Honors Chemistry Nuclear Chemistry Chemical reactions involve changes

Honors Chemistry Nuclear Chemistry Chemical reactions involve changes with electrons. ________ Nuclear reactions involve changes in atomic nuclei. ________ radiation and Spontaneously-changing nuclei emit ____ radioactive are said to be _____. energy and/or particles

Radioactivity nucleons: protons (p+) and neutrons (n 0) mass number: (p+ + n 0) in a given atom isotopes: species having the same number of p+, but different numbers of n 0 radioisotopes -- radioactive ones are called ______ nuclide: a nucleus w/a specified number of p+ and n 0 radionuclides -- radioactive ones are called ______ atomic number: (Z); # of p+ these are unstable and emit radiation

Radioactive Decay For nuclear equations, mass (top) and charge (bottom) must balance. alpha (a) decay: (go DOWN two #s on Table) 234 92 U 230 4 90 Th + 2 He a-particle (i. e. , a He nucleus): massive, slow-moving; stopped by skin

234 91 Pa beta (b) decay: (go UP one # on Table) 234 0 92 U + – 1 e b-particle (i. e. , a fast-moving electron): little mass; stops ~1 cm into body In b-decay, the effect is that a n 0 is converted into a p+, ejecting an e– from the nucleus. 1 0 n 0 1 – 1 e + 1 p NOTE: There are no e– in the nucleus. The ejected e– is formed when energy released from the E = mc 2 nucleus “congeals” into mass, via _______.

gamma radiation: consists of high-energy photons -- can penetrate to internal organs -- gamma ray: 0 g (or just g) 0 emitted when nucleons rearrange into a more stable configuration -- The shell model says that nucleons reside in shells, similar to… how e– reside in shells. -- gamma radiation often accompanies other nuclear decays 234 92 U 230 4 0 90 Th + 2 He + 2 0 g

positron decay: 23 Mg 12 23 0 0 11 Na + 1 e + 0 n positron: identical to an e–, but (+) neutrino: “massless, ” chargeless particle electron capture: nucleus captures orbiting e– 11 0 6 C + – 1 e 11 0 5 B + 0 n The effect of positron decay and electron capture is to turn a p+ into a n 0. 1 1 p 0 1 1 e + 0 n POSITRON DECAY 1 0 1 p + – 1 e 1 0 n ELECTRON CAPTURE

Nuclear Stability strong force Nucleons are held together by the _____. ~1. 5 n 0 : 1 p+ Band (or Belt) of Stability # of n 0 ~1 n 0 : 1 p+ 0 Z (i. e. , # of p+) 83 At Z > 83, none are stable (i. e. , all are radioactive).

Nuclei that… …have too many… …and stabilize by… …are above belt… n 0 …are below belt… p+ b-emission positron emission (or e– capture) …have Z > 83… p+ and n 0 a-emission Examples: (a) 242 94 Pu 4 238 2 a + 92 U (b) 163 64 Gd 0 163 – 1 b + 65 Tb 145 65 Tb 0 145 1 e + 64 Gd (positron emission or e– capture) 145 0 65 Tb + – 1 e 145 64 Gd

A radioactive series is the decay sequence a radionuclide goes through to become stable. a b e. g. , U-238 Th-234 Pa-234 , etc. -- there are three basic series, ending with… Pb-206, Pb-207, and Pb-208.

Radioactive materials will continue to decay until they reach a stable material (Usually with an atomic number less than 83. ) Stable Isotope

Nuclear Transmutations These are induced by a bombarding particle, and are typically written in the following order: 27 13 Al + target nucleus 4 2 He 30 15 P bombarding particle product nucleus This reaction is abbreviated… 27 30 13 Al (a, n) 15 P Ernest Rutherford was the first to artificially transmute elements. + 1 0 n ejected particle

Write the shorthand for 27 1 Al + 13 0 n 24 4 Na + 11 2 He 27 24 13 Al (n, a) 11 Na Write the equation for 14 17 7 N (a, p) 8 O 14 4 7 N + 2 He 17 1 8 O + 1 H 14 4 7 N + 2 a 17 1 8 O + 1 p

Rates of Radioactive Decay Each radioisotope has a unique rate of decay, its half-life, t 1/2, which is the time required for half of a sample of a radioisotope to decay into something stable. An isotope’s half-life is: (1) independent of T, P, and its state of chemical combination (2) useful in radioactive dating. “Otzi” the Iceman lived circa 3300 B. C. , according to radiocarbon dating analyses.

Half-lives can be as short as a fraction of a second or billions of years. Different nuclei have different decay patterns, depending on why they are unstable (i. e. too many protons, too many neutrons, )

In general, the further away an isotope is from the “band, ” the shorter its half life. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Molybdenum-99 has a half-life of 2. 79 days. How much of a 16. 80 mg sample of Mo-99 is left after 8. 37 days? start 16. 80 mg after one t 1/2 8. 40 mg = 3 half-lives after two t 1/2 4. 20 mg after three t 1/2 2. 10 mg (this is the amount of radioactive Mo-99 left; i. e. , 14. 70 mg is now stable mat’l) ½ ½ ½ etc.

Example: Carbon-14 emits beta radiation and decays with a half-life of 5730 y. Assume you start with a mass of 2. 00 g of carbon-14. a. How long is 3 half-lives? b. How many grams will remain at the end of 3 half lives? c. How many years will it take for only 0. 0625 g to remain? a. t 1/2 = 5730 y, 3(5730 y) = 17190 y 1 b. 2. 00 g c. 2. 00 g 5 1. 00 g 4 2 0. 50 g 3 3 0. 25 g t 1/2 = 5730 y, 5(5730 y) = 28650 y 0. 25 g 2 0. 125 g 1 0. 0625 g

t½ equation 2. 00 g 1 1. 00 g 2 0. 50 g 3 0. 25 g 2. 00 g(1/2)(1/2) or 2. 00(1/2)3 = 0. 25 g N 0 = initial amount N 0(1/2)n = N N = final amount n = # of half lives If 150. 0 g of a radioactive substance undergoes 25 half lives, how many g will remain? 150. 0 g (1/2)25 = 4. 47 x 10 -6 g

Energy Changes in Nuclear Reactions Energy and mass are two sides of the same coin. E = mc 2 c = 3. 00 x 108 m/s m = mass , in kg E = energy, in J When a system loses/gains energy, it loses/gains mass. In chemical reactions, this mass change is nearly undetectable, so we speak of mass as being “conserved, ” when it really isn’t. The amount of “mass-and-energy-together, ” however, IS conserved. Mass changes in nuclear reactions are much larger than in chemical reactions.

Nuclear Binding Energy mass of nucleus < mass of individual nucleons in nucleus rest masses: n 0 = 1. 00866 amu = 1. 67493 x 10– 24 g p+ = 1. 00728 amu = 1. 67262 x 10– 24 g mass of mass defect = constituent – nucleus nucleons (or “mass deficiency”) “Tighter, lighter. " “Separate, heavier. " This “missing” mass is converted into energy, which is used to hold the nucleus together.

Use mass defect, E = mc 2, and # of nucleons to calculate binding energy per nucleon (BE/n). -- large BE/n means great nuclear stability -- BE/n is largest for Fe-56, meaning that nuclei _____ than Fe-56… (1) LARGER…decay OR can undergo fission + ENERGY (2) SMALLER…can undergo fusion + ENERGY Both fission and fusion are exothermic.

Calculate the binding energy per nucleon of N-14, which has a nuclear mass of 13. 999234 amu. 7 p+ (1. 00728 amu) = 7. 05096 amu 7 n 0 (1. 00866 amu) = 7. 06062 amu 14. 11158 amu m. d. = 14. 11158 – 13. 999234 = 0. 11235 amu = 1. 8656 x 10– 28 kg Most BE’s are measured in electron-volts… = 1. 20 x 10– 12 J/nucleon 1 e. V = 1. 60 x 10– 19 J This is 7. 50 x 106 e. V, or 7. 50 Me. V.

Nuclear Fission requires… slow-moving neutrons. distance too big; strong force weakens; +/+ repulsion takes over slow fast nn 00 released n 0; free to split more nuclei Important fissionable nuclei: U-233, U-235, Pu-239 chain reaction: one nuclear reaction leads to one or more others

critical mass: the mass of fissionable material required to maintain a chain reaction at a constant rate supercritical mass: the mass above which the chain reaction accelerates safe critical mass supercritical mass (reaction maintained at constant rate) (“Ah jes’ felt lahk runn. ING. ”) Little Boy, later dropped onrun!”) Hiroshima (“Run, Forrest,

Nuclear Reactors -- fuel is… ~3% U-235 (natural U is ~0. 7% U-235) -- control rods of B or Cd… absorb n 0, slow down the reaction -- moderator: slows down n 0 to cause fission reactors in former USSR: graphite in the rest of the world: water WHY? low-molar-mass materials -- water is heated to steam, which spins electricalgenerating turbines control room at a nuclear power plant

Schematic of a Nuclear Power Plant EIW = Emergency Injection Water PORV = Pressure Release Valve

Main benefits: n 0 + U-238 (1) no air pollution; does NOT contribute to global warming (2) small volume of material consumed (3) breeder reactors: reactors that generate new fissionable mat’l at a greater rate than the original fuel is consumed -- non-fissionable U-238 is transmuted into fissionable Pu-239 U-239 b Np-239 b Pu-239 Main problem: What to do with waste?

Schematic of Breeder Reactor

Nuclear Fusion -- also called thermonuclear reactions -- products are generally NOT radioactive -- requires high temperatures (> 40, 000 K) !!!! WHY? -- the tokamak uses magnetic fields to contain and heat the reaction

Radon -- an a-emitter from the decay of radium in rocks and soil -- very dense; seeps into basements and is readily inhaled Ra-226 a radon formed Rn-222 a Po-218 radon this, too, is an emits a radiation a-emitter -- estimated to be responsible for ____ 10% of U. S. lung cancer deaths

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