Ch. 20: Radioactivity and Nuclear...
Transcript of Ch. 20: Radioactivity and Nuclear...
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Ch. 20: Radioactivity and Nuclear Chemistry
Dr. Namphol Sinkaset Chem 201: General Chemistry II
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I. Chapter Outline
I. Introduction II. Types of Radioactivity III. The Valley of Stability IV. Radiometric Dating V. Nuclear Fission VI. Nuclear Fusion VII. Transmutation VIII. Radiation and Life
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I. Introduction
• Antoine-Henri Becquerel discovered radioactivity accidentally while studying x-rays and phosphorescence (the “glow” in “glow in the dark”).
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I. Introduction • Crystals of
potassium uranyl sulfate were used to try and prove that phosphorescence occurred with x-ray emission.
• His experiments involved sunlight, photographic plates, and a black cloth.
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I. Introduction • Becquerel concluded that the uranium
caused the exposure and called the emissions uranic rays.
• Marie Curie studied uranic rays for her doctoral thesis and discovered they weren’t unique to uranium.
• She discovered 2 new elements that had the same emissions and renamed the phenomenon radioactivity.
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I. Introduction • The Curies and
Becquerel won the Nobel Prize in physics for the discovery of radioactivity in 1903.
• Marie Curie also won the Nobel in chemistry in 1911 for discovering Ra and Po.
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II. 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 (γ) ray emission positron emission electron capture
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II. Review of Atomic Symbols
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II. 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.
p 1 1
n 0 1 e -1
0
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II. Shedding Helium
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II. 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 Particles – 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 Electron
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II. 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 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!!
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II. 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 Summary
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II. Sample Problems 20.1
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?
• 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 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 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. Sample Problem 20.2
• If a nuclide has an N/Z ratio that is too high, what nuclear process is most likely to occur?
• If a nuclide has an N/Z ratio that is too low, what nuclear process is most likely to occur?
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III. 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 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.
• Some steps involve gamma decay to remove extra energy.
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IV. Detecting Radioactivity
Film-badge dosimeter
Geiger-Müller counter
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IV. 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 1st 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 Problem 20.3
• How long would it take for a 1.35-mg sample of Pu-236 to decay to 0.100 mg if it has a half-life of 2.87 years?
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IV. Rate of Decay and Amount are Interchangeable
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IV. Radiocarbon Dating • Radioactive C-14 is continuously taken
up by living organisms, so the amount is in equilibrium with the amount in the atmosphere (created by neutron bombardment of N-14).
• 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 Problem 20.4
• 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? Note that the half-life for C-14 is 5730 years.
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IV. Uranium/Lead Dating
• C-14 dating is only good for things that are less than 50,000 years old.
• Can use other known radioactive decays to date older things.
• U-238 decays into Pb-206 with a half-life of 4.5 × 109 years.
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IV. Sample Problem 20.5
• A rock contains a Pb-206 to U-238 mass ratio of 0.145:1.00. Assuming that the rock did not contain any Pb-206 at the time of its formation, determine its age. Note that the half-life of U-238 is 4.5 × 109 years.
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V. 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 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 Reaction
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V. 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 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.
• Mass is converted to energy to hold nucleus together!
• The nuclear binding energy is the energy needed to break up a nucleus into its component nucleons.
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V. 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 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 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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VI. Tokamak Fusion Reactor
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VII. Making New Elements
• Why did the alchemists fail at turning lead into gold?*
• Changing one element into another is known as transmutation.
• Early work in transmutation involved bombarding nuclides w/ alpha particles.
• Al-27 + alpha particle P-30 + neutron
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VII. Linear Accelerators
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VII. Sample Problem 20.6
• Write a balanced nuclear reaction for the creation of element 107 and one neutron from the collision of bismuth-209 with chromium-54.
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VIII. 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 clearly seen in humans, even Hiroshima survivors.
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VIII. 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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VIII. Sources of Radiation
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VIII. Results of Radiation Exposure
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VIII. 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