Jeffrey Mack California State University, Sacramento Chapter 23 Nuclear Chemistry.
Post on 30-Mar-2015
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Jeffrey Mack California State University, Sacramento Chapter 23 Nuclear Chemistry Slide 2 Images of a human heart before and after stress detecting gamma rays from radioactive Tc-99m Nuclear Chemistry Slide 3 ProtonsProtons (+1) electrical charge mass = 1.672623 10 24 g mass = 1.007 atomic mass units (amu) ElectronsElectrons negative electrical charge relative mass = 0.0005 amu NeutronsNeutrons no electrical charge mass = 1.009 amu Atomic Composition Slide 4 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: 10 B 11 B Isotopes Slide 5 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 Slide 6 -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 Slide 7 Penetrating Ability Slide 8 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 Slide 9 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 Slide 10 Beta emission In emission, the mass number (A) remains unchanged and the atomic number (Z) decreases by 1. Nuclear Reactions Slide 11 Radioactive Decay Series Slide 12 Positron (positive electron) emission 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 Slide 13 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 Slide 14 Hydrogen: 1 1 H, protium 2 1 H, deuterium 3 1 H, tritium (radioactive) Helium, 4 2 He Lithium, 6 3 Li and 7 3 Li Boron, 10 5 B and 11 5 B Iron 54 26 Fe, 5.82% abundant 56 26 Fe, 91.66% abundant 57 26 Fe, 2.19% abundant 58 26 Fe, 0.33% abundantIsotopes Slide 15 209 Bi 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 Slide 16 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 Slide 17 Out of > 300 stable isotopes:Even Odd Odd Even Z N 15752 505 31 15 P 19 9 F 2 1 H, 6 3 Li, 10 5 B, 14 7 N, 180 73 Ta Stability of Nuclei Slide 18 The trend suggests some PAIRING of NUCLEONS There are nuclear magic numbers 2 He28Ni 8 O50Sn 20 Ca82 Pb Even Odd Odd Even Z N 15752 505 Stability of Nuclei Slide 19 Band of Stability and Radioactive Decay Isotopes with low n/p ratio, below band of stability decay, decay by positron emission or electron capture Slide 20 The energy required to separate the nucleus of an atom into protons and neutrons. For deuterium, 2 1 H 2 1 H 1 1 p + 1 0 nE b = 2.15 10 8 kJ/mol E b per nucleon = E b /2 nucleons = 1.08 10 8 kJ/mol nucleons Binding Energy, E b Slide 21 For deuterium, 2 1 H: 2 1 H 1 1 p + 1 0 n Mass of 2 1 H: = 2.01410 g/mol Mass of proton: = 1.007825 g/mol Mass of neutron:= 1.008665 g/mol m:= 0.00239 g/mol From Einsteins equation: E b = (m)c 2 = 2.15 x 10 8 kJ/mol E b per nucleon = E b /2 nucleons = 1.08 10 8 kJ/mol nucleons For deuterium, 2 1 H: 2 1 H 1 1 p + 1 0 n Mass of 2 1 H: = 2.01410 g/mol Mass of proton: = 1.007825 g/mol Mass of neutron:= 1.008665 g/mol m:= 0.00239 g/mol From Einsteins equation: E b = (m)c 2 = 2.15 x 10 8 kJ/mol E b per nucleon = E b /2 nucleons = 1.08 10 8 kJ/mol nucleons Calculate Binding Energy Slide 22 Binding Energy/Nucleon Slide 23 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 Slide 24 After each successive half-life, one half of the original amount remains. Half-Life Slide 25 Activity (A) = Disintegrations/time = (k)(N) where N is the number of atoms Decay follows first order kinetics: The half-life of radioactive decay ist 1/2 = 0.693/k Kinetics of Radioactive Decay Slide 26 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 Slide 27 Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation: 14 N + 1 0 n 14 C + 1 H The C-14 is oxidized to CO 2, which circulates through the biosphere. When a plant dies, the C-14 is not replenished. But the C-14 continues to decay with t 1/2 = 5730 years. Activity of a sample can be used to date the sample. Radiocarbon Dating Slide 28 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 4 He and 11 B. 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 Slide 29 An Example of a n, reaction is production of radioactive 32 P. 32 P is used in studies of phosphorous uptake in the body. Artificial Nuclear Reactions Slide 30 Elements beyond 92 (transuranium) are made via n, reactions. Transuranium Elements Slide 31 106 Sg Transuranium Elements & Glenn Seaborg Slide 32 Nuclear Fission Slide 33 Fission chain reaction has three general steps: Initiation: Reaction of a single atom starts the chain (e.g., 235 U + neutron) Propagation: 236 U fission releases neutrons that initiate other fissions Termination. Consumption of the fissionable material is completed Nuclear Fission Slide 34 109 Mt Nuclear Fission & Lise Meitner Slide 35 Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. 17% of the worlds energy comes from nuclear. Nuclear Fission & Power Slide 36 Curie: 1 Ci = 3.7 10 10 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 10 10 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 Slide 37 Effects of Radiation Slide 38 Slide 39 Nuclear Medicine: Imaging Slide 40 Technetium-99m is used in more than 85% of the diagnostic scans done in hospitals each year. Synthesized on-site from Mo-99. 99m 43 Tc decays to 99 43 Tc 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 Slide 41 Imaging of a heart using Tc-99m before and after exercise. Nuclear Medicine: Imaging Slide 42 10 B isotope (not 11 B) 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 10 B 7 Li neutron capture reaction (that produces a photon and an alpha particle) 10 B + 1 n 7 Li + 4 He + photon BNCT Boron Neutron Capture Therapy Slide 43 Food can be irradiated with rays from 60 Co or 137 Cs. 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