Transcript Introduction to Nuclear Chemistry

Introduction to
Nuclear Chemistry
The Nucleus
• Remember that the nucleus is comprised of the
two nucleons, protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together is
effectively the mass of the atom.
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Atomic Composition
• Protons
– (+1) electrical charge
– mass = 1.672623  1024 g
– mass = 1.007 atomic mass units (amu)
• Electrons
– negative electrical charge
– relative mass = 0.0005 amu
• Neutrons
– no electrical charge
– mass = 1.009 amu
Isotopes
• Isotopes are atoms of the same element (same Z)
but different mass numbers (A).
10
• Boron-10:
5 protons and 5 neutrons:
5B
• Boron-11:
5 protons and 6 neutrons: 115B
11B
10B
Radioactive Isotopes
• Isotopes of certain unstable elements that
spontaneously emit particles and energy from
the nucleus.
• Henri Beckerel 1896 accidentally observed
radioactivity of uranium salts that were fogging
photographic film.
• His associates were Marie and Pierre Curie.
Radioactivity
Marie and Pierre Curie
• The isolation and
characterization of radium &
polonium by Marie Curie was
one of milestones of
chemistry.
• It is a credit to her skills as a
chemist that she was able to
isolate only a single gram of
radium from 7 tons of
uranium ore.
Radioactivity
• It is not uncommon for some nuclides of an
element to be unstable, or radioactive.
• We refer to these as radionuclides.
• There are several ways radionuclides can
decay into a different nuclide.
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Types of
Radioactive Decay
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Natural Radioactivity
• -particles can be stopped by paper.
• -particles require at least a cm of lead (Pb).
• -particles require at least 10 cm of lead (Pb).
Energy:  >  > 
Penetrating Ability
Nuclear Reactions
Alpha emission
• Nucleons must be conserved in any nuclear reaction.
• In  emission, the mass number (A) decreases by 4 and
the atomic number (Z) decreases by 2.
Alpha Decay
Alpha decay is the loss of an -particle.
(a helium nucleus) 4
2
238
92
U

He
234
90Th
+
4
2
He
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Nuclear Reactions
Beta emission
In  emission, the mass number (A) remains unchanged
and the atomic number (Z) increases by 1.
1
1
n
neutron
®
0
-1
e
+
electron
1
1
p
pronton
Beta Decay
Beta decay is the loss of a -particle (a high
energy electron).
0
−1
131
53
I

or
131

54
0
−1
e
Xe
+
0
−1
e
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Gamma Emission
• This is the loss of a -ray, which is high-energy
electromagnetic radiation that almost always
accompanies the loss of a nuclear particle.
• -ray have no mass and no charge.
0
0

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Positron Emission
Some nuclei decay by emitting a positron, a
particle that has the same mass as but an
opposite charge to that of an electron.
0
1
11
6
C

e
11
5
B
+
0
1
e
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Electron Capture (K-Capture)
Addition of an electron to a proton in the
nucleus is known as electron capture or Kcapture.
– The result of this process is that a proton is
transformed into a neutron.
1
1
p
+
0
−1
e
1

0
n
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Field Effect
• It is useful to understand how the three main types of
radiation are effects when they are passed through an
electric field and a magnetic field.
– The Beta particle’s flight is skewed the most, as it is the lightest.
– The alpha’s path is also altered, but due to it being about 7200 times
more massive its path is not altered as much as beta’s.
– The gammas path is not altered at all, why? It has not charge, and
therefore is not effected by either a magnetic field or electric field.
Radioactive Decay Reactions
Band of Stability
And
Nuclear Decay
Number of Neutrons, (N)
Number of Protons (Z)
Neutron-Proton Ratios
• Any element with more than one
proton (i.e., anything but
hydrogen) will have repulsions
between the protons in the
nucleus.
• A strong nuclear force helps keep
the nucleus from flying apart.
• Neutrons play a key role
stabilizing the nucleus.
• Therefore, the ratio of neutrons
to protons is an important factor.
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Neutron-Proton Ratios
• Neutrons play a key role
stabilizing the nucleus.
• Therefore, the ratio of
neutrons to protons is an
important factor.
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Band of Stability and
Radioactive Decay
a emission reduces Z
243
95
Am ® 24a +
239
93
Np
b emission increases Z
60
27
Co ®
0
-1
b +
60
28
Ni
Isotopes with low n/p
ratio, below band of
stability decay, decay by
positron emission or
electron capture
Radioactive Series
• Large radioactive nuclei
cannot stabilize by
undergoing only one
nuclear transformation.
• They undergo a series of
decays until they form a
stable nuclide (often a
nuclide of lead).
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Half-Life
• The HALF-LIFE of an isotope is the time it takes
for 1/2 a sample to decay from its initial amount.
• The rate of a nuclear transformation depends only
on the “reactant” concentration.
• The decay and half-life for a nuclear reaction
follows first order kinetics.
Half-Life
After each successive half-life, one half of the
original amount remains.
Kinetics of Radioactive Decay
Activity (A) = Disintegrations/time = (k)(N)
where N is the number of atoms
Decay follows first order kinetics:
æAö
ln ç ÷ = -kt
è Ao ø
The half-life of radioactive
decay is t1/2 = 0.693/k
Common Radioactive Isotopes
Isotope
Half-Life
Radiation
Emitted
Carbon-14
5,730 years
, 
Radon-222
3.8 days

Uranium-235
7.0 x 108 years
, 
Uranium-238
4.46 x 109 years

Applications of Nuclear Chemistry
Radiocarbon Dating
Willard Libby (1908-1980)
Libby received the 1960 Nobel
Prize in chemistry for developing
carbon-14 dating techniques. He
is shown here with the
apparatus he used. Carbon-14
dating is widely used in fields
such as anthropology and
archeology.
Artificial Nuclear Reactions
• New elements or new isotopes of known
elements are produced by bombarding an atom
with subatomic particles such as a protons or
neutrons, or even a heavier particles such as 4He
and 11B.
• Reactions using neutrons are called
n, reactions because a -ray is usually emitted.
• Radioisotopes used in medicine are often made
by n, reactions.
Transuranium Elements & Glenn Seaborg
106Sg
Nuclear Fission
Nuclear Fission
Fission chain reaction has three general steps:
1. Initiation:
Reaction of a single atom starts the chain (e.g., 235U +
neutron)
2. Propagation:
236U
fission releases neutrons that initiate other fissions
3. Termination.
Consumption of the fissionable material is completed
Nuclear Fission
• How does one tap all that energy?
• Nuclear fission is the type of reaction carried out
in nuclear reactors.
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Nuclear Fission
• Bombardment of the radioactive nuclide with a
neutron starts the process.
• Neutrons released in the transmutation strike
other nuclei, causing their decay and the
production of more neutrons.
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Nuclear Fission & Lise Meitner
109Mt
Nuclear Fission & Power
• Currently about 103 nuclear power plants in the U.S. and
about 435 worldwide.
• 17% of the world’s energy comes from nuclear.
Nuclear Medicine: Imaging
Nuclear Medicine: Imaging
• Technetium-99m is used in more than 85% of the
diagnostic scans done in hospitals each year. Synthesized
on-site from Mo-99.
99
42
Mo ®
Tc + b
99m
43
0
-1
• 99m43Tc decays to 9943Tc giving off a  -ray.
• The half-life of the radioisotope is 6.01 hrs.
• Once ingested, the Tc-99m concentrates in areas of high
activity such as the thyroid.  -ray imagining detects its
presence.
Food Irradiation
• Food can be irradiated with  rays from 60Co or 137Cs.
• Irradiation retards the growth of bacteria, molds and
yeasts.
• Irradiated milk has a shelf life of 3 mo. without
refrigeration.
• USDA has approved irradiation of meats and eggs.