Transcript Chapter 21 - Richsingiser.com

Daniel L. Reger
Scott R. Goode
David W. Ball
http://academic.cengage.com/chemistry/reger
Chapter 21
Nuclear Chemistry
Review
• Most naturally occurring elements are
mixtures of isotopes, which are represented
by symbols of the form
A
Z
X
where X is the symbol of the element,
A = mass number, and Z = atomic number.
• Nuclide is the nucleus of a specific isotope.
• A nucleon is a proton or neutron.
Definitions
• A stable isotope is one that does not
spontaneously decompose into another
nuclide.
• An radioactive nuclide is one that
spontaneously decomposes into another
nuclide.
Stable Nuclide Characteristics
1 The number of neutrons is equal to or
greater than the number of protons
(except for 1H and 3He).
2 Up to Z = 20, the number of neutrons
and protons are nearly equal; above 20
the ratio of n/p increases slowly to about
1.6:1.
3 Nuclear stability is greater for nuclides
containing even numbers of protons,
neutrons, or both.
Stable Nuclide Characteristics
4 Certain numbers of protons and
neutrons (called magic numbers)
confer unusual stability: 2, 8, 20, 26,
28, 50, 82, and 126.
5 The zone of stability contains all of
the stable nuclides, but some
nuclides in this band are unstable.
6 Tc (Z=43), Pm (Z=61), and all
elements beyond Bi (Z=83) have no
stable isotopes.
Radioactivity
• There are three major kinds of emissions
from radioactive.
α particles are high-energy 4He nuclei.
β particles are high-energy electrons that
originate from the nucleus.
γ rays are very short wavelength (high
energy) electromagnetic radiation.
Nuclear Equations
• A nuclear equation describes any
process in which a nuclide
undergoes change.
 In a balanced nuclear equation the
sum of the mass numbers and atomic
numbers on the reactant and product
sides of the equation must be equal.
238
92
U
Th  
234
90
4
2
Z: 92 = 90 + 2 A: 238 = 234 + 4
Alpha Decay
• An alpha decay decreases the
atomic number by 2 and the mass
number by 4.
232
228
4
90 Th  99 Ra  2 
• Alpha decay is seen in all elements
heavier than bismuth (Z > 83).
Beta Decay
• Beta decay increases the atomic number
by one, without changing the mass
number.
14C  14N + 0
7
6
-1
• The β particle does not exist in the
nucleus, but is created at the instant of its
emission.
• Beta emission is observed in nuclides that
have too many neutrons to be stable.
Positron Emission
• A positron is identical to an electron, except its
charge is positive.
• Positron emission decreases the atomic
number by one, without changing the mass
number.
40
19
K
40
18
Ar  β
0
1
• The symbol for the positron and beta particle is
the same, except for the sign of Z.
• Positron emission is seen in nuclides that have
too many protons to be stable.
Electron Capture
• In electron capture an electron in a low energy
orbital of the atom is capture by the nucleus
and converts a proton to a neutron.
44
0
44
22Ti  1e  21Sc
• X rays (not g rays) accompany electron
capture, because the atom produced is in an
excited electronic state.
• Electron capture and positron emission both
decrease the atomic number by 1.
• Both processes occur when the nuclide
contains too many protons to be stable.
Predicting Modes of Decay
For radioactive elements
• When Z > 83, an
emission is often
observed.
• If A > atomic mass
0
of element 1
decay occurs.
• If A < atomic mass
of element 01
decay or electron
capture occurs.
Example Problem
• Predict the mode of decay, and write the
nuclear equation for each radioactive
nuclide:
(a)
226
88
Ra
(b)
22
11
Na
(c)
130
53
I
Radioactive Series
• Among the heavier elements
radioactive decay series are common.
Detecting Radiation
• Radiation detection is based on the
ionization caused by high energy
particles and light and includes:
• Exposure of photographic film.
• Geiger counters.
• Scintillation counters.
Decay Rates
• Radioactive decays obey a first order
rate law:
- N
rate =
 kN
t
where N is the number of radioactive
nuclei.
• Usually the half-life, t1/2, is given rather
than k.
ln 2 0.693
t1/2 
k

k
Nuclear Transmutation
• A nuclear transmutation is a
reaction in which two particles or
nuclides produce nuclides that are
different than the reactant species.
• Two of the first nuclear
transmutations observed were:
14
7
9
4
N   O p
4
2
17
8
1
1
Be    C  n
4
2
12
6
1
0
Nuclear Reactions of Charged Particles
• When both reactants in a nuclear
transmutation are positively charged,
very high energies are needed because
of electrostatic repulsion.
• Devices used to produce these high
energy particles include cyclotrons and
linear accelerators.
Energy and Mass
• The energy equivalent of mass is
calculated from
E = mc2
where E is energy, m is mass, and c is
the speed of light.
• When a nuclear change occurs a
measurable difference in the mass of the
products and reactants is observed.
Nuclear Binding Energy
• Nuclear binding energy is the amount
of energy required to keep the protons
and neutrons together in the nucleus.
• Some of the mass of the nucleons is
converted into binding energy.
• The mass of a nuclide is always less than
the masses of the neutrons and protons
present.
Mass Defect
• The mass defect is the difference
between the mass of the nucleons and
the mass of the nuclide.
• The larger the mass defect, the greater is
the nuclear binding energy.
Calculating Mass Defect
• The mass of a 7Li atom is 7.016003 u.
Calculate the mass defect (in u) given
the following masses:
1H 1.007825 u
1 n 1.008665 u
o
Nuclear Binding Energy from Mass Defect
• The 7Li nuclide has a mass defect of
0.042132 u. Calculate the binding energy
of this nuclide, in kJ/mol, using the
equation
DE = Dmc2
Nuclear Fission
• Nuclear fission forms two nuclei of
roughly similar size from a single heavy
nucleus.
• Fission reactions release very large
quantities of energy (“exergonic”) and
produce several neutrons in addition to
two nuclides.
236
141
92
92 U  56 Ba + 36 Kr
+3
1
0n
Fission Yield Curve
• More than 370 nuclides, with A = 72 to
161, are found from the fission of 235U.
Chain Reaction of
•
235U
235U
absorbs a neutron which induces
fission, and a chain reaction.
Critical Mass
• The critical mass is the minimum mass
needed for a nuclear chain reaction to
maintain a self-sustaining reaction.
Nuclear Power Reactor
• In a nuclear power reactor, conditions
maintain criticality at a constant power
level.
Nuclear Power and Safety
• Long term storage of radioactive fission
products and fear of disastrous accidents
are the major deterrents to increased use
of nuclear power.
• Current technology incorporates the
radioactive waste in glass loaded into
stainless steel containers, which are buried
deep underground.
• Strict government regulations are enforced to
assure the safe operation of reactors.
Reactor Designs
• Nuclear fusion is the combination of
two light nuclides to form a larger one.
• The energy produced by the sun
comes from fusion reactions, such as
1
1
2
0
7
E = -9.9  10 kJ/mol
1H  1H  1H  1n
1
2
3
8
E = -5.2  10 kJ/mol
1H  1H  2 He
3
1
4
0
9
2 He + 1H  2 He + 1 β E = -1.9  10 kJ/mol
Reactor Designs
• The possibility of fusion reactors is at
least several decades away from reality.
• An international consortium of the U.S.,
Japan, Russia, and the European
Community are jointly designing a
experimental thermonuclear power
reactor.
Units of Radioactivity and Radiation Dose
Radioactivity
Name
Abbrev Definition or conversion
SI unit
Becquerel Bq
Common unit
Curie
Ci
1 disintegration /second
3.7 1010 disintegrations
/second
rad
Quantity of radiation that
transfers 1 10-2 J of energy
per kilogram of matter
Sv
rem
1 Sv = 100 rem
1 rem = 0.01 Sv
Radiation Dose
SI unit
rad
Effective Radiation Dose
SI unit
Common unit
Sievert
rem
Most Radiation has a Natural Source
Radon
•
222Ra
is produced by the decay of natural
238U found in rocks such as granite.
• Home radon test kits are sold because of
the great public awareness and the
potential dangers of radon accumulation.
• Since radon generally enters homes
through the basement, one of the most
effective means of eliminating the
radiation danger is to add ventilation fans
in the basement.
Nuclear Medicine
• When a patient has an overactive thyroid
gland, he or she is often given a dose of
131I, which is a beta-emitter.
• Iodine concentrates in the thyroid gland,
and the beta radiation from the 131I
isotope reduces the amount of hormone
produced by the thyroid gland.
CT Scans
• Radiopharmaceuticals are synthesized
with isotopes that emit gamma radiation
coupled to organic molecules that are
taken up specifically in target organs.
• Modern scanners use gamma ray
cameras that take measurements at
thousands of different locations.
• Computerized tomography is often
abbreviated CT and referred to as a CT
scan.
Gamma Radiation Scans - 99mTc
• Technicium is a transition metal with a
variety of stable oxidation states that
makes it an excellent species to use to
react with organic molecules.
• Heart imaging is done with a compound
formed by technicium(I) and six isonitrile
ligands.
• Other compounds have been developed
for mapping the brain, lungs, etc.
• Over 100 different nuclear medicine
diagnoses are available to clinicians.
Positron Emission Tomography (PET) Scans
• A fluorinated sugar (fluorodeoxyglucose)
that contains 18F is often used.
• This sugar is taken up most by the organs
that are subdividing most quickly. And
cancer cells are among the fastest.
• PET scans are particularly effective in
diagnosing lung, head and neck,
colorectal, esophageal, lymphoma,
melanoma, breast, thyroid, cervical,
pancreatic, and brain cancers.