1	Chapter 21 Nuclear Chemistry
2	Atomic Nucleus Very, very small Diameter on the order of a couple of fm wide (1 fm = 10^-15 m) 100,000 times smaller than the atom Contains practically all of the mass and all of the positive charge in the atom Protons and neutrons reside in the nucleus. Atomic number: number of protons Mass number: number of protons plus the number of neutrons.
3	Symbolic notation
4	Nuclear Radioactivity Discovered in 1896 by Henri Becquerel Uranium ores glowed in the dark. Radium and other elements could be isolated from uranium even after it had been purified. The “glowing” was a form of radiation emitted by the uranium. It is a property of the uranium atom. During emission of this radiation, the uranium changed its identity to another element. Radioactive atoms are unstable atoms which decompose into other atoms with emission of nuclear radioactivity. Referred to as radionuclides.
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6	Alpha (a) Decay Consisted of a high energy beam of ⁴He²⁺ ions. During alpha decay, the reactant nucleus, called the parent, emits an alpha particle and a product nucleus, called the daughter, which has a mass number 4 less than the parent and an atomic number 2 less than the parent.
7	Beta (b) Decay Consist of a beam of high energy electrons. During beta decay, the parent nucleus emits an electron resulting in a daughter nucleus with the same mass number but with an atomic number that is one more than the parent.
8	Positron Emission Positron Looks like an electron, has the same mass as an electron, but has a positive charge. The antimatter form of the electron. When it comes in contact with an electron, they annihilate each other producing 2 photons of light in the gamma (g) region.
9	Positron Emission During positron emission, the parent nucleus emits a positron resulting in a daughter with the same mass number but with its atomic number one less than the parent. (Previously called Beta plus decay)
10	Electron Capture During electron capture, the parent nucleus consumes one of its electrons resulting in a daughter that has the same mass number but with an atomic number one less than the parent nucleus. (Both positron emission and electron capture result in the same daughter nucleus)
11	Positron Emission vs. Electron Capture Some radionuclides are capable of doing both processes. Positron emission: 2 g rays emitted from annihilation of positron. Electron capture: A large amount of x-rays are emitted from the hole created by the consumed electron.
12	Gamma (g) decay The daughter nucleus produced in a nuclear decay is usually in a very high energy (or excited state). The nucleus relaxes to the ground state by emitting a photon of light in the gamma (g) region. No change in the identity or mass number of the nucleus during gamma decay.
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14	Modes of Decay Give the daughter nucleus and write the balanced nuclear reaction for the following nuclear decays: (a) Thorium-232 decaying by alpha emission; (b) Phosphorus-32 decaying by beta emission; (c) Fluorine-18 decaying by positron emission; (d) Indium-106 decaying by electron capture.
15	Nuclear Reactions Represent a nuclear reaction Identity of atoms can change unlike a chemical reaction. Sum of mass numbers and the sum of atomic numbers is the same on both sides of the arrows.
16	Nuclear stability
17	Nuclear stability A plot showing the neutron number versus the atomic number for all the stable non-radioactive isotopes is shown on the previous slide. The stable isotopes show that there is a particular region for stability called the belt of stability. There is an ideal ratio of neutrons to protons for a nucleus to be stable. About 1:1 for light elements Goes up to about 1.5:1 for heaviest elements.
18	Predicting modes of decay If a nucleus has too many neutrons to be stable (a high neutron to proton ratio) It can convert neutrons into protons by emitting an electron (beta decay)
19	Predicting modes of decay If a nucleus has too few neutrons to be stable (too low of a neutron to proton ratio), it can convert protons into neutrons by either emitting a positron or by capturing an electron.
20	Predicting modes of decay If a nucleus is just too heavy to be stable, it loses mass by emitting alpha particles.
21	Predicting modes of decay Too many neutrons: beta emission Too few neutrons: positron emission or electron capture Too heavy: alpha emission
22	Predicting the mode of decay Predict the mode of decay for the following nuclides: ²³⁸U; ⁸B; ⁶⁸Cu
23	Radioactive series ²³⁸U, the most common isotope of uranium, decays in a series of 14 steps to ²⁰⁶Pb. Called a radioactive series. Other series ²³⁵U to ²⁰⁷Pb ²³²Th to ²⁰⁸Pb
24	More observations about nuclear stability Magic numbers Nuclides with the following numbers of protons are extra stable: 2, 8, 20, 50, 82 Nuclides with the following number of neutrons are extra stable: 2, 8, 20, 28, 50, 82, 126 Nuclides with a magic number of protons and a magic number of neutrons are doubly magic and are extra, extra stable!!
25	More observations about nuclear stability 157 stable isotopes with an even number of protons and an even number of neutrons. 53 stable isotopes with an even number of protons and an odd number of neutrons. 50 stable isotopes with an odd number of protons and an even number of neutrons. 5 stable isotopes with an odd number of protons and an odd number of neutrons. Protons and neutrons like to be paired up in even numbers!!!
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27	Nuclear Transmutations In 1919, Rutherford bombarded a sample of nitrogen gas and converted a nitrogen atom into an oxygen atom by the nuclear reaction shown above. Nuclear transmutation: changing the identity of an atom through a nuclear reaction.
28	Nuclear Reactions Fill in the missing part of the following nuclear reactions:
29	Particle accelerators Charged particles, such as protons, alpha particles, or any bombarding particle (except neutrons) are accelerated in particle accelerators which speed up the particle using magnetic fields. Atom smashers: cyclotron, synchotron, supercollider.
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31	Neutrons Neutrons have no charge. They don’t need to be accelerated to cause a reaction. Many radioactive isotopes are created by neutron bombardment.
32	Transuranium elements Elements above uranium in the periodic table are man-made. Initially, they were made by neutron bombardment. The heavier transuranium elements are created in particle accelerators with larger bombarding particles
33	Transuranium elements
34	Rates of nuclear decay Nuclear decays obey first order kinetics. Rate constant, k, called a decay constant. Generally, each radioactive nuclide has its own value of half-life, t_1/2.
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36	Rates of nuclear decay ¹⁹⁸Au is a radioactive isotope of gold with a half-life of 2.69 d. How many grams of ¹⁹⁸Au will be left from a 2.50 g sample of ¹⁹⁸Au after 4.00 d?
37	Activity, A Activity is the number of decays per unit time. Used as a measure of radioactivity. Units: SI unit: Becquerel (Bq) equals 1 disintegration per second. Curie (Ci) equal to the activity of 1 gram of radium chloride: 1 Ci = 3.7 × 10¹⁰ disintegrations/second.
38	Dating The activity due to certain radioactive isotopes may be used to estimate the age of various objects. Carbon-14 dating ¹⁴C is formed in the upper atmosphere by cosmic radiation (neutrons) colliding with nitrogen: ¹⁴N + ¹n → ¹⁴C + ¹p ¹⁴C is radioactive with a half-life of 5715 yr: ¹⁴C → ¹⁴N + ⁰e The amount of ¹⁴C in the atmosphere is constant. Living organisms have a constant ¹⁴C activity since they exchange carbon with the atmosphere. Once the living organism dies, ¹⁴C activity decreases. Comparing the ¹⁴C activity in formerly living material to ¹⁴C activity in living organisms may give the age of the object. A=A₀e^-kt
39	Dating A mammoth skeleton has a ¹⁴C activity of 0.85 disintegrations per minute per gram of carbon. Assuming that the ¹⁴C activity of living material is equal to 15.3 disintegrations per minute per gram of carbon, what is the age of the mammoth skeleton?
40	Dating Other dating techniques: ²³⁸U decays eventually to ²⁰⁶Pb. The relative amounts of ²⁰⁶Pb and ²³⁸U in a rock may be used to estimate the age of the rock. ⁴⁰K decays to ⁴⁰Ar. The age of a rock may be estimated by measuring the amount of argon trapped in the rock.
41	Mass changes in nuclear reactions Nuclear reactions are accompanied by a change in mass: ²³⁵U → ²³¹Th + ⁴He ∆m = mass of products minus mass of reactants ∆m = (231.036299 amu + 4.002603 amu) – 235.043924 amu ∆m = –0.00502 amu
42	Mass changes converted to energy changes In nuclear reactions, mass is converted to energy using the equation: E = mc² m: mass in units of kg c = speed of light = 2.998 × 10⁸ m/s For the reaction in the previous slide:
43	Mass defect Mass of ¹⁶O atom: 15.994915 amu Mass of proton: 1.007276 amu Mass of neutron: 1.008665 amu Mass of electron: 0.000548 amu Mass of ¹⁶O atom = 8×mass of proton + 8×mass of neutron??
44	Mass defect
45	Binding energy Convert the mass defect into an energy using E = mc². This is called the binding energy. This is the energy that holds the nucleus together. Multiply by Avogadro’s Number to get an energy in J/mol. VERY LARGE ENERGY!!
46	Binding energy per nucleon In order to get a binding energy to compare among atoms, divide the total binding energy by the number of nucleons. A nucleon is a proton or a neutron. Number of nucleons equal to the mass number. For ¹⁶O:
47	Binding Energy per nucleon The mass of a ³⁷Cl atom is 36.965903 amu. Given the mass of a proton as 1.007276 amu and the mass of a neutron as 1.008665 amu, calculate the mass defect of ³⁷Cl as well as its binding energy (in J) and binding energy per nucleon (in J/nucleon).
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49	Nuclear Fission Fission is the breaking apart of a very large nucleus to produce 2 medium sizes fragments plus a number of neutrons plus a large amount of energy. A very few nuclei undergo spontaneous fission. Some nuclei undergo induced fission.
50	Induced Fission
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52	Nuclear chain reaction The product neutrons may hit other ²³⁵U atoms to cause additional fission reactions. Called a nuclear chain reaction. If, on average, one emitted neutron from a fission reaction produces a new fission reaction, the chain reaction is self-sustaining (at a constant rate) which is called a critical mass. If, on average, more than one emitted neutron from a fission reaction produces a new fission reaction, the chain reaction increases it’s rate. This is called a supercritical mass.
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55	Nuclear bomb Uncontrolled supercritical mass Fuel: almost 100% ²³⁵U or ²³⁹Pu Slammed together using chemical explosives. 20 kiloton bomb
56	Nuclear reactors Fuel is “enriched” uranium: 3% ²³⁵U in the form of UO₂ pellets encased in zirconium or stainless steel tubes. Fuel is used as a source of heat to generate steam to drive turbines to produce electricity.
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58	Parts of a nuclear reactor Control rods Neutron absorbers (cadmium or boron) Pulled in or out of reactor to control the rate of the nuclear chain reaction. Moderator Used to slow down the emitted neutrons from fission reactions to make them more efficient at inducing more fissions. Water or graphite Coolant Used to carry off the heat of the reaction to produce the steam for the turbines. Water or liquid sodium metal.
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60	Problems with nuclear reactors Fission products Fuel must be reprocessed or replaced to remove the products of the fission reactions. These products are highly radioactive and will remain so for a long period of time (600 y). What is to be done with nuclear waste products?? If the chain reaction goes too fast, the fuel may overheat which may cause sufficient damage to let radioactivity escape. Nuclear fuel in a reactor will never explode due to a nuclear reaction.
61	Nuclear fusion Combining of very light nuclei to give a moderate sized nucleus plus a large amount of energy. The sun and most stars obtain their energy from the fusion of hydrogen to produce helium. Called “thermonuclear reactions”.
62	Thermonuclear reactions
63	Thermonuclear reactions There is a very large activation energy barrier for this reaction to occur. The temperatures necessary for fusion reactions to occur around 10⁷-10⁸ K. Lowest temperature required for deuterium (²H) and tritrium (³H) fusion reactions (abount 40,000,000 K)
64	Fusion reactions Hydrogen bomb (thermonuclear warhead): a fission bomb is used to get the temperatures necessary for fusion. Bombs of strengths measured in megatons of TNT. Tokamak uses magnetic confinement of a collection of ions called a plasma.
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66	Biological effects of radiation Nuclear radioactivity ionizes any material it passes through. In living tissue, radioactivity is capable of ionizing water which produces species with unpaired electrons called free radicals. Damage produced depends on activity of radiation, the energy of the radiation, the length of exposure, whether the exposure is inside or outside the body.
67	Biological effects of radiation Penetrating ability Alpha particles stopped by your skin. Beta particles penetrate about 1 cm of tissue. Gamma rays go right through your body. Ionizing ability Alpha particles produce a lot of ions. Beta particles are intermediate. Gamma rays do not produce many ions.
68	Measuring dosage of radiation Dosage: amount of energy deposited by the radiation per kg of body tissue. SI unit: Gray (Gy) 1 J of energy per kilogram Rad: 0.01 J of energy per kilogram (100 rad = 1 Gy) Different types of radiation have different effects. Relative biological effectiveness (RBE): a multiplication factor representing the effect of a particular type of radiation 1 for gamma rays and beta particles 10 for alpha particles
69	Exposure to radiation Exposure is the dosage times the RBE factor. S. I. unit: sievert (Sv) = Gy ´ RBE rem (roentgen equivalent for man) rem = rad ´ RBE Short term exposures 0-25 rem: no detectable effects. 25-50 rem: slight decrease in white blood cell counts 100-200 rem: nausea: marked decrease in white blood cell counts. 500 rem: death to 50% of population within 30 days. Natural exposure: 360 mrem per year
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71	Radon Rn(g) produced from uranium contained in the ground. It is an alpha emitter which can be breathed into the lungs where it can do significant damage. Homes that are sealed up from the outside well may collect dangerous levels of radon gas inside.