Chapter 24
Nuclear Reactions and Their
Applications
The behavior of three types
of radioactive emissions in
an electric field
Types of Radioactive Decay: Balancing Nuclear Equations
Total A
Total Z
Reactants = Total ATotal Z Products
Alpha decay - A decreases by 4 and Z decreases by 2. Every element
heavier than Pb undergoes a decay.
Beta decay - ejection of a b particle from the nucleus from the conversion
of a neutron into a proton and the expulsion of 0-1b. The product nuclide
will have the same Z but will be one atomic number higher.
Positron decay - a positron (01b) is the antiparticle of an electron. A
proton in the nucleus is converted into a neutron with the expulsion of the
positron. Z remains the same but the atomic number decreases.
Electron capture - a nuclear proton is converted into a neutron by the
capture of an electron. Z remains the same but the atomic number
decreases.
Gamma emission - energy release; no change in Z or A.
Sample Problem 1
PROBLEM:
Writing Equations for Nuclear Reactions
Write balanced equations for the following nuclear reactions:
(a) Naturally occurring thorium-232 undergoes a decay.
(b) Chlorine-36 undergoes electron capture.
Nuclear Stability and Mode of Decay
•Very few stable nuclides exist with N/Z < 1.
•The N/Z ratio of stable nuclides gradually increases a Z increases.
•All nuclides with Z > 83 are unstable.
•Elements with an even Z usually have a larger number of stable
nuclides than elements with an odd Z.
•Well over half the stable nuclides have both even N and even Z.
Predicting the Mode of Decay
•Neutron-rich nuclides undergo b decay.
•Neutron-poor nuclides undergo positron decay or electron capture.
•Heavy nuclides undergo a decay.
A plot of neutrons vs. protons for the stable nuclides
Sample Problem 2
PROBLEM:
Predicting Nuclear Stability
Which of the following nuclides would you predict to be stable
and which radioactive? Explain.
(a)
18
10Ne
(b)
32
16S
(c)
236
90Th
(d)
123
56Ba
Sample Problem 3
PROBLEM:
Predicting the Mode of Nuclear Decay
Predict the nature of the nuclear change(s) each of the following
radioactive nuclides is likely to undergo:
(a)
12
5B
(b)
234
92U
(c)
74
33As
(d)
127
57La
The 238U decay series
Decay rate (A) = DN/Dt
SI unit of decay is the becquerel (Bq) = 1d/s.
curie (Ci) =
number of nuclei disinegrating each second in 1g of radium-226 =
3.70x1010d/s
Nuclear decay is a first-order rate process.
Large k means a short half-life and vice versa.
Decrease in the number of 14C
nuclei over time
Sample Problem 4
Finding the Number of Radioactive Nuclei
PROBLEM: Strontium-90 is a radioactive by-product of nuclear reactors that
behaves biologically like calcium, the element above it in Group 2A(2).
When 90Sr is ingested by mammals, it is found in their milk and eventually
in the bones of those drinking the milk. If a sample of 90Sr has an activity of
1.2x1012 d/s, what are the activity and the fraction of nuclei that have
decayed after 59 yr (t1/2 of 90Sr = 29 yr)
Radiocarbon dating for determining the age of artifacts
Sample Problem 5
Applying Radiocarbon Dating
PROBLEM: The charred bones of a sloth in a cave in Chile represent the earliest
evidence of human presence in the southern tip of South America. A
sample of the bone has a specific activity of 5.22 disintegrations per
minute per gram of carbon (d/min*g). If the ratio of 12C:14C in living
organisms results in a specific activity of 15.3 d/min*g, how old are
the bones? (t1/2 of 14C = 5730 yr)
A linear accelerator
The linear accelerator operated by
Stanford University, California
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The cyclotron accelerator
Penetrating power of
radioactive emissions
Penetrating power is
inversely related to the mass
and charge of the emission.
Nuclear changes
cause chemical
changes in
surrounding matter
by excitation and
ionization.
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The use of radioisotopes to image the
thyroid gland
asymmetric scan
indicates disease
normal
PET and brain activity
normal
Alzheimer’s
The increased shelf life of irradiated food
The Interconversion of Mass and Energy
E = mc2
DE = Dmc2
Dm = DE / c2
The mass of the nucleus is less than
the combined masses of its nucleons.
The mass decrease that occurs when
nucleons are united into a nucleus is
called the mass defect.
The mass defect (Dm) can be used to
calculate the nuclear binding
energy in MeV.
1 amu = 931.5x106 eV = 931.5MeV
Sample Problem 6
PROBLEM:
Calculating the Binding Energy per Nucleon
Iron-56 is an extremely stable nuclide. Compute the binding
energy per nucleon for 56Fe and compare it with that for 12C
(mass of 56Fe atom = 55.934939 amu; mass of 1H atom =
1.007825 amu; mass of neutron = 1.008665 amu).
The variation in binding energy per nucleon
Induced fission of 235U
A chain reaction of 235U
Diagram of an atomic bomb
The tokamak design for
magnetic containment
of a fusion plasma
Detection of radioactivity by an ionization counter
Vials of a scintillation “cocktail” emitting light
Element synthesis in the life cycle of a star
Figure B24.3
A view of Supernova 1987A