NAME________________________________ PER ________ DATE DUE ___________________
ACTIVE LEARNING I N C HEMISTRY E DUCATION
CHAPTER 28
NUCLEAR
CHEMISTRY
(Part 1)
28-1
©1997, A.J. Girondi
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© 1997 A.J. Girondi, Ph.D.
505 Latshmere Drive
Harrisburg, PA 17109
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Website: www.geocities.com/Athens/Oracle/2041
28-2
©1997, A.J. Girondi
SECTION 28.1
Nuclear Notation and Isotopes
Nuclear chemistry involves changes that occur in the nucleus of an atom. These changes in a
nucleus often result in the release of great amounts of energy – much greater than the amount of energy
released in any chemical reactions. You will recall that chemical reactions involve the formation and
breaking of bonds between atoms. In addition to the release of energy, certain types of particles are
emitted from a nucleus during nuclear reactions. Before going on, there are a few basic facts which you
should know:
1. Most of the mass of an atom is found in the nucleus. This is a result of the relatively close
"packing" of the protons and neutrons in it.
2. All protons carry a positive charge.
3. Because they all carry a positive charge, the protons in a nucleus repel each other with a strong
force; yet, the nucleus of a stable atom does not fall apart.
You may recall that it is the number of protons in the nucleus of an atom (the atomic number) that
determines what element the nucleus represents. A nuclear change sometimes involves a change in the
number of protons in the nucleus. When this happens, a nucleus of one element is changed into a
nucleus of a different element. This is called a transmutation.
In a previous chapter, you were
introduced to nuclear notation. Let's review it now to refresh your memory. The general form for nuclear
notation can be represented by the expression shown below:
mass number
(sum of protons
and neutrons)
atomic number
A
X
Z
symbol of the element
(number of protons)
What would the expression A minus Z, or A - Z, represent? {1}_________________________________
If the value of Z changes, will X change? {2}_______________
happen to the value of A?
If Z changes by a value of 2, what will
{3}_________________________________________________________
You learned previously that atoms of an element can exist in different forms known as isotopes.
Isotopes are atoms of an element that contain different numbers of neutrons. Therefore, isotopes have
different masses and different mass numbers – although they have the same atomic number. Some
elements have many isotopes, while others have only a few. In addition, some isotopes of elements are
naturally-occurring while others are man-made. Some isotopes are unstable, meaning that they
decompose or break apart on their own. Many elements possess both stable and unstable isotopes.
Unstable isotopes are said to be radioactive. They give off energy and/or nuclear particles when they
decompose. In Table 28.1, the mass numbers of isotopes of some selected elements are shown.
If ALL of the isotopes of an element happen to be radioactive, then the element itself is
categorized as being radioactive. With this in mind, which of the selected elements listed in Table 28.1
should be categorized as radioactive? {4}____________________________ How many radioactive isotopes does
carbon (C) have?{5}_________
How many nonradioactive isotopes does nitrogen (N) have?
_________
How
many
man-made
radioactive isotopes does helium (He) have?{7}_________
All
{6}
elements on the periodic table with an atomic number of 84 or greater are radioactive. These elements are
shown as they occur on the periodic table in Table 28.2.
28-3
©1997, A.J. Girondi
Table 28.1
Isotopes of Some Selected Elements
In this table, mass numbers of naturally-occurring nonradioactive isotopes are given in plain type; mass
numbers of naturally-occurring radioactive isotopes are double-underlined; mass numbers of any other
isotopes are single-underlined. Naturally-occurring isotopes are listed in their order of abundance. All other
isotopes are listed in order of decreasing half-life which is discussed later in this chapter.
Element
Mass numbers of isotopes
H
1, 2, 3
He
4, 3, 6, 8
Be
9, 10, 7, 11, 6
B
11, 10, 8
C
12, 13, 14, 11, 10, 15, 16, 9
S
32, 34, 33, 36, 35, 38, 37, 31, 30, 29
N
14, 15, 13, 16, 17, 18
Ca
40, 44, 42, 48, 43, 46, 41, 45, 47, 49, 50, 39, 38, 37
Sn
120, 118, 116, 119, 117, 124, 122, 112, 114, 115, 126, 123, 113,
125, 121, 110, 127, 128, 111, 109, 108, 129, 131, 130, 132
U
238, 235, 236, 234, 233, 232, 230, 237, 231, 240, 229, 239, 228, 227
Lr
260, 256, 255, 254, 257, 256, 252, 251, 258
The simplest element, hydrogen, has three isotopes. The most common form of
hydrogen (protium) has one proton and no neutrons in its nucleus. Its atomic number is 1,
and its mass number is 1. The nuclear notation for protium is shown at right. In nature
approximately 99.985% of all hydrogen atoms are protium.
The remaining 0.015% of hydrogen consists of deuterium atoms.
1
H
1
Also known as heavy
hydrogen, deuterium differs from protium in that it has one neutron in the nucleus in addition to one
proton.Using the
{8}_______________
letter D instead of H as the symbol, write the nuclear notation for deuterium:
Protium and deuterium are both stable, naturally-occurring isotopes.
Water (H2O)
molecules which contain deuterium instead of protium are known as "heavy water" which is sometimes
represented as D2O. About two water molecules in every billion are "heavy." A third form of hydrogen is
man-made and is radioactive. It is known as tritium, and it is a common by-product of the nuclear reactions
that occur in a nuclear power plant. Tritium has two neutrons in its nucleus. Using the letter T instead of H
as the symbol, write the nuclear notation for tritium. {9}________________
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©1997, A.J. Girondi
Table 28.2
The Radioactive Elements
1A
8A
( All of their isotopes are radioactive)
2A
3A
4A
5A
6A
7A
43
Tc
84
85
86
Po At
87
Fr
88
89
104
105
106 107
108
Rn
109 110 111
Ra Ac Unq Unp Unh Uns Uno Une Uun Uuu
61
Pm
90
91
92
Th Pa U
93
94
95
96
97
98
Np Pu Am Cm Bk Cf
99
101 102
103
Es Fm Md No
100
Lr
It is common to identify which particular isotope of an element is being discussed by writing the
mass number after the name of the element with a dash in between. For example, protium is hydrogen-1,
while deuterium is hydrogen-2.
Following this method, how would tritium be
written?
{10}_______________ What is meant by mass number? {11}___________________________________________________
SECTION 28.2
Four Types of Nuclear Reactions
The equation at right represents a nuclear change. We will refer to
it as a nuclear equation. More specifically, it depicts the change of an atom
of carbon-14 into an atom of nitrogen-14:
14
14
6
7
C ----->
Nuclear equations often include a special type of notation to represent subatomic
particles such as electrons, protons, and neutrons. This notation looks similar to
nuclear notation which represents a nucleus, but it is not the same. The
notations describing an electron, a proton, and a neutron are shown below. Note
that the superscripts represent the mass numbers of each particle. The mass
number of an electron is zero. However, the subscripts represent the charge on
the particle. Note that neutrons have no charge, so the subscript for them is zero.
Thus, the difference between nuclear notation and the notation for these
subatomic particles lies in the meaning of the subscript.
28-5
0
N +
-1
0
electron:
-1
proton:
1
+1
neutron:
1
0
e
e
p
n
©1997, A.J. Girondi
mass number
mass number
14
6
atomic number
1
C
+1
charge
NUCLEAR NOTATION
p
SUBATOMIC PARTICLE NOTATION
According to the data in Table 28.1, is carbon-14 a radioactive isotope?{12}_________ How about
nitrogen-14?{13}_________ Note that if the atomic number changes during a nuclear reaction, the
identity of the resulting element changes, too. In the equation shown below, a nucleus of carbon
becomes a nucleus of nitrogen as the atomic number changes from 6 to 7. An electron is also given off as
a product. But hey! If the atomic number changes from 6 to 7, this means that one addition proton is now
present. Where did it come from? Hmmmm.
14
14
6
7
C ----->
N +
0
-1
e
Electrons are sometimes called beta particles (pronounced "bay-ta"). So, the giving off of an
electron in a nuclear reaction is called a beta emission. In order for carbon-14 to change to nitrogen-14,
there was an increase in the number of {14}_________________ in the nucleus. When a neutron
decomposes, the products are a proton and an electron. The new proton causes the atomic number to
increase by one, and the electron is given off. When the decomposition of a neutron produces a proton,
the mass number remains unchanged. Since one element is changed into another in this reaction, this
particular type of nuclear reaction is called a {15}____________________________.
There are four types of nuclear reactions that release energy:
1. Natural Radioactive Decay
Natural radioactive decay refers to the ability of a nucleus to decompose (decay) and give off
energy spontaneously (without any external stimulation). As a result, the number of {16}______________
(atomic number) in the nucleus may increase or decrease, depending on the type of radioactive decay.
The equation below in which carbon-14 is converted to nitrogen-14 represents a natural radioactive
decay.
14
14
6
7
C ----->
N +
0
-1
e
2. Artificial Transmutation
During artificial transmutation, a nucleus changes its identity as a result of some external
stimulation created by man. For example, an external particle such as a neutron could be used to bombard
the nucleus, causing it to decompose. This kind of nuclear disintegration results in the formation of an
artificial (man-made) isotope of the element. The equation below shows the conversion of natural
nonradioactive cobalt-59 to radioactive cobalt-60 by a process known as slow neutron bombardment.
59
1
27
0
Co +
n
---->
60
Co
27
Notice that since a neutron is being added to the nucleus, the mass number of the nucleus increases by
one, from 59 to 60. The atomic number remains unchanged since the number of {17}______________________ is
unchanged. Since the atomic number remains unchanged, the identity of the nucleus (cobalt) remains
the same. What we have done here is to change one isotope of cobalt into a different isotope of cobalt.
28-6
©1997, A.J. Girondi
3. Fission
In fission, a nucleus with a large mass splits into two nuclei with smaller masses. To cause fission,
man bombards certain nuclei with special particles. The fission process is used to generate heat in nuclear
power plants, and is the kind of reaction which occurs during the explosion of an atomic bomb. Let's see
where this energy comes from. Look at the equation below which represents the fission of uranium- 235.
Find the total of the mass numbers of the two particles on the left side of the equation: {18}__________.
235
U +
92
1
0
n ---->
138
95
1
36
0
Kr + 3 n + energy
Ba +
56
Next, find the total of the mass numbers of the five particles on the right side: {19}__________. How do
these totals compare? {20}______________________________ As a result, you would think that mass
the amount of matter) is conserved (neither created nor destroyed). However, this is a bit misleading.
Keep in mind that the mass number is the total number of the protons and neutrons in the nucleus, not
their exact total mass. Remember that masses of atoms and subatomic particles are expressed in very tiny
units called atomic mass units (amu). The mass of an atom of U-235 is actually a little greater than 235 amu,
and the masses of Ba-138 and Kr-95 are actually a little less than 138 and 95, respectively. Therefore, in
the equation above, there is a small loss of mass which appears as a great amount of energy. In other
words, some mass is converted into energy. An atomic bomb gives off a tremendous amount of heat
because some mass is converted into energy. A tiny amount of mass can produce a tremendous amount
of energy. When the uranium nucleus splits into smaller nuclei, the energy which was needed to hold the
whole thing together in the first place is no longer needed. This is the energy which is given off.
4. Fusion
When fusion occurs, the nuclei of two lower mass elements are combined to form a nucleus with a
greater mass representing a different element. Exceedingly high temperatures are needed to cause
fusion to occur, since the two nuclei repel each other due to their similar positive charges. Fusion
reactions are the source of the sun's energy where hydrogen nuclei combine to form helium nuclei. The
equation below shows the fusion of 2 deuterium nuclei to form one helium-4 nucleus (also called an alpha
particle).
2
2
1
1
H +
H
------->
4
2
He +
energy
Fusion reactions were used in weapons such as the hydrogen bomb. Scientists are experimenting with
fusion reactions in devices known as breeder reactors which may someday replace fission reactors in
nuclear power plants. Fusion, like fission, results in a loss of mass which is converted into a great amount
of energy. However, fusion releases much more energy per gram of fuel than fission does.
Problem 1. Let's practice writing nuclear notation. Keep in mind that the superscript is the mass
number (sum of protons and neutrons) and the subscript is the atomic number (number of protons) if the
particle is a nucleus. If the particle is a subatomic particle (proton, electron, or neutron,) then the subscript
is the charge on the particle. Write the nuclear notation for each of the following:
a.
b.
c.
d.
an isotope of carbon (C) which contains 6 protons and 8 neutrons
an isotope of helium (He) which contains 2 protons and 4 neutrons
an isotope of uranium (U) which contains 92 protons and has a mass number of 233
an isotope of tin (Sn) which contains 50 protons and 60 neutrons
a.____________
b.____________
c.____________
28-7
d.____________
©1997, A.J. Girondi
Now, let's try working with some nuclear equations. Keep in mind that in a balanced nuclear
equation, the total of the superscripts of all particles must be equal on both sides of the equation. The
sum of the subscripts of all particles must also be equal on both sides. For example, consider the
equation below.
226
222
88
86
Ra ----->
Rn +
4
2
He
In this example, an isotope of radium (Ra) decomposes into an isotope of radon (Rn), and this
decomposition is accompanied by the emission of a helium nucleus which is also called an alpha particle.
What is the sum of the superscripts on the right side of the equation? {21}_______________ How does this
compare with the superscript on the left side? {22}_____________________ What is the sum of the
subscripts on the right side? {23}__________________ How does this compare to the subscript on the left side?
{24}___________________________________ Is this nuclear equation balanced?{25}_________________
Problem 2. Complete the following transmutation reactions, indicating in each case, the nuclear
notation of the element formed. What element is formed in the first equation below? Well, if you check it
out, the atomic number of the missing particle will have to be 6. What element has an atomic number of 6?
{26}________________________ Therefore, what element symbol will the missing particle have?{27}________________
a.
9
4
Be +
28
b.
Si
4
27
c.
Al
13
+
+
Mn +
25
+
D
----->
+
n
----->
+
D
----->
+ 2
2
1
0
1
24
e.
----->
2
2
Na
0
1
0
n
n
4
2
0
+
----->
11
1
He
1
55
d.
4
-1
He
1
0
n
e
Complete the following equations indicating in nuclear notation, in each case, what particle - if any - was
ejected. Answers may include:
electron:
14
f.
7
N
9
g.
4
+
Be +
1
1
-1
e
proton:
1
+1
p
neutron:
n
----->
+
D
----->
+
0
2
0
11
28-8
5
1
0
n
alpha particle:
4
2
He
B
10
5
B
©1997, A.J. Girondi
h.
i.
27
+
Al
13
239
92
4
2
U
He
----->
----->
+
30
15
239
93
P
Np +
The radioactive elements with atomic numbers 84 through 92 (up to and including uranium) have
some naturally-occurring radioactive isotopes. The elements beyond uranium (with atomic numbers
greater than 92) do not have any naturally occurring isotopes. These elements beyond uranium are
known as the transuranium elements. They are all synthetic elements since all of their isotopes are manmade. Most of the radioactive elements (with atomic numbers 84 and above) are too unstable to be
assigned an atomic mass (atomic weight). If you look at a periodic table, you will notice that the atomic
masses of these elements are given in parentheses. (Check this out on a periodic table now.) The
number in the parentheses represents the atomic mass of the single most stable isotope. You will recall
that atomic mass is defined as the average mass of the various naturally occurring isotopes of an element
in the proportions in which they occur in nature. The radioactive isotopes of elements with atomic
numbers 84 and above are constantly decomposing. These isotopes have different half-lives, which
means that they are decomposing at different rates. Use this information to explain why these elements
cannot have an atomic mass as defined above:
{28}________________________________________
______________________________________________________________________________
Most elements with atomic numbers smaller than 84 are stable because NONE of their naturallyoccurring isotopes are radioactive. There are some exceptions to this rule. For example, K-40 and Ca-46
are radioactive. Most of the elements below atomic number 84 are stable enough to be assigned an
atomic mass. (Elements #43 and #61, Technetium and Promethium, are exceptions.) Man-made
radioactive isotopes have been synthesized for many of these elements, but synthetic isotopes are not
included in the calculation of atomic masses since they are not found in nature.
Section 28.3 Early Studies of Radioactivity
In1896, a French scientist by the name of Henri Becquerel accidentally discovered natural
radioactivity while conducting experiments with a uranium compound called potassium uranyl. In one of
his experiments, Becquerel wrapped a photographic plate in black, lightproof paper and placed some of
the uranium compound on top of the covered plate. He then placed this arrangement in the sunlight.
Although the sunlight could not pass through the lightproof paper, the plate became exposed in the area
of the uranium compound, as indicated by a dark area on the photograph. Becquerel thought that
perhaps energy from the sun had been changed into some more penetrating form which was able to pass
through the paper. He then attempted to repeat the experiment, but cloudy weather prevented him from
doing so at that time. He decided to store his second set-up in a closed drawer. Later, on a sunny day,
Becquerel repeated the experiment using a fresh photographic plate instead of the one he had stored in
the closed drawer. He then developed both of the photographic plates. Since the stored plate had not
been exposed to sunlight, Becquerel expected the developed photograph to be blank or almost blank.
Instead, he found that it had a dark area like that of the fresh plate which had been exposed to sunlight.
Becquerel reasoned that the uranium compound must have emitted some type of energy on its own
without the stimulation of sunlight. This ability of a nucleus to emit energy spontaneously (without
external stimulation) is called natural radioactivity. Uranium ore exhibits natural radioactivity with the
greatest amount of energy coming from its most abundant naturally-occurring isotope, U-238.
28-9
©1997, A.J. Girondi
Becquerel also discovered that as the energy is emitted from a radioactive nucleus and passes
through molecules of oxygen and nitrogen in the air, it causes these molecules to lose electrons, forming
positively charged ions. As a result, the air becomes ionized. The fact that radioactive nuclei can ionize
gases is a principle used in the construction of equipment which can detect the presence of radioactivity.
You probably have a smoke detector in your home. The most common form of smoke detector contains a
small sample of a radioactive element (probably americium). The radiation emitted is capable of ionizing
small particles in the air. When enough particles are present (as during a fire), the ions which are produced
allow an electric current to form and the alarm goes off.
An electroscope is a device which can detect and store an electric charge. See Figure 28.1. A
simple electroscope can be constructed by attaching two pieces of thin metal foil to a metal rod. This
apparatus is then sealed inside a glass container such as a jar. When the electroscope is in its normal
"uncharged" state, the two pieces of metal foil will hang beside each other. To convert the electroscope
to its "charged" state, we have to supply it with an excess of electrons. How do you do this? Well, there
are many ways. Even by combing your hair and then touching the comb to the metal rod on the
electroscope will do it. The electrons on the comb (which came from your hair) will flow into the rod and
into the two pieces of metal foil. At that point, both pieces of foil would carry a negative charge and they
would repel each other. The greater the amount of charge they hold, the more they repel each other. So,
an electroscope is a crude device for detecting and measuring an electrical charge. The air around the foil
in the electroscope acts as an insulator, helping to prevent the electroscope from losing its stored charge
right away. It is much harder for electrons to flow through air than through metal. If you touch the metal rod
on the electroscope with any substance which is a good "acceptor" or conductor of electrons (such as a
piece of metal), the excess electrons will flow out of the electroscope which will then lose its charge.
discharged
weakly charged
highly charged
Figure 28.1
An Electroscope
When nuclear radiation ionizes the air forming positively-charged particles, these positive particles
can draw negatively-charged electrons away from an electroscope in which they might be stored. It is
possible to measure the rate at which radioactive emissions occur by measuring the rate at which an
electroscope loses its charge. Marie Sklodowska, a student of Becquerel, used an electroscope to study
the radioactivity of uranium and its various ores. She found that one uranium ore, pitchblende, gave off
28-10
©1997, A.J. Girondi
much more radioactivity than even pure uranium. After her marriage to the physicist Pierre Curie, they
both studied the radioactivity of pitchblende. The Curies discovered that the increased radioactivity of
pitchblende was due to the presence of two elements in the ore. Madame Curie called the first radioactive
element which they discovered in the ore "polonium" after her native land, Poland. Find polonium (Po) on
the periodic table. What is its atomic number?{29}_______________ On the periodic table, the mass number of
polonium is (210). What is so special about Po-210 and why is this mass number given in parentheses?
It took the Curies four years to complete the processing of the ore from which they extracted only 0.1 gram
of the second radioactive element, radium, in the form of radium chloride. Radium (Ra) has what atomic
number on the periodic table?{30}___________ Its mass number is given as (226). Both polonium and
radium were found to be more radioactive than uranium. Although the use of the electroscope allowed
the Curies to measure the rates at which radiation was emitted, it did not provide any indication as to the
nature of the radiation. In other words, it did not indicate whether the radiation consisted of energy, or
particles, or both.
In 1903, Ernest Rutherford performed an experiment which provided some new information
about the properties of radiation. He placed a piece of pitchblende into a hole drilled deep into a block of
lead. (See Figure 28.2) Most of the radiation emitted by the pitchblende was absorbed by the lead. Only
the radiation that was traveling in a straight line through the hole could escape. A photographic plate was
positioned in the path of the escaping radiation. When the plate was developed, a small single spot
appeared where it was struck by the radiation.
Next, Rutherford placed the poles of a U-shaped magnet at right angles to the stream of radiation.
This forced the radiation to pass through a magnetic field. Since a magnetic field deflects oppositely
charged particles in opposite directions, it was possible to determine the charge of any particles in the
radiation. Streams of radiation which do not contain particles would not be affected by the magnetic field.
When the magnetic field was used, three distinct spots were produced. (See figure 26.2.) The three
spots indicated that the magnet had separated the radiation into three distinct streams. Two streams were
deflected in opposite directions, whereas one stream was not deflected at all. How many of these three
streams contained particles?{31}__________ Why were the two affected streams deflected in opposite
directions? {32}___________________________________________________________________
The two deflected streams are called alpha (∝ ) and beta (ß) radiation in Figure 28.2. The unaffected
stream was called gamma (∂) radiation. What must be true about the stream of gamma radiation that was
not deflected? {33}_________________________________________________________________________________________________
photographic plate
photographic plate
(–)
(+)
3 spots
formed
single
spot
formed
magnet
radiation
radiation
pitchblende
Lead
pitchblende
Lead
Figure 28.2
Rutherford's Study of Radiation from Pitchblende
28-11
©1997, A.J. Girondi
The particles which were deflected only slightly in a
direction indicating a positive charge were called alpha particles.
The Greek symbol for alpha is: ∝. The fact that they were only
slightly deflected indicated that they had a relatively large mass
compared to beta particles. In later experiments, it was shown
that alpha particles were actually bundles composed two protons
and two neutrons. They have the same structure as helium
nuclei. You can say that the term alpha particle is another name
for a helium nucleus. Alpha particles are, therefore, designated Nuclear Notation for Helium-4
by the same nuclear notation as is the most common isotope of
or for an Alpha Particle
helium which is helium-4. Alpha particles travel at 10,000 to
20,000 miles per second, but can be stopped by a sheet of
paper. They have a great ability to cause ionization by knocking
electrons loose from atoms or molecules through which they
pass.
4
He
2
The very low mass particles were deflected much more than the alpha particles and in the
opposite direction. Apparently, they were negatively charged. Rutherford called them beta particles. The
Greek symbol for beta is: ß . They were later shown to be electrons which travel at a rate of up to 100,000
miles per second! Their ability to penetrate matter when they strike it is much greater than that of alpha
particles; nevertheless, they still cannot penetrate more than a few inches of solid material. Beta particles
cause much less ionization than alpha particles.
The radiation emitted between the alpha and beta streams was not deflected at all by the magnetic
field and, therefore, carries no electric charge. This stream was called gamma radiation. The Greek symbol
for gamma is: ∂. Gamma rays are similar to x-rays, but are higher in energy. Their penetrating power is
much greater than either alpha or beta radiation, and they can penetrate almost one foot of solid lead!
Gamma rays travel at the speed of light (186,000 miles per second). They cause practically no ionization at
all when they interact with atoms or molecules. Table 28.3 summarizes some of the information
presented about the three forms of radioactivity. Complete the column headed "Penetrating Power" by
inserting the terms high, low, and moderate in the proper slots. Next, complete the column headed
"Ionizing Power" by inserting the terms high, moderate, and almost none in the proper slots.
Table 28.3
The Three Forms of Natural Radioactivity
Decay Product
alpha particle
Symbol
4
2
beta particle
He
0
-1
gamma rays
e
none
Charge
Penetrating
Power
Ionizing
Power
+2
{34}_________
{37}_________
–1
{35}_________
{38}_________
none
{36}_________
{39}_________
In general, a radioactive isotope of an element emits alpha particles or beta particles, but not both. The
emission of gamma rays generally accompanies both alpha emissions and beta emissions. Which of the
three kinds of radioactive emissions is needed in order for a transmutation to occur? {40}______________
Explain:
{41}_____________________________________________________________________
______________________________________________________________________________
28-12
©1997, A.J. Girondi
Name three radioactive elements found in pitchblende: {42}___________________________________
SECTION 28.4
Methods of Detecting Radiation
Electroscopes
Radioactivity has an effect on matter as it passes through it. We can, therefore, study radioactivity
by recording and measuring these effects. You already know that nuclear emissions can expose
photographic plates and can ionize gases. Some measuring devices make use of the fact that gases will
conduct electricity when they become ionized as a result of exposure to radiation. For example, the
electrical charge stored in an electroscope can be lost when the air inside and around the electroscope
becomes ionized. See Figure 28.3 below.
molecules
of air
ions of air
inside here
incoming radiation
ionizes the air
charge
lost
charged
foil strips
Figure 28.3
Effect of Radiation on Stored
Charge
Ionization chambers
In an ionization chamber, radiation passes through a gas. The radiation causes the gas particles to
be split into pairs of ions which are then collected on the surfaces of oppositely charged electrodes. The
number of pairs of ions produced can be measured. An example of a measuring instrument using this
principle is the self-reading dosimeter. With such a device, radiation can be measured in units called
Roentgens. This may sound a bit complicated, but a Roentgen is the amount of gamma radiation required
to produce 1.61 X 1012 pairs of ions when it is absorbed by 1 gram of air.
Geiger Counter
A Geiger counter (more accurately known as a Geiger–Mueller counter) consists of a sealed tube
containing argon gas at a low pressure. One end of the tube contains a thin glass window. There are two
electrodes in the tube (see Figure 28.4). The negative electrode is a metal cylinder located just inside the
tube. The positive electrode is a wire which runs down the center of the cylindrical tube. A high voltage
exists between these electrodes, but electric current does not flow, since the uncharged (un-ionized)
argon gas atoms cannot carry the current from one electrode to the other. When radiation enters through
the thin window, it ionizes some of the argon atoms, forming argon ions and free electrons. The argon
ions become conductors of electric current between the electrodes. The electrical impulses are then sent
into an amplifier. From there they may be sent to a counter or to an amplifier to be converted into sounds
or flashes of light.
28-13
©1997, A.J. Girondi
negative
electrode
positive
electrode
incoming
radiation
argon gas
thin glass
window
To
amplifier or
counter
1000 Volts
Figure 28.4
Geiger Counter
Photographing Particle Trails
As you know, fast moving charged particles such
as those present in radioactive emissions can cause the
formation of ions when they collide with molecules through
which they pass. If this process occurs in a container which
is saturated with water vapor, the water molecules can
condense on ions forming tiny spots of fog. This fog forms
along the paths of the radioactive emissions since that is
where the ions form. These foggy paths are visible to the
eye. They are called trails. Photographs of these particle
trails enable scientists to study how certain decays occur.
The device in which all this takes place is called a cloud
chamber.
In Figure 28.5, the curved vertical line
represents the path of a subatomic particle passing
through a thin sheet of lead. The path is curved due to the
presence of a strong magnetic field in the cloud chamber.
Figure 28.5
Particle Trails in a Cloud Chamber
Scintillation Counter
When radiation strikes fluorescent substances (known as phosphors) it causes flashes of light to
be emitted. This is what happens in a fluorescent light bulb or on a television screen. There are
instruments which can count these small flashes of light, and in this way, measure radiation. The process
of producing light flashes is called scintillation. The devices are called scintillation counters.
28-14
©1997, A.J. Girondi
Section 28.5 More Practice With Nuclear Equations
Problem 3. Complete the equations below, and make sure that they are balanced.
a.
b.
c.
d.
14
7
N
+
4
2
9
4
4
2
Be +
3
1
He ----->
He ----->
3
H ----->
23
3
11
2
Na +
2
3
+
8
12
6
O +
C +
He +
He ----->
e.
17
2
1
1
He ----->
H +
1
13
0
7
n +
N
Now, complete the equation below. Does anything appear strange? An electron with a positive charge!
f.
30
0
15
+1
P ----->
e
+
Yes, there is such a thing as an electron with a positive charge. It's call a positron. As you can imagine,
there's a lot more to know about nuclear chemistry!
SECTION 28.6
Learning Outcomes
This is the end of Chapter 28. The subject of nuclear chemistry is continued in Chapter 26.
Review the learning outcomes below. When you feel that you have mastered them, arrange to take the
exam on Chapter 26, and then move on to Chapter 27.
_____1. Define and /or describe nuclear terms including: isotope, transmutation, alpha particle, beta
particle, gamma rays, fission, fusion, radioactivity, Geiger counter, scintillation counter, and cloud
chamber.
_____2. Write the nuclear notation of nuclear particles and of the nuclei of atoms given mass numbers,
atomic numbers, or other relevant data.
_____3. Describe the historical contributions of Becquerel, Madame and Pierre Curie, and Rutherford.
_____4. Given sufficient information, complete and balance nuclear equations.
_____5. Be able to locate the radioactive elements on the periodic table.
28-15
©1997, A.J. Girondi
SECTION 28.7
Answers to Questions and Problems
Questions:
{1} number of neutrons; {2} yes; {3} it will also change by a value of 2; {4} U and Lr; {5} six; {6} two;
{7} none; {8} 21D; {9} 31T; {10} hydrogen–3; {11} sum of protons and neutrons in nucleus; {12} yes;
{13} no; {14} protons; {15} transmutation; {16} protons; {17} protons; {18} 236; {19} 236 (note that there
are three neutrons represented); {20} they are equal; {21} 226; {22} equal; {23} 88; {24} equal;
{25} yes; {26} carbon; {27} C; {28} since some isotopes are decomposing, the average mass of the
isotopes is changing; {29} 84; {30} 88; {31} two; {32} they contained particles with opposite charges;
{33} it contains no particles; {34} almost none; {35} moderate; {36} high; {37} high; {38} moderate;
{39} almost none; {40} alpha or beta; {41} alpha emission results in loss of 2 protons, while beta emission
results in formation of one proton; {42} polonium, radium, uranium
Problems:
1. a.
14
6
2. a.
12
6
3. a.
1
6
2
29
C
P
+1
b.
C
b.
b.
5
1
0
n
He
B c.
c.
c.
24
11
0
-1
e
Na
233
92
U
d.
55
d.
26
25
d.
12
Fe
Mg
110
50
Sn
24
e.
12
11
e.
28-16
5
Mg
B
f.
f.
30
14
4
2
He
g.
1
n h
0
1
0
n i. e
0
-1
Si
©1997, A.J. Girondi