Nuclear Chemistry

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Nuclear Chemistry
Pictures of human heart before and after stress
using gamma rays from radioactive Tc-99m
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ATOMIC COMPOSITION

• Protons
• electrical charge
• mass 1.672623 x 10-24 g
• relative mass 1.007 atomic mass units (amu)
• Electrons
• negative electrical charge
• relative mass 0.0005 amu
• Neutrons
• no electrical charge
• mass 1.009 amu

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Isotopes

• One of two or more atoms of the same element
  (having same atomic , Z) but different mass
  number (A) diff of neutrons.
• Boron-10 (10B) has 5 p and 5 n 105B
• Boron-11 (11B) has 5 p and 6 n 115B

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Radioactivity

• One of the pieces of evidence for the fact that
  atoms are made of smaller particles came from the
  work of Marie Curie (1876-1934).
• Radioactivity, the spontaneous disintegration of
  some elements into smaller pieces, accompanied by
  the emission of ?, and ?-particles, and
  ?-radiation.

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Types of Radiation
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Penetrating Ability
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Which symbol represents a beta particle?
a) 01e b) 0-1e c) 10n d) 00? e)
42He
Answer b
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Nuclear Reactions

• Ernest Rutherford found Ra forms Rn gas when
  emitting an alpha particle.
• 1902Rutherford and Soddy proposed radioactivity
  is the result of the natural change of the
  isotope of one element into an isotope of a
  different element.

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Nuclear Reactions

• Alpha emission

Note that mass number (A) goes down by 4 and
atomic number (Z) goes down by 2.
The nucleons (nuclear particles protons
neutrons) are rearranged but conserved.
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Nuclear Reactions

• Beta emission

Note that mass number (A) is unchanged and atomic
number (Z) goes up by 1.
How does this happen?
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Radioactive Decay Series
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Other Types of Nuclear Reactions

• Positron (01b) emission a positive electron

K-capture the capture of an electron from the
first or K shell
An electron and proton combine to form a
neutron. 0-1e 11p ? 10n
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Stability of Nuclei
CD-ROM Screens 23.3 and 23.4
- stable isotopes fall in a very narrow range
called the band of stability.
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Isotopes

• Except for 11H the mass number is always at least
  2 ? atomic number.
• Repulsive forces between protons must be
  moderated by neutrons.

• Hydrogen
• 11H, protium
• 21H, deuterium
• 31H, tritium (radioactive)
• Helium, 42He
• Lithium, 63Li and 73Li
• Boron, 105B and 115B

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Stability of Nuclei

• Heaviest naturally occurring non-radioactive
  isotope is 209Bi with 83 protons and 126 neutrons
• There are 83 x 126 10,458 possible isotopes.
  Why so few actually exist?

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Stability of Nuclei

• Up to Z 20 (Ca), n p (except for 73Li,
  115B, 199F)
• Beyond Ca, n gt p (A gt 2 Z)
• Above Bi all elements are radioactive
  (unstable). Fission leads to smaller particles,
  the heavier the nucleus the greater the rate.
• Above Ca elements of EVEN Z (protons) have
  more isotopes and most stable isotope has EVEN N
  (neutrons).

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Stability of Nuclei

• Out of gt 300 stable isotopes

Most of the stable isotopes have an even atomic
number (Z).
More stable isotopes have an even number of
neutrons (N) than have an odd number.
Only 5 stable isotopes have odd numbers of
protons and neutrons.
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Stability of Nuclei

• Suggests some PAIRING of NUCLEONS
• There are nuclear magic numbers
• 2 He 28 Ni
• 8 O 50 Sn
• 20 Ca 82 Pb

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Stability of Nuclei
Unstable nuclei decay in a manner that bring them
to a stable n-p ratio (towards the band of
stability).
Predict type of radioactive decay observed
3 types.
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Band of Stability and Radioactive Decay (p.1118)
Elements beyond Bi are are unstable. A process
is needed to reduce Z
Nuclei above the band of stability are neutron
rich. b emission increases Z to give a lower n/p
ratio.
6027Co ? 0-1b 6028Ni
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Band of Stability
Isotopes with low n/p ratio (proton rich), below
band of stability, decay by positron emission or
electron capture
137N ? 01b 136C
4120Ca 0-1e ? 4119K
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If californium-251 decays by successive a,
a, ß emissions, what nucleus is produced?
a) b) c)
d)
e)  
Answer a
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Binding Energy, Eb
Eb is the energy required to separate the nucleus
of an atom into protons and neutrons. For
stability, Eb gt electrostatic repulsive forces
between protons. For deuterium, 21H 21H ? 11p
10n Eb 2.15 ? 108 kJ/mol Eb per nucleon Eb/2
nucleons 1.08 ? 108 kJ/mol nucleons
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Calculate Binding Energy
For deuterium, 21H 21H ? 11p 10n Mass
of 21H 2.01410 g/mol Mass of proton 1.007825
g/mol Mass of neutron 1.008665 g/mol ?m mass
defect 2.01649 2.01410 0.00239 g/mol From
Einsteins equation Eb (?m)c2 2.15 x 108
kJ/mol Eb per nucleon Eb/2 nucleons 1.08 x
108 kJ/mol nucleons
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Binding Energy/Nucleon
The greater the binding energy/nucleon, the
greater the stability of the nucleus.
ChemNow Screen 23.5 Fig. 23.4
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Half-LifeSection 15.4 Screen 23.6

• HALF-LIFE is the time it takes for ½ the sample
  to disappear.
• The rate of a nuclear transformation depends only
  on the reactant concentration.
• Concept of HALF-LIFE is especially useful for 1st
  order reactions (Ch. 15).

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Half-Life
After 3 half lives 2.50 mg 15O
After 5 half lives 0.625 mg 15O
Decay of 20.0 mg of 15O. What remains after 3
half-lives? After 5 half-lives?
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Kinetics of Radioactive Decay

• Activity (A) Disintegrations/time (k)(N)
• where N is the number of atoms k is the
  rate constant (decay constant).
• Decay is first order, and so
• ln (A/Ao) -kt
• The half-life of radioactive decay is
• t1/2 0.693/k

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Section 15.4 pg. 719
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Using the Integrated Rate Law

• All 1st order reactions have a straight line plot
  for ln A vs. time.

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Q.35. The half-life of 131I is 8.04 days. How
much time is required for the activity to
decrease to 35 of its original value?
Solution
Substitute into
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Q.35. The half-life of 131I is 8.04 days. How
much time is required for the activity to
decrease to 35 of its original value?
Rate constant k 0.0862 d-1
Fraction remaining
Substitute into
t 12.2 days
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Radiocarbon Dating

• Radioactive C-14 is formed in the upper
  atmosphere by nuclear reactions initiated by
  neutrons in cosmic radiation
• 14N 1on ? 14C 1H
• The C-14 is oxidized to CO2, which circulates
  through the biosphere.
• When a plant dies, the C-14 is not replenished.
• But the C-14 continues to decay with t1/2 5730
  years.
• Activity of a sample can be used to date the
  sample.

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Radiocarbon Dating
Willard Libby (1908-1980) received the 1960 Nobel
Prize in chemistry for developing C-14 dating
techniques.
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Artificial Nuclear Reactions

• New elements or new isotopes of known elements
  are produced by bombarding an atom with a
  subatomic particle such as a proton or neutron --
  or even a much heavier particle such as 4He and
  11B.
• Reactions using neutrons are called n,g
  reactions because a g ray is usually emitted.
• Radioisotopes used in medicine are often made by
  n,g reactions.

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Artificial Nuclear Reactions

• An example of a n,g reaction is production of
  radioactive 31P for use in studies of P uptake in
  the body.
• 3115P 10n ? 3215P g

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Transuranium Elements

• Elements beyond 92 (transuranium) made starting
  with an n, g reaction
• 23892U 10n ? 23992U g
• 23992U ? 23993Np 0-1b
• 23993Np ? 23994Pu 0-1b

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Transuranium Elements Glenn Seaborg
synthesized 10 new transuranium elements Pu - Lr
Th,etc (Actinides) fit under Lanthanides
106Sg
1951 Nobel Prize in Chemistry
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Nuclear Fission
-the splitting of a heavy nucleus of an atom
into two or more fragments Pu, U Th!
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Nuclear Fission

• Fission chain rxn 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. 235U is used up, or 10n
  escape

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Nuclear Fission Lise Meitner
-first used and explained the term nuclear
fission.
109Mt
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Nuclear Fission POWER

• Currently about 103 nuclear power plants in the
  U.S. and about 435 worldwide.
• 17 of the worlds energy comes from nuclear.

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Units for Measuring Radiation

• Curie 1 Ci 3.7 ? 1010 distintegrations/s
• SI unit is the becquerel 1 Bq 1 dps
• Rad measures amount of energy absorbed
• 1 rad 0.01 J absorbed/kg tissue
• Rem based on rad and type of radiation.
  Quantifies biological tissue damage.
• Usually use millirem to quantify normal
  exposures.

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Effects of Radiation (rem)
0 25 no effects 500 Death
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Nuclear Medicine Imaging
Applications of Nuclear Chemistry
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Food Irradiation

• Food can be irradiated with g rays from 60Co or
  137Cs.
• Irradiated milk has a shelf life of 3 mo. without
  refrigeration.
• USDA has approved irradiation of meats and eggs.

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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. 9942Mo ? 99m43Tc
0-1b 99m43Tc decays to 9943Tc giving off g
ray.
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Nuclear Medicine Imaging
Imaging of a heart using Tc-99m before and after
exercise.
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BNCTBoron Neutron Capture Therapy

• 10B isotope (not 11B) has the ability to capture
  slow neutrons
• In BNCT, tumor cells preferentially take up a
  boron compound, and subsequent irradiation by
  slow neutrons kills the cells via the energetic
  10B ? 7Li neutron capture reaction (that
  produces a photon and an alpha particle)
• 10B 1n ? 7Li 4He photon