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Chapter 23 Nuclear Chemistry. Nuclear Chemistry. Images of a human heart before and after stress detecting gamma rays from radioactive Tc-99m . Atomic Composition. Protons (+1) electrical charge mass = 1.672623 10 24 g mass = 1.007 atomic mass units ( amu ) Electrons
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Nuclear Chemistry Images of a human heart before and after stress detecting gamma rays from radioactive Tc-99m
Atomic Composition • 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
11B 10B Isotopes • 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:
Radioactivity • 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
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: > >
Nuclear Reactions • 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.
207 207 Other Types of Nuclear Reactions Positron (positive electron) emission • 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.
Stability of Nuclei • 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.
Isotopes • Hydrogen: • 11H, protium • 21H, deuterium • 31H, tritium (radioactive) • Helium, 42He • Lithium, 63Li and 73Li • Boron, 105B and 115B • Iron • 5426Fe, 5.82% abundant • 5626Fe, 91.66% abundant • 5726Fe, 2.19% abundant • 5826Fe, 0.33% abundant
Stability of Nuclei • 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.
Out of > 300 stable isotopes: N Even Odd Z 157 52 3115P Even Odd 50 5 21H, 63Li, 105B, 147N, 18073Ta 199F Stability of Nuclei
N Even Odd Z 157 52 Even Odd 50 5 Stability of Nuclei • The trend suggests some PAIRING of NUCLEONS • There are “nuclear magic numbers” • 2 He 28 Ni • 8 O 50 Sn • 20 Ca 82 Pb
Band of Stability and Radioactive Decay Isotopes with low n/p ratio, below band of stability decay, decay by positron emission or electron capture
Binding Energy, Eb • The energy required to separate the nucleus of an atom into protons and neutrons. • For deuterium, 21H • 21H 11p + 10n Eb = 2.15 108 kJ/mol • Eb per nucleon = Eb/2 nucleons • = 1.08 108 kJ/mol nucleons
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: = 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
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: The half-life of radioactive decay is t1/2 = 0.693/k
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.
Radiocarbon Dating Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation: 14N + 10n 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.
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.
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.
Transuranium Elements Elements beyond 92 (transuranium) are made via n, reactions.
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 & Power • Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. • 17% of the world’s energy comes from nuclear.
Units for Measuring Radiation • Curie: 1 Ci = 3.7 1010distintegrations/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”.
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 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.
Nuclear Medicine: Imaging Imaging of a heart using Tc-99m before and after exercise.
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
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.