jeffrey mack california state university, sacramento chapter 23 nuclear chemistry
TRANSCRIPT
Jeffrey MackCalifornia State University,
Sacramento
Chapter 23
Nuclear Chemistry
Images of a human heart before and after stress detecting gamma rays from radioactive Tc-99m
Nuclear Chemistry
• Protons– (+1) electrical charge– mass = 1.672623 1024 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
Atomic Composition
• Isotopes are atoms of the same element (same Z) but different mass numbers (A).
• Boron-10:5 protons and 5 neutrons: • Boron-11: 5 protons and 6 neutrons:
10B
11B
Isotopes
• 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.
Marie and Pierre Curie
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: > >
Natural Radioactivity
Penetrating Ability
• Ernest Rutherford isolated Radium forms Radon gas while studying alpha particle emission.
• 1902 Rutherford and Soddy proposed radioactivity was the result of the natural change of the isotope of one element into an isotope of a different element.
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.
Nuclear Reactions
Beta emission
In emission, the mass number (A) remains unchanged and the atomic number (Z) decreases by 1.
Nuclear Reactions
Radioactive Decay Series
Positron (positive electron) emission
207 207
• Positrons have the mass of an electron, but positive charge. They are the antimatter analog of an electron.
• Positron emission arises from “electron capture”.• An inner shell electron is absorbed by the nucleolus
converting a proton into a neutron along with an emitted positron.
Other Types of Nuclear Reactions
• H is most abundant element in the universe.• H represents 88.6% of all atoms• He represents 11.3% of all atoms• Together 99.9% of all atom & 99% of mass of
the universe.
Stability of Nuclei
• Hydrogen: – 1
1H, protium
– 21H, deuterium
– 31H, tritium
(radioactive)• Helium, 4
2He
• Lithium, 63Li and 7
3Li
• Boron, 105B and 11
5B
• Iron– 54
26Fe, 5.82% abundant
– 5626Fe, 91.66%
abundant– 57
26Fe, 2.19% abundant
– 5826Fe, 0.33%
abundant
Isotopes
• 209Bi with 83 protons and 126 neutrons is the heaviest naturally occurring non-radioactive isotope.
• There are 83 x 126 = 10,458 possible isotopes.
• Why do so few exist in nature?
Stability of Nuclei
• Up to Z = 20 (Ca) stable isotopes often have the same # of neutrons and protons. Only H and He-3 have more protons than neutrons.
• Beyond Ca, the ratio of neutrons to protons is >1.• As Z increases, the n:p ratio deviates further from 1:1• Above Bi all isotopes are radioactive. Fission leads to
smaller particles, the heavier the nucleus the greater the rate.
• Above Ca: elements of EVEN Z have more stable isotopes than ODD Z elements.
• The more stable isotopes have an EVEN number of neutrons.
Stability of Nuclei
Out of > 300 stable isotopes:
Even Odd
Odd
Even
ZN
157 52
50 5
3115P
199F
21H, 6
3Li, 105B, 14
7N, 18073Ta
Stability of Nuclei
• The trend suggests some PAIRING of NUCLEONS
• There are “nuclear magic numbers”2 He 28 Ni8 O 50 Sn20 Ca 82 Pb
Even Odd
Odd
Even
ZN
157 52
50 5
Stability of Nuclei
Band of Stability and Radioactive Decay
Isotopes with low n/p ratio, below band of stability decay, decay by positron emission or electron capture
• The energy required to separate the nucleus of an atom into protons and neutrons.
• For deuterium, 21H
21H 1
1p + 10n Eb = 2.15 108 kJ/mol
• Eb per nucleon = Eb/2 nucleons
= 1.08 108 kJ/mol nucleons
Binding Energy, Eb
For deuterium, 21H: 2
1H 11p + 1
0n
Mass of 21H: = 2.01410 g/mol
Mass of proton: = 1.007825 g/mol
Mass of neutron: = 1.008665 g/mol
∆m: = 0.00239 g/mol
From Einstein’s equation: Eb = (∆m)c2
= 2.15 x 108 kJ/mol
Eb per nucleon = Eb/2 nucleons
= 1.08 108 kJ/mol nucleons
Calculate Binding Energy
Binding Energy/Nucleon
• 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.
Half-Life
Activity (A) = Disintegrations/time = (k)(N) where N is the number of atomsDecay follows first order kinetics:
The half-life of radioactive decay is t1/2 = 0.693/k
Kinetics of Radioactive Decay
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.
Radiocarbon Dating
Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation:14N + 1
0n 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.
Radiocarbon Dating
• 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.
Artificial Nuclear Reactions
• An Example of a n, reaction is production of radioactive 32P.
• 32P is used in studies of phosphorous uptake in the body.
Artificial Nuclear Reactions
Elements beyond 92 (transuranium) are made via n, reactions.
Transuranium Elements
106Sg
Transuranium Elements & Glenn Seaborg
Nuclear Fission
Fission chain reaction has three general steps:
Initiation:
Reaction of a single atom starts the chain (e.g., 235U + neutron)
Propagation:236U fission releases neutrons that initiate other fissions
Termination.
Consumption of the fissionable material is completed
Nuclear Fission
109Mt
Nuclear Fission & Lise Meitner
• Currently about 103 nuclear power plants in the U.S. and about 435 worldwide.
• 17% of the world’s energy comes from nuclear.
Nuclear Fission & Power
• Curie: 1 Ci = 3.7 1010 distintegrations/s (dps)
• SI unit is the becquerel: 1 Bq = 1 dps• Rad: measures amount of energy absorbed
1 rad = 0.01 J absorbed/kg tissue• Rem: “roentgen equivalent man” based on
amount and type of radiation. • Quantifies biological tissue damage, usually
represented “millirems”.
• Curie: 1 Ci = 3.7 1010 distintegrations/s (dps)
• SI unit is the becquerel: 1 Bq = 1 dps• Rad: measures amount of energy absorbed
1 rad = 0.01 J absorbed/kg tissue• Rem: “roentgen equivalent man” based on
amount and type of radiation. • Quantifies biological tissue damage, usually
represented “millirems”.
Units for Measuring Radiation
Effects of Radiation
Effects of Radiation
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.
99m43Tc decays to 99
43Tc 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.
Nuclear Medicine: Imaging
Imaging of a heart using Tc-99m before and after exercise.
Nuclear Medicine: Imaging
• 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
BNCTBoron Neutron Capture Therapy
• 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.
Food Irradiation