Transcript Nuclear Chemistry

NUCLEAR CHEMISTRY
CHAPTER 19
• Isotopes with unstable atomic nuclei are
radioactive and are called radionuclides
• Radioactive isotopes undergo nuclear reactions as
they emit high-energy particles and/or
electromagnetic radiation
• By the early 1900’s physicist realized that the mass of
a particle could not always be treated as constant
• The mass (m) of a particle depends on its
velocity (v) relative to an observer
m
mo
1  (v / c ) 2
c = speed of light = 3.00x108 m/s
v = speed of the particle in m/s
mo = particle’s rest mass (mass at v = 0)
• The rest mass of the particle can be measured in
the lab when the sample is at rest (from our
viewpoint)
• The Law of Conservation of Mass-Energy
replaces the two independent laws:
The sum of all the energy in the universe and
the sum of all the mass (expressed as an
equivalent in energy) is a constant
• The Einstein equation relates the change in rest
mass to an energy change
E  mo c
2
• Because c is so large an enormous amount of
energy accompanies even a small change is
rest mass
ENERGY AND MASS
• When a system gains or loses energy it also gains or
loses a quantity of mass.
E = mc2
Δm = mass defect
ΔE = change in energy
• If ΔE is negative (exothermic), mass is lost from
the system.
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MASS DEFECT (ΔM)
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• Calculating the mass defect for 2 He:
•
Since atomic masses include the masses of the electrons,
we must account for the electron mass.
4
4
4.0026 = mass of 2 He atom = mass of 2 He nucleus + 2me
1
1
1.0078 = mass of 1H atom = mass of 1H nucleus + me
•
4
2
He nucleus is “synthesized” from 2 protons and two
neutrons.
m =  4.0026  2me  
m =  0.0304 amu
2 1.0078  me  + 2 1.0087
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• This energy is also called the binding energy
because this amount of energy would need to
be added to break the nucleus apart
• The higher the binding energy, the more stable
the nucleus
• Iron-56 is the most stable nucleus and has a binding
energy of 8.79 MeV.
• The binding energy can be calculated using the
Einstein equation and the mass defect
• Consider the case of one helium-4 nucleus:
rest mass of an isolated proton  1.007276470 u
rest mass of an isolated neutron  1.008664904 u
rest mass of one 4 He nucleus  4.001506 u
The mass defect is then
mo  2(1.007276470 u)  2(1.008664904 u) - 4.001506 u
 0.030377 u
Using the Einstein equation
1.6605403 10-27 kg
E  0.030377 u 
 (3.00 108 m s 1 ) 2
1u
 4.54 10 -12 J
For the formation of 1 mol 4 He :
4.54 10-12 J 6.02 10 23 nuclei
E 

 2.73 1012 J mol 1
nucleus
mol
TYPES OF RADIOACTIVE DECAY
• Alpha production (α):
• Beta production (β):
Gamma ray production (γ):
Positron production:
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TYPES OF RADIOACTIVE DECAY
• Electron capture:
Inner-orbital electron
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• Yet another type of nuclear reaction is electron
capture
• It is very rare among natural isotopes but
common among synthetic radionuclides
• Vanadium-50 can capture K shell (principle
quantum number n = 1) or L shell (n= 2)
electrons and change to stable atoms of
titanium
V  10e  5022Ti  X rays  v
50
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Net change :
1
1
p  10e  01n
DECAY SERIES (SERIES OF ALPHA AND BETA
DECAYS)
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•
•
Nuclear equations are used to
symbolize the decay of a nucleus
• The decay of uranium-238 to
thorium-234 is: 238
234
4
U

U

92
90
2 He
Rules for balancing nuclear reactions
are different from those for chemical
reactions:
1) The sums of the mass numbers on
each side of the arrow are equal.
2) The sums of the atomic numbers on
each side of the arrow are equal.
• Another rule for nuclear stability is based on
magic numbers of nucleons
• Isotopes with specific numbers of protons or neutrons are
more stable than the rest
• The magic numbers are: 2, 8, 20, 28, 50, 82, and 126
• Magic numbers do not cancel the need for a favorable
neutron:proton ratio
• Magic numbers supports the hypothesis that a
nucleus has a shell structure with energy levels
like those for electrons
THE ZONE OF STABILITY
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• In nuclear science, a half-life is the period of
time it takes for a given sample of a radionuclide
to decay to one-half of its initial amount
• Radioactive decay is a first-order process so the
period of time taken by one half-life is
independent of the number of nuclei
• Half-lives vary greatly (see Table 19.3)
• For example the half-life of oxygen-15 is 124 s, radon-222 is
3.82 day, radium-226 is 1590 yr, and potassium-40 is 1.3x109
yr.
RATE OF DECAY
Rate = kN
• The rate of decay is proportional to the number of
nuclides. This represents a first-order process.
• Time required for the number of nuclides to reach
half the original value.
t1/ 2
ln  2  0.693
=
=
k
k
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CARBON–14 DATING
• Used to date wood and cloth artifacts.
• Based on carbon–14 to carbon–12 ratio.
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• The conversion of one isotope into another is
called transmutation
• Transmutation can be caused by radioactive
decay or by the bombardment of nuclei with
high-energy particles such as:
• Alpha particle (from natural emitters)
• Neutrons from atomic reactors
• Protons made by stripping electrons from hydrogen
• Beta particles (electrons) are not normally used
because they are repelled by the electrons around the
target nuclei
NUCLEAR FISSION AND FUSION
• Fusion – Combining two light nuclei to form a
heavier, more stable nucleus.
• Fission – Splitting a heavy nucleus into two nuclei
with smaller mass numbers.
1
0
n+
235
92
U
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142
56
Ba +
91
36
1
0
Kr + 3 n
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NUCLEAR FISSION
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NUCLEAR FUSION
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FISSION PROCESSES
• A self-sustaining fission process is called a chain
reaction.
Neutrons
Causing
Fission
Event
Event
subcritical
<1
critical
=1
supercritical
>1
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Result
reaction stops
sustained reaction
violent explosion
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