mass, but the two samples together have a combined mass that is slightly larger than the critical
mass required to generate the chain reaction.
Our society has found a way to harness the heat energy generated during a nuclear reaction to
produce electricity. We use shielding (radiation-absorbing materials) to contain radioactive
emissions that occur. Control rods absorb neutrons, thereby interfering with the chain reaction
when they are inserted into the fissionable material (usually uranium-235). If the neutrons are
moving too quickly, a chain reaction cannot be initiated, so moderators are inserted into the
uranium fuel to slow them down. Make sure you look at Figure 12 on page 655 for a diagram of
a nuclear fission power plant.
Fusion Reactions
When energy is released during a nuclear reaction between two or more light nuclei, to form one
or more nuclei of smaller total mass, we say that a fusion reaction occurs. Fusion reactions must
give off energy if the total mass has been reduced.
CC2 Unit CC Notes: Nuclear Chemistry
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The fusion of hydrogen isotopes to form helium nuclei is responsible for two things that are part
of our daily life: the sun and thermonuclear devices such as the hydrogen bomb. Because
hydrogen nuclei are both positively charged, they normally repel each other. In order to overcome
this repulsive force, the hydrogen nuclei must have a lot of kinetic energy (TRANSLATE THAT
AS MOVE VERY FAST). The only way nuclei can reach such large kinetic energies is at very
high temperatures (~2  107 Kelvins), like those found on the sun and other stars. Because they
occur at very high temperatures, fusion reactions are often called THERMONUCLEAR
REACTIONS. The chief source of light given off by the sun is likely due to the energy released
during the conversion of hydrogen nuclei to helium nuclei. It is a multi-step process with a net
reaction of:
 
4 11 H  24 He 2 10 e  Q , where Q is energy.
What’s really interesting is that the first time scientists realized the existence of helium was when
examining the absorption spectra of the gases surrounding the sun. Atomic bombs called
thermonuclear devices are probably based on the fusion of two radioactive isotopes of hydrogen:
2
1
H 13H  24 He  01n  Q
As might be imagined, fusion reactions only initiate at very high temperatures. To provide the
high temperatures necessary for a fusion device, a fission bomb could be used. Fusion bombs
release more energy than fission devices.
When fusion was first discovered, it was viewed as a potential source of extremely clean energy
that wouldn’t pollute the environment. Scientists have encountered great difficulty, however, in
building reaction vessels that could simultaneously produce the required high initiation
temperatures and subsequently not melt.
Russian scientists developed the first tokomak fusion test reactor which uses magnetic fields to
contain the plasma of an ionized gas while it is heated to extremely high temperatures. When the
temperature is high enough, the energy output from the fusion process equals the energy
expended to start those reactions. Scientists the world over are looking for a cold fusion process
that occurs at lower temperatures and which will generate more energy than they consume for
initiation.
CC-4 Reading: Radioactive Dating and Half-life Calculations
The most common (and potentially most damaging) forms of radiation are alpha, beta, and gamma
emissions. We discussed in the previous reading assignment notes the relative penetrating ability and
resulting damage that can occur from these types of radiation. Different types of shielding must be used in
order to protect from these different forms of radiation, but first we must know how radiation is measured.
Because all of the three types of radiation are capable of ionizing air, we measure the amount of radiation
in terms of its ionizing ability—a roentgen is the amount of radiation that produces 2  109 ion pairs when
it passes through 1 cm3 of dry air. We define 1 rem as the quantity of ionizing radiation that does as much
damage to human tissue as is done by 1 roentgen of high voltage X-rays. We use exposure of film badges
to measure the amount of radiation to which we are exposed, and we use Geiger-Müller counters to detect
the presence of radiation—the ions generated by exposure to radiation can generate electrical pulses that
can be counted by this detector. The more pulses counted, the stronger the radiation source. Finally, ionized
gas particles can give up their energy to certain absorbing materials that subsequently give up the energy in
the form of light. This is called scintillation, and the greater the amount of light emitted, the stronger the
radiation source.
Although radiation can damage the DNA in living tissue (usually causing cancer in some form), it can also
be used many ways in science and medicine: The ability of radioactivity to damage tissue is used to destroy
cancer tumors. Because radioactive isotopes of an element combine with other elements in the same way as
CC2 Unit CC Notes: Nuclear Chemistry
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non-radioactive nuclides, we can use radioactive carbon, nitrogen, and phosphorus atoms to serve as
tracers in biochemical pathways and determine DNA sequences.
While both of the above techniques are rather high-end science, most people have heard about the use of
radioactive dating to determine the age of old things. The way it works goes like this: The relative
abundance of isotopes of a given element, both radioactive and non-radioactive, in a natural sample is a
constant on Earth. Scientists have determined these percentages. Living things utilize these elements every
day. Because the radioactive elements combine with different elements to form compounds in exactly the
same way as their non-radioactive counterpart, their relative abundance in these compounds in living tissue
is identical to their relative abundance as elements in nature.
Let us use carbon as an example: Plants and animals all utilize carbon—it is considered to be one of the
building blocks of life, as it is present in almost all living tissue in the form of sugars and DNA. You will
remember, I’m sure, that plants utilize carbon dioxide, CO 2, to make glucose and oxygen via
photosynthesis. A small percentage of the CO2 in the air contains the radioactive isotope carbon-14, so the
plants incorporate the radioactive CO2 molecules just as they would the non-radioactive ones. The carbon14 is then present in the living plants in exactly the same percentage as it appears in the CO 2 molecules in
the air. Other animals eat these plants, and they are subsequently consumed by other animals. Thus the
carbon-14 will be present in all life forms in exactly the same relative abundance in which it appears in the
CO2 molecules in the air.
Now, here’s how radioactive dating works: When a life form dies, it stops eating, so the replenishment of
all nutrients, including carbon-based substances, also stops. The carbon-14 starts to decay by beta emission
to form nitrogen-14:
14
6
Scientists know the starting ratio of
carbon14
carbon12
C 147 N  10 e
in the environment, and subsequently in all living tissues.
They also know the half-life of carbon-14 (~5700 years). By measuring the relative amount of carbon-14 in
an organic sample (e.g., paper, wood, clothing, etc.) and comparing it to the amount that was originally
present, we can estimate the age of an artifact up to about an age of 60,000 years. For example, if the
carbon14
carbon12 ratio of the artifact is half that found in the environment, we can estimate its age as ~5700 years.
The procedure we use is the following. First, we construct a graph of counts (a measure of the amount of
radiation emitted) per minute based on given data, or on half-life information. We connect the data points
14
with a “best-fit” curve. We then determine the carbon
carbon12 ratio by measuring the amount of radiation being
given off at a given instant in time, and interpolate on the graph (go between data points) to find the
estimated age of the artifact. If you can determine the age of an artifact found at an archeological site, you
can also estimate the age of the civilization that you are studying.
Make sure to read SAMPLE PROBLEM B on page 660.
Half-life
The rate of radioactive decay is independent of temperature and pressure. We define the time it takes for
half of the atoms in a radioactive sample to decay as the half-life, t½, of that sample. Each type of
radioactive isotope will decay at a different rate, thus each will have a different half-life. An example of a
how the concept of half-life works would be the following: If, for example, the half-life of isotope X is 5
years, and the starting size of the sample is 50 grams, the following will be the progression of decay:
Mass of isotope
X sample:
Time:
# of half-lifes
50.0 g
25.0 g
12.5 g
6.25 g
t = 0 (start)
t = 5 years
t = 10 years
t = 15 years
0
1
2
3
CC2 Unit CC Notes: Nuclear Chemistry
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A graphical illustration is shown in Figure 17 on page 661. Additionally, it is important to recognize that as
the original radioactive isotope disappears, atoms of other elements are produced, as well as the different
types of radiation.
Sample Problem:
The half-life of technetium-99 (Tc-99) is 6.00 hours. What mass of Tc-99 remains from a 10.0-g sample
after 24 hours? We can do a chart like the one above and fill in the values to find out how much Tc-99
remains after 24 hours:
Mass of Tc99 sample
Time:
# of half-lifes
10.0 g
_______
_______
_______
_______
t=0
t = 6 hours
t = 12 hours
t = 18 hours
t = 24 hours
0
1
2
3
4