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Transcript
NUCLEAR
CHEMISTRY
Nuclear Radiation
• Nuclear chemistry is the study of the
structure of atomic nuclei and the
changes they undergo.
The Discovery of Radioactivity
• Marie Curie named the
process by which
materials such as
uranium give off rays
radioactivity; the rays
and particles emitted by
a radioactive source are
called radiation.
Types of Radiation
• As you may recall, isotopes are atoms
of the same element that have different
numbers of neutrons.
• Isotopes of atoms with unstable nuclei
are called radioisotopes.
Types of Radiation
• These unstable nuclei emit radiation
to attain more stable atomic
configurations in a process called
radioactive decay.
• During radioactive decay, unstable
atoms lose energy by emitting one of
several types of radiation.
Types of Radiation
• The three
most
common
types of
radiation are
alpha (α),
beta (β), and
gamma (γ).
Types of Radiation
Alpha, Beta or Gamma?
??
??
??
Types of Radiation
• Negatively charged beta particles are
deflected toward the positively charged
plate.
??
??
??
Types of Radiation
• Positively charged alpha particles are
deflected toward the negatively charged
plate.
• Alpha particles are deflected less than beta
rays because they are more massive.
??
??
Types of Radiation
• Gamma rays, which have no electrical
charge, are not deflected.
??
Alpha
• An alpha particle (α) has the same
composition as a helium nucleus—two
protons and two neutrons—and is
therefore given the symbol
.
• The charge of an alpha particle is 2+ due
to the presence of the two protons.
Alpha
• Because of their mass and charge, alpha
particles are relatively slow-moving
compared with other types of radiation.
• Thus, alpha particles are
not very penetrating—a
single sheet of paper
stops alpha particles.
Beta
• A beta particle is a very-fast moving
electron that has been emitted from a
neutron of an unstable nucleus.
• Beta particles are represented by the
symbol
The zero superscript indicates the
insignificant mass of an electron in
comparison with the mass of a nucleus.
Beta
• The –1 subscript denotes the negative
charge of the particle.
• Beta radiation consists of a stream of
fast-moving electrons.
Beta
• Because beta particles are both
lightweight and fast moving, they have
greater penetrating power than alpha
particles.
• A thin metal foil is
required to stop beta
particles.
Gamma
• Gamma rays are high-energy (short
wavelength) electromagnetic radiation.
They are denoted by the symbol .
• As you can see from the symbol, both
the subscript and superscript are zero.
Gamma
• Thus, the emission of gamma rays does
not change the atomic number or mass
number of a nucleus.
• Gamma rays almost always accompany
alpha and beta radiation, as they
account for most of the energy loss that
occurs as a nucleus decays.
Types of Radiation
Name
alpha
beta
gamma
Symbol Formula
α
4
He
2
β
0
e
-1
γ
Mass
4
0
0
Charge Description
+2
helium
nuclei
-1
high speed
electrons
0
high energy
radiation
Nuclear Stability
• Radioactive nuclei undergo decay in
order to gain stability.
• All elements with atomic numbers
greater than 83 are radioactive.
Balancing a Nuclear Equation
• Nuclear equations are used to show
nuclear transformations. Balanced
nuclear equations require that both the
atomic number and the mass number
must be balanced.
Balancing a Nuclear Equation
Mass number
X
X
Z
Z
A
A
Atomic
number
Element
symbol
Balancing a Nuclear Equation
When beryllium-9 is bombarded with
alpha particles (helium nuclei), a neutron
is produced. The balanced nuclear
reaction is given as:
9 Be +
4
4He  1 n +
2
0
???
Balancing a Nuclear Equation
9
Be
+
4
4 He  1 n + ???
0
2
• On the reactant side, the mass
numbers equal (9 + 4) = 13.
• On the product side, the mass number
equals 1.
• The product side needs an additional
12 for the mass number.
Balancing a Nuclear Equation
9
Be
+
4
4 He  1 n + ???
2
0
• On the reactant side, the atomic
numbers equal (4 + 2) = 6.
• On the product side, the atomic
number equals 0.
• The product side needs an additional 6
for the atomic number.
Balancing a Nuclear Equation
9
4 Be +
4 He  1 n + 12
???
0
6
2
• The atomic number (the number on the
bottom) determines the identity of the
element.
Balancing a Nuclear Equation
9
4 Be +
4 He  1 n + 12
0
6C
2
• The element with an atomic number of
6 is carbon.
Balancing a Nuclear Equation
When nitrogen-14 is bombarded with a
neutron, a proton is produced. The
balanced nuclear equation can be written
as:
14
1
1
???
N
+
n

p
+
7
0
1
Balancing a Nuclear Equation
14
7N +
1
1
n

p
+
0
1
???
• On the reactant side, the mass
numbers equal (14 + 1) = 15.
• On the product side, the mass number
equals 1.
• The product side needs an additional
14 for the mass number.
Balancing a Nuclear Equation
14
N
+
7
1
1
???
n

p
+
0
1
• On the reactant side, the atomic
numbers equal (7 + 0) = 7.
• On the product side, the atomic
number equals 1.
• The product side needs an additional 6
for the atomic number.
Balancing a Nuclear Equation
14N + 1 n  1 p +
7
1
0
14
???
6
• The atomic number (the number on the
bottom) determines the identity of the
element.
Balancing a Nuclear Equation
14N +
7
1n  1 p +
0
1
14
6C
• The element with an atomic number of
6 is carbon.
Balancing a Nuclear Equation
230
Th
90

226
???
Ra
88
+
4
He
2
• Balance the nuclear reaction above.
Balancing a Nuclear Equation
234
U
92

230
???
Th
90
+
4
He
2
• Balance the nuclear reaction above.
Balancing a Nuclear Equation
50
Co
27

50
???
Ni
28
+
0
e
-1
• Balance the nuclear reaction above.
Balancing a Nuclear Equation
• Sometimes you must use atoms other
than those on the periodic table to
balance nuclear reactions.
Balancing a Nuclear Equation
Question
What element is formed when
undergoes beta decay? Give the atomic
number and mass number of the
element.
Question
Write a balanced nuclear equation for
the alpha decay of the following
radioisotope.
Question
Nitrogen-12 decays into a positron and
another element. Write the balanced
nuclear equation.
12
N
7

12
C
6
+
0
e
1
Question
Uranium-238 is bombarded with a
neutron. One product forms along with
gamma radiation. Write the balanced
nuclear equation.
238
U
92
+
1
n
0

239
U
92
+ γ
Question
Nitrogen-14 is bombarded with
deuterium (hydrogen-2). One product
forms along with an alpha particle. Write
the balanced nuclear equation.
14
7
N +
2
H
1

12
C
6
+
4
He
2
Radioactive Decay Rates
• Radioactive decay rates are measured in
half-lives.
• A half-life is the time required for onehalf of a radioisotope’s nuclei to decay
into its products.
Radioactive Decay Rates
• For example, the half-life of the
radioisotope strontium-90 is 29 years.
• If you had 10.0 g of strontium-90 today,
29 years from now you would have 5.0 g
left.
• The decay continues until negligible
strontium-90 remains.
Radioactive Decay Rates
• The graph shows the
percent of a stontium90 sample remaining
over a period of four
half-lives.
• With the passing of
each half-life, half of
the strontium-90
sample decays.
Calculating Amount of
Remaining Isotope
• Iron-59 is used in medicine to diagnose
blood circulation disorders.
• The half-life of iron-59 is 44.5 days.
• How much of a 2.000-mg sample will
remain after 133.5 days?
(0.2500 mg)
Question
• Cobalt-60 has a half-life of 5.27 years.
How much of a 10.0 g sample will remain
after 21.08 years?
(0.625 g)
Question
• Carbon-14 has a half-life of 5730 years.
How much of a 250. g sample will remain
after 5730 years?
(125 g)
Nuclear Fission
• Heavy atoms (mass number > 60) tend
to break into smaller atoms, thereby
increasing their stability.
• Using a neutron to split a nucleus into
fragments is called nuclear fission.
• Nuclear fission releases a large
amount of energy and several
neutrons.
Nuclear Fission
• Since neutrons are products, one fission
reaction can lead to more fission
reactions, a process called a chain
reaction.
• A chain reaction can occur only if the
starting material has enough mass to
sustain a chain reaction; this amount is
called critical mass.
Nuclear Fusion
• The combining of atomic nuclei is called
nuclear fusion.
• For example, nuclear fusion occurs
within the Sun, where hydrogen atoms
fuse to form helium atoms.
Nuclear Fusion
• Fusion reactions can release very large
amounts of energy but require extremely
high temperatures. For this reason, they
are also called thermonuclear
reactions.
Question
What is the difference between nuclear
fusion and nuclear fission?
Nuclear fusion is the
combining of nuclei to form a
single nucleus. Nuclear
fission is the splitting of a
nucleus into fragments.
Applications and Effects of
Nuclear Reactions
• Geiger counters, scintillation counters,
and film badges are devices used to
detect and measure radiation.
Applications and Effects of
Nuclear Reactions
Applications of Nuclear
Reactions
• Geiger counters
use ionizing
radiation, which
produces an
electric current in
the counter, to
rate the strength
of the radiation on
a scale.
Applications of Nuclear
Reactions
• Film badges are
often used to
monitor the
approximate
radiation exposure
of people working
with radioactive
materials.
Applications of Nuclear
Reactions
• Scintillation
counters
measure ionizing
radiation.
Applications of Nuclear
Reactions
• With proper safety procedures, radiation
can be useful in industry, in scientific
experiments, and in medical procedures.
Applications of Nuclear
Reactions
• Nuclear power plants
use the process of
nuclear fission to
produce heat in nuclear
reactors.
• The heat is used to
generate steam, which is
then used to drive
turbines that produce
electricity.
Nuclear Reactors
Applications of Nuclear
Reactions
• A radiotracer is a radioisotope that emits
non-ionizing radiation and is used to
signal the presence of an element or of a
specific substance.
• Radiotracers are used to detect diseases
and to analyze complex chemical
reactions.
Applications of Nuclear
Reactions
Applications of Nuclear
Reactions
• Ionizing radiation has many uses. An
X-ray is ionizing radiation, and ionizing
radiation can be used in medicine to kill
cancerous cells.
Applications of Nuclear
Reactions
• Most medical devices require
sterilization after they are packaged, and
another trend has been the move to
sterilization by gamma radiation as
opposed to other methods such as
ethylene oxide gas. Advantages of
gamma irradiation include speed, costeffectiveness, and the elimination of the
need for special packaging.
Applications of Nuclear
Reactions
• Chemical reaction rates are greatly
affected by changes in temperature,
pressure, and concentration, and by the
presence of a catalyst.
• In contrast, nuclear reaction rates remain
constant regardless of such changes.
• In fact, the half-life of any particular
radioisotope is constant.
Applications of Nuclear
Reactions
• Because of this, radioisotopes, especially
carbon-14, can be used to determine the
age of an object.
• The process of determining the age of
an object by measuring the amount of
a certain radioisotope remaining in that
object is called radiochemical dating.
Radiochemical Dating
Effects of Nuclear Reactions
• Any exposure to radiation can damage
living cells.
• Gamma rays are very dangerous
because they penetrate tissues and
produce unstable and reactive
molecules, which can then disrupt the
normal functioning of cells.
Effects of Nuclear Reactions
• The amount of radiation the body
absorbs (a dose) is measured in units
called rads and rems.
• Everyone is exposed to radiation, on
average 100–300 millirems per year.
A dose exceeding 500 rem can be fatal.