ch. 19: radioactivity and nuclear chemistry dr. namphol sinkaset chem 201: general chemistry ii
TRANSCRIPT
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Ch. 19: Radioactivity and Ch. 19: Radioactivity and Nuclear ChemistryNuclear Chemistry
Dr. Namphol Sinkaset
Chem 201: General Chemistry II
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I. Chapter OutlineI. Chapter Outline
I. Introduction
II. Types of Radioactivity
III. The Valley of Stability
IV. Radiometric Dating
V. Nuclear Fission
VI. Nuclear Fusion
VII. Radiation and Life
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I. IntroductionI. Introduction
• Antoine-Henri Becquerel discovered radioactivity when he placed some rock crystals on a photographic plate.
• He called the rays that were emitted uranic rays because they came from uranium in the crystals.
• Marie Curie changed the name to radioactivity when she discovered polonium and radium.
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II. Types of RadioactivityII. Types of Radioactivity
• Ernest Rutherford and others worked on figuring out what radioactivity was.
• Discovered that radioactive emissions were produced from unstable nuclei.
• Several types of radioactivity alpha (α) decay beta (β) decay gamma () ray emission positron emission electron capture
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II. Review of Atomic SymbolsII. Review of Atomic Symbols
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II. Subatomic ParticlesII. Subatomic Particles
• The term nuclide is used to refer to a particular isotope of an element.
• Each nuclide is composed of subatomic particles.
• Each subatomic particle has its own representation in nuclear chemistry.
p11
n01 e-1
0
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II. Shedding HeliumII. Shedding Helium
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II. Nuclear EquationsII. Nuclear Equations
• In a nuclear reaction, elements change their identity.
• Nuclear equations are balanced by ensuring the sum of mass numbers and the sum of atomic numbers on both sides are equal.
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II. α Partcles – Dangerous?II. α Partcles – Dangerous?
• Alpha particles are the most massive particles emitted by nuclei.
• They have the potential to interact with and damage other molecules.
• Alpha radiation has the highest ionizing power, but it has the lowest penetrating power.
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II. Emitting an ElectronII. Emitting an Electron
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II. Dangers of Beta ParticlesII. Dangers of Beta Particles
• Beta particles are less massive than alpha particles, so they have less ionizing power.
• However, they have greater penetrating power. Sheet of metal or thick block of wood needed to stop them.
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II. Gamma Ray EmissionII. Gamma Ray Emission
• This type of radiation involves emission of high-energy photons, not particles.
• Gamma rays have no mass and no charge as they are a type of EM radiation.
• Gamma rays can be emitted along with other types of radiation.
• Gamma rays have low ionizing power, but very high penetrating power.
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II. Antiparticles of Electrons!!II. Antiparticles of Electrons!!
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II. Electron CaptureII. Electron Capture
• Instead of emitting particles, a nucleus can pull in an e- from an inner orbital.
• When an e- combines with a proton in the nucleus, a neutron is formed. proton + electron neutron
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II. Radioactive Decay SummaryII. Radioactive Decay Summary
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II. Sample ProblemsII. Sample Problems
a) Write a nuclear equation for the positron emission of sodium-22.
b) Write a nuclear equation for electron capture in krypton-76.
c) Potassium-40 decays into argon-40. Identify the type of radioactive decay.
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III. Why Is There Radioactivity?III. Why Is There Radioactivity?
• When a nuclide undergoes radioactive decay, it becomes more stable.
• The strong force binds protons and neutrons together, but it only works at very short distances.
• Stability of nucleus is a balance between +/+ repulsions and the strong force attraction.
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III. Importance of NeutronsIII. Importance of Neutrons
• Neutrons are key to nuclei stability because they increase strong force attractions, but lack charge repulsion.
• However, since neutrons occupy energy levels like e-, cannot just stuff nucleus with neutrons.
• Nuclear stability is indicated by the ratio of neutrons to protons (N/Z).
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III. The Valley of StabilityIII. The Valley of Stability
• For lighter elements, N/Z for stable isotopes is about 1.
• For Z > 20, stability requires higher N/Z.
• No stable isotopes above Z = 83.
• Thus, nuclides decay to get back to the valley of stability.
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III. Magic NumbersIII. Magic Numbers• Nucleons occupy energy levels in the nucleus,
so certain numbers of nucleons are stable.• N or Z = 2, 8, 20, 28, 50, 82, and N = 126 are
uniquely stable and are called magic numbers.
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III. Journey to Valley of StabilityIII. Journey to Valley of Stability
• Atoms w/ Z > 83 undergo decay in one or more steps to become stable.
• The successive decays to become stable are known as a decay series.
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IV. Radioactivity is EverywhereIV. Radioactivity is Everywhere
• Everything around us contains at least some nuclides which are radioactive.
• Radioactivity is found in the ground, in our food, in our air.
• Radioactivity is in our environment because of some long decay times, and the constant production of radioactive nuclides through various decay series.
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IV. Radioactivity is 1IV. Radioactivity is 1stst Order Order
• All radioactive nuclides follow 1st order kinetics.
• Thus, ln Nt/N0 = -kt.• Since decay is 1st
order, half lives are independent of initial concentration.
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IV. Sample ProblemIV. Sample Problem
• How long would it take for a 1.35-mg sample of Pu-236 to decay to 0.100 mg?
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IV. Rate of Decay and Amount IV. Rate of Decay and Amount are Interchangeableare Interchangeable
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IV. Radiocarbon DatingIV. Radiocarbon Dating
• Radioactive C-14 is continuously taken up by living organisms, so the amount is in equilibrium with the amount in the atmosphere.
• When the organism dies, it no longer takes in C-14. The C-14 continuously decays in the remains.
• Age can be determined by comparing rate of decay in remains to rate of decay in atmosphere.
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IV. Sample ProblemIV. Sample Problem
• An ancient scroll is claimed to have originated from Greek scholars in about 500 B.C. A measure of its C-14 decay rate gives a value that is 89% of that found in living organisms. How old is the scroll? Could it be authentic?
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V. Making New ElementsV. Making New Elements
• Enrico Fermi attempted to synthesize a new element by bombarding U-238 with neutrons.
• He detected beta particles, but never confirmed the chemical products.
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V. Nuclear FissionV. Nuclear Fission
• Meitner, Strassmann, and Hahn repeated Fermi’s experiment.
• They discovered that elements lighter than uranium were produced w/ a lot of energy.
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V. Nuclear Chain ReactionV. Nuclear Chain Reaction
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V. Source of Energy in FissionV. Source of Energy in Fission
• U-235 + n Ba-140 + Kr-93 + 3n
• If we look at exact masses, we find that mass of products is 235.86769 amu and mass of reactants is 236.05258 amu.
• Mass is not conserved!!
• In nuclear reactions, mass can be converted into energy via E = mc2.
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V. The Mass DefectV. The Mass Defect
• All stable nuclei have masses less than their components which is known as the mass defect.
• When the mass defect is used in E = mc2, the nuclear binding energy is calculated.
• The nuclear binding energy is the energy needed to break up a nucleus into its component nucleons.
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V. Calculating Binding EnergiesV. Calculating Binding Energies
• A useful conversion between mass and energy is 1 amu = 931.5 MeV. Note that 1 MeV = 1.602 x 10-13 J.
• The mass defect of a helium nucleus is 0.03038 amu, so its binding energy is 28.30 MeV.
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V. Comparing Nuclei StabilityV. Comparing Nuclei Stability
• In order to see which nuclei are more stable than others, the binding energy per nucleon is calculated.
• This is simply the binding energy divided by the number of nucleons in the nuclide.
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VI. Nuclear FusionVI. Nuclear Fusion
• Smaller nuclides can combine into more stable nuclides in a process called fusion.
• Fusion is the energy source of the sun and used in hydrogen bombs.
• High temps are needed to overcome the +/+ repulsions.
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VII. Radiation RisksVII. Radiation Risks
• There are 3 categories of radiation effects. Acute radiation damage: large amounts of
radiation in short time. Immune and intestinal cells most susceptible.
Increased cancer risk: low dose over time. Damage occurs to DNA.
Genetic defects: high radiation exposure to reproductive cells causing problems in offspring. Not seen in humans, even Hiroshima survivors.
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VII. Measuring ExposureVII. Measuring Exposure
• There are several ways to measure exposure to radiation. curie (Ci): exposure to 3.7 x 1010 decay
events per second. gray (Gy): exposure to 1 J/kg body tissue.
Also have the rad (radiation absorbed dose) which is 0.01 J/kg body tissue.
rem (roentgen equiv. man): multiplication of rads by the biological effectiveness factor, which depends on the type of radiation.
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VII. Sources of RadiationVII. Sources of Radiation
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VII. Results of Radiation VII. Results of Radiation ExposureExposure
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VII. Applications of RadioactivityVII. Applications of Radioactivity
• Medicine Use of radiotracers to track movement of
compound or mixture in body. I-131 for thyroid, labeled antibodies to locate infection, P-32 for cancer.
Gamma rays to kill cancer cells.
• Kill microorganisms Sterilize medical devices. Kill bacteria and parasites in food.
• Sterilize harmful insects