Nuclear Chemistry
Review
Name_______________________________
Date________ Block____
What is radioactive decay?
Not all atomic nuclei are stable. Many decay spontaneously and produce a new nucleus and
some type of nuclear particle.
The cobalt-60 nucleus is unstable. It spontaneously decays to produce nickel-60 and an electron
(the electron is also called a beta particle since it originates in the nucleus of the atom).
The reaction is written:
60
27Co
→
60
28Ni
+
0
-1
e
When writing nuclear decay reactions and equations, ALWAYS write the atomic mass and the
atomic number for each element and each particle.
Nuclear equations MUST be balanced:
1) the sum of the atomic masses must be the same on both sides of the equation.
2) the sum of the atomic numbers must be the same on both sides of the equation
In the equation above, the atomic mass is 60 on the left side, and (60 + 0 = 60) on the right side.
The atomic number is 27 on the left side and (28 – 1 = 27) on the right side.
Both sides are balanced.
What types of radioactive decay are there?
Different types of radioactive decay are defined by the type of particle produced in the reaction.
If an alpha (α) particle (42He) is emitted, the decay is called alpha emission (α emission).
When an α-particle is emitted, the net result for the new nucleus is a loss of 4 in the mass number
and a loss of 2 in the atomic number.
211
83
Bi →
4
2He
+
207
81Tl
If an electron or beta particle (0-1e or 0-1β) is emitted, the decay is called beta emission
(β emission). When a beta particle is emitted, a neutron has been changed to a proton. The net
result of beta emission for the new nucleus is no change in the mass number and an increase of 1
in the atomic number.
10
4Be
→
0
-1e
+
10
5B
If a gamma ray (00 γ) is emitted, usually another particle is emitted also. A γ-ray is a high
energy photon (particle) of light energy with no mass or atomic number.
226
88Ra
→
222
86Rn
+
4
2He
+
0
0γ
A positron (01e or 01β) has very little mass and a positive charge. It is sometimes called a
“positively charged electron or beta-particle.” The net result on the new nucleus is no change in
mass and a decrease of 1 in the atomic number.
15
8O
→
15
7N
+
0
1β
Sometimes, the nucleus of an unstable atom can capture one of its own inner orbital electrons.
This process is called electron capture. The result is a new nucleus with no change in mass
number and a decrease of 1 in the atomic number.
37
18Ar
+
0
-1e
→
37
17Cl
What is nuclear transformation?
Nuclear transformation is the change of one element to another as the result of a nuclear change
or transformation. While nuclear decay and electron capture are natural occurrences, a nuclear
transformation results artificially: a nucleus is bombarded with high speed particles while inside
a particle accelerator. The most common particles used to cause nuclear transformation are
α-particles and neutrons (α-particles are large in mass compared with beta-particles, positrons,
and gamma rays; neutrons have no electrical charge and are not affected by external electrical
charge).
How are radioactive nuclei used?
Radioactive dating of old objects.
Some elements have naturally occurring radioactive nuclei in some of their atoms. One example
is carbon. Every living organism known contains carbon atoms. One isotope of carbon is
carbon-14 which is radioactive. This isotope is present in all living organisms in a fixed ratio to
nonradioactive carbon-12. Carbon-14 decays to carbon-12 very slowly: if you start with 1.00 g
of C-14, after 5730 years, you will have 0.500 g C-14 and 0.500 g C-12.
The 5730 years is the half-life of C-14. One half-life is the time it takes for one-half of the
radioactive nuclei that are present to decay. Half-lives are expressed in units of time: seconds,
minutes, hours, days, or years. By knowing how many C-14 are left in an old piece of wood, and
by knowing how many C-14 nuclei would have been there originally, by using the half-life of C14, it is possible to date the age of the piece of wood.
Radioactive tracers in medicine.
Certain radioactive nuclei are used to diagnose or treat certain diseases. Each body organ uses
the different elements in the body in different amounts. By selecting the appropriate isotope that
will be “picked up” by the body organ being studied, the amount of the isotope taken up by the
tissues of that organ determine whether the tissue is healthy or diseased. Other isotopes may be
implanted into diseased tissue to kill cancer cells in a localized area. Such isotopes are chosen
for relatively short half-lives so that healthy tissue is not damaged to any great extent.
Nuclear Energy
Nuclear energy is the energy that is released by unstable nuclei that either have been split by
fission or have been combined by fusion.
Nuclear fission involves the splitting of a heavy (massive) nucleus into two lighter (less massive)
nuclei. Nuclear fusion involves the combining of two relatively light nuclei to produce a
heavier nucleus. Both processes release a tremendous amount of energy.
One example of nuclear fission is the relatively heavy uranium-235 nucleus being bombarded
with neutrons and then splitting into two lighter nuclei. U-235 can split into many different pairs
of smaller nuclei, but usually produces barium-142 and krypton-91 along with 3 neutrons.
1
0n
+
235
92U
→
142
56Ba
+
91
36Kr
+ 3
1
0n
The released neutrons can, in turn, collide with more U-235 atoms causing fission and the release
of more neutrons. As more and more fission reactions occur, the process is called a chain
reaction. In order for a chain reaction to occur, there must be a certain amount of fissionable
atoms present to sustain the reaction. This amount of required fissionable material is called the
critical mass. If there is not critical mass, there will be no chain reaction.
Can a chain reaction be controlled?
Chain reactions usually occur in a reactor core. A reactor core is the “center” of a nuclear
reactor. The core is made of a material that allows the neutrons to pass through without much
resistance (the material does not absorb neutrons so the neutrons can collide with the fissionable
material.) The core material has deep holes drilled at specific distances from each other in which
the fuel rods (metal rods that contain the fuel pellets of the fissionable material) are inserted.
Between the fuel rod holes are other holes that contain control rods. Control rods are made of a
material that absorbs some of the neutrons and prevents them from passing through to collide
with the fuel atoms. By controlling the number of control rods inserted into the core and the
depth to which the control rods are inserted, the chain reaction can be controlled to release a
fairly constant amount of energy.
Tremendous amounts of heat energy are released as a result of fission. The heat is removed from
the core by a liquid, usually water under pressure, and the liquid is circulated through pipes to
another area where the heat is transferred to water to produce steam to drive electrical generators.
“Meltdown” can occur if the control rods are not inserted properly and the chain reaction is
allowed to “runaway” at an ever-increasing rate until all of the fissionable material is used up. A
runaway reaction releases a tremendous amount of energy in a very short time period. The heat
released is great enough to melt concrete, steel, and many minerals in rocks. The “China
syndrome” is the theoretical melting of the ground beneath a runaway reactor resulting in a
“tunnel” through the earth to China (assuming that the reactor is directly opposite China on the
globe).
The accident at Chernobyl in Russia in the 1980s resulted from a loss of circulating coolant to
the reactor core. The nuclear reactions continued uncontrolled and the heat produced melted the
bottom of the reactor vessel and core. The heat also caused an explosion due to an increase in
pressure of steam formed by the water in the cooling system (that was not being circulated). The
explosion blew off the top of the reactor vessel and the concrete and steel roof of the reactor
building. Fuel rods containing radioactive material were ejected through the hole in the roof. A
cloud of radioactive material from the explosion drifted northward and westward. As the
particles cooled they “rained” to earth and contaminated the soil and lake water in several
countries. Even though there had been a meltdown, no tunnel to China or to a place opposite
Chernobyl on the globe was formed.
How does nuclear radiation damage living tissues?
Energy in many forms including nuclear radiation can damage living tissue. There are two kinds
of damage: somatic and genetic. Somatic damage results when the tissue themselves are
damaged. Sunburn is an example of somatic damage to the skin. Depending on the extent of the
damage, sickness or death can occur. Sometimes the damage is not noticeable until years later
when cancer develops. Genetic damage occurs when the genetic material in eggs and sperm is
damaged. Such damage results in death or birth defects in children born to parents who were
exposed to damaging radiation.
What factors control the amount of damage to living tissues?
1) The energy level of the radiation: high energy radiation (called ionizing radiation) does more
damage than low energy radiation (nonionizing radiation).
2) The ability of the radiation to penetrate body tissues. The higher the penetrating ability, more
damage can occur. Alpha particles do not penetrate the skin; beta particles can penetrate 1 cm
into the body; gamma rays can penetrate deep into the body.
3) The ionizing ability of the radiation. Some radioactive particles can bump electrons away
from atoms resulting the formation of ions (electrically charged atoms or molecules). Ions doe
not behave in the same way as neutral atoms or molecules, so body functions are affected.
Gamma-rays penetrate deep into body tissues but do not cause much ionization. Alpha particles
cause a great deal of ionization even though they do not penetrate past the skin. Beta particles do
penetrate the skin up to 1 cm but do cause a lot of ionization.
4. The chemical properties of the radiation source. Radionuclides (radioactive atoms) can be
ingested with contaminated food (food grown in soil that has been contaminated with radioactive
fallout will contain radioactive atoms.) Some of the radionuclides can react chemically with
molecules in the body to form compounds that remain in the body for long periods of time. The
chances of damage to tissue is greater if the radionuclides stay in the body than if they pass
through the body without reacting.
REVIEW QUESTIONS
1. In a balanced nuclear equation, which two quantities must be the same on both sides of the
equation?
___________________________________ and ______________________________
2. What is atomic number and mass number of each of the particles given below?
Particle
gamma ray
positron
alpha particle
beta particle
Atomic number
Mass number
3. Write balanced nuclear equations for the decay of the radioactive particles below.
a)
226
86Rn
decays to produce a new nucleus and an alpha particle and a gamma ray.
________________________________________________________________________
b)
70
31Ga
c)
144
60Nd
d)
234
92U
decays to produce a new nucleus and a beta particle.
decays to produce a new nucleus and a beta particle.
decays to produce a new nucleus and an alpha particle and a gamma ray.
4. Complete and balance these nuclear reactions.
a)
161
67Ho
b)
10
4Be
+ _________ →
→ __________ +
c) __________ +
d)
253
e)
59
f)
55
99Es
+
4
2He
0
-1e
→
→
1
1H
29Cu
→ _______ +
25Mn
+
1
1H
0
161
66Dy
-1e
44
21Sc
+ ________
59
28Ni
→ _______ +
1
0n
5. Show the product nucleus formed when each of the nuclides below is combined.
a)
238
92U
+
12
6C
→ _________ + 4 10n
b)
238
+
14
7N
→ _________ + 5 10n
92U
6. When each of the particles listed below is emitted during nuclear decay, what is the effect on
atomic mass and mass number of the nucleus?
a) beta particle: ____________________________________________________________
b) alpha particle: ____________________________________________________________
c) gamma ray: ______________________________________________________________
7. In a sample of each of the nuclides below, which nuclide would exhibit the greatest number of
decay events during a fixed period of time? _______________________________
nuclide
potassium-42
hydrogen-3
plutonium-239
half-life
12.4 hours
12.5 years
2.44 x 104 years
8. Most radioactive nuclides used in medicine have short half-lives. Why?
9. What are the differences between somatic and genetic damage caused by radioactive
radiation?
10. What type of radioactive particles causes the most ionization? _______________________
11. Which statement about radioactive decay is TRUE? ______
A) loss of a gamma-ray results in a decrease by 1 in the mass number
B) loss of a beta particle results in a decrease by 1 in the atomic number
C) loss of a positron results in an increase by 1 in the mass number
D) loss of an alpha particle results in a decrease of 4 in mass number
E) loss of a beta particle results in an increase in the mass number
12. Which is NOT a factor in determining the biological damage caused by a source of
radiation?
A) the mass of the radionuclide
B) the chemical properties of the radiation source
C) the penetrating ability of the radiation
D) the energy of the radiation
E) the ionizing ability of the radiation
Answers
1. The sum of the atomic numbers and the sum of the mass numbers must be the same on both sides of the equation.
2. gamma ray: atomic number of 0; mass number of 0
positron:
atomic number of 1+; mass number of 0
alpha particle: atomic number of 2+; mass number of 4
beta particle: atomic number of 1-; mass number of 0
3. a)
b)
c)
d)
86Rn
→ 42He + 00γ + 22284Po
0
70
Ga
→
31
-1e +
32Ge
144
0
144
Nd
→
e
+
60
-1
61Pm
234
4
0
230 Th
92U → 2He + 0γ +
90
4. a)
b)
c)
d)
e)
f)
+ 0-1e → 16166Dy
10
0
5B + -1e
4Be →
44
0
44
22Ti + -1e →
21Sc
253
4
1
256
100Fm
99Es + 2He →
1H +
59
0
59
+ 28Ni
1e
29Cu →
55
1
55
1
26Fe + 0n
25Mn + 1H →
5. a)
b)
238
226
70
161
67Ho
10
92U
238
92U
+
+
6C
14
7N
12
→
→
246
98Cf
247
99Es
+ 4 10n
+ 5 10n
6. a) emission of beta particle has no effect on atomic mass, and increases atomic number by 1
b) emission of alpha particle decreases mass number by 4, and decreases atomic number by 2
c) emission of a gamma ray has no effect of either mass number or atomic number
7. Since potassium-42 has the shortest half-life, it would have the greatest number of decay events in a fixed period
of time.
8. Most radioactive nuclides used in medicine have short half-lives because using any radioactive material inside
the body poses some risk of damage by the high-energy decay particles. Radioactive nuclides with short half-lives
decay rapidly into stable nonradioactive nuclei.
9. Somatic damage is the damage done directly to the tissues of an organism and usually shows up soon after
exposure to the radiation source. Genetic damage is the damage done to the genetic material in the eggs and sperm.
The damage occurs at the time of exposure to the radiation source but the effects usually will not show up until the
birth of offspring. Genetic damage may cause an individual to become sterile (unable to produce eggs or sperm that
function normally to produce an embryo.)
10. Alpha particle cause the most ionization.
11. (D) is true. The loss of an alpha particle results in a decrease of 4 in the mass number.
12. (A) is not a factor in determining the biological damage caused by a source of radiation.
The mass of the nuclide does not determine chemical properties, penetrating ability, energy, or ionizing ability
of the radiation.