Learn about the nucleus, atomic number, and mass in this chapter. Complete HW questions 1-3, 7, 11, 13, 17, 19, 27, 29 by tomorrow. Also, work on odd questions 31-35, and 41, 57, 59, and 61. From "The Central Science" by Brown, LeMay, Bursten, and Bookstaver, 10th edition.
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About Nuclear Chemistry
PowerPoint presentation about 'Nuclear Chemistry'. This presentation describes the topic on Learn about the nucleus, atomic number, and mass in this chapter. Complete HW questions 1-3, 7, 11, 13, 17, 19, 27, 29 by tomorrow. Also, work on odd questions 31-35, and 41, 57, 59, and 61. From "The Central Science" by Brown, LeMay, Bursten, and Bookstaver, 10th edition.. The key topics included in this slideshow are . Download this presentation absolutely free.
Slide1NuclearChemistry Chapter 21 Nuclear Chemistry Chemistry, The Central Science , 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten John D. Bookstaver St. Charles Community College St. Peters, MO 2006, Prentice Hall, Inc.
Slide2NuclearChemistry February 3 • Nuclear chemistry • HW • 1,2,3,7,11,13,17,19,27,29 for tomorrow • 31 to35 odd, 41,57,59,61
Slide3NuclearChemistry The Nucleus • Remember that the nucleus is comprised of the two nucleons , protons and neutrons. • The number of protons is the atomic number. • The number of protons and neutrons together is effectively the mass of the atom.
Slide4NuclearChemistry Isotopes • Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. • There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235 Uranium-238
Slide6NuclearChemistry Radioactivity • It is not uncommon for some nuclides of an element to be unstable, or radioactive . • We refer to these as radionuclides . • There are several ways radionuclides can decay into a different nuclide.
Slide7NuclearChemistry Types of Radioactive Decay
Slide9NuclearChemistry SeparationAlphaBetaGamma.MO V Separation of Radiation
Slide13NuclearChemistry Nuclear Reactions • The chemical properties of the nucleus are independent of the state of chemical combination of the atom. • In writing nuclear equations we are not concerned with the chemical form of the atom in which the nucleus resides. • It makes no difference if the atom is as an element or a compound. • Mass and charges MUST BE BALANCED!!!
Slide14NuclearChemistry Alpha Decay: Loss of an -particle (a helium nucleus) He 4 2 U 238 92 Th 234 90 He 4 2 +
Slide15NuclearChemistry Alpha Decay • Mass changes by 4 • The remaining fragment has 2 less protons • Alpha radiation is the less penetrating of all the nuclear radiation (it is the most massive one!)
Slide16NuclearChemistry Beta Decay: Loss of a -particle (a high energy electron) 0 − 1 e 0 − 1 or I 131 53 Xe 131 54 + e 0 − 1
Slide17NuclearChemistry Beta Decay • Involves the conversion of a neutron in the nucleus into a proton and an electron. • Beta radiation has high energies, can travel up to 300 cm in air. • Can penetrate the skin
Slide18NuclearChemistry Beta decay • Write the reaction of decay for C-14
Slide19NuclearChemistry Gamma Emission: Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 0 0
Slide20NuclearChemistry Positron Emission: Loss of a positron ( particle with same mass, but opposite charge than an electron) e + 0 1 C 11 6 B 11 5 + e 0 1
Slide21NuclearChemistry Positron emission • Involves the conversion of a proton to a neutron emitting a positron. • The atomic number decreases by one, mass number remains the same.
Slide22NuclearChemistry Electron Capture (K-Capture) Capture by the nucleus of an electron from the electron cloud surrounding the nucleus. As a result, a proton is transformed into a neutron. p 1 1 + e 0 − 1 n 1 0
Slide23NuclearChemistry Electron capture • Rb-81 • Note that the electron goes in the side of the reactants. Electron gets consumed. Rb 81 37 Kr 81 36
Slide24NuclearChemistry Patterns of nuclear Stability • Any element with more than one proton ( all but hydrogen) will have repulsions between the protons in the nucleus. • A strong nuclear force helps keep the nucleus from flying apart.
Slide25NuclearChemistry Neutron-Proton Ratios • Neutrons play a key role stabilizing the nucleus. • The ratio of neutrons to protons is key to determine the stability of a nucleus .
Slide26NuclearChemistry Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.
Slide27NuclearChemistry Neutron-Proton Ratios For smaller nuclei ( Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
Slide28NuclearChemistry Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so- called belt of stability .
Slide29NuclearChemistry Stable Nuclei • Nuclei above this belt have too many neutrons. • They tend to decay by emitting beta particles. ( neutron becomes proton )
Slide30NuclearChemistry Above the belt of stability Beta particle emission • Too many neutrons. The nucleus emits Beta particles, decreasing the neutrons and increasing the number of protons.
Slide31NuclearChemistry Stable Nuclei • Nuclei below the belt have too many protons. • They tend to become more stable by positron emission or electron capture (both lower the number of protons)
Slide32NuclearChemistry Stable Nuclei • Elements with low atomic number are stable if # proton = # neutrons • There are no stable nuclei with an atomic number greater than 83. • These nuclei tend to decay by alpha emission.
Slide33NuclearChemistry Below the stability belt Increase the number of neutrons (by decreasing # protons) • Positron emission more common in lighter nuclei. • Electron capture common for heavier nuclei.
Slide35NuclearChemistry Radioactive Series • Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. • They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).
Slide36NuclearChemistry Predicting modes of nuclear decay C-14 Xe-118 Pu-239 In-120
Slide37NuclearChemistry • beta decay • Positron emission or electron capture • Alpha decay (too heavy, loses mass) • Beta decay (ratio too low, gains protons)
Slide38NuclearChemistry MAGIC NUMBERS 2, 8, 20, 28, 50, or 82 Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.
Slide39NuclearChemistry Some Trends Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.
Slide40NuclearChemistry Shell model of the nucleus • Nucleons are described a residing in shells like the shells for electrons. • The numbers 2,8,18,36,54,86 correspond to closed shells in nuclei. • Evidence suggests that pair of protons and pairs of neutrons have special stability
Slide41NuclearChemistry Transmutations • To change one element into another. • Only possible in nuclear reactions never in a chemical reaction. • In order to modify the nucleus huge amount of energy are involved. • These reactions are carried in particle accelerators or in nuclear reactors
Slide42NuclearChemistry Nuclear transmutations • Alpha particles have to move very fast to overcame electrostatic repulsions between them and the nucleus. • Particle accelerators or smashers are used. They use magnetic fields to accelerate the particles.
Slide43NuclearChemistry Particle Accelerators (only for charged particles!) These particle accelerators are enormous, having circular tracks with radii that are miles long.
Slide44NuclearChemistry Cyclotron Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.
Slide45NuclearChemistry Neutrons • Can not be accelerated. They do not need it either (no charge!). • Neutrons are products of natural decay, natural radioactive materials or are expelled of an artificial transmutation. • Some neutron capture reactions are carried out in nuclear reactors where nuclei can be bombarded with neutrons.
Slide46NuclearChemistry Representing artificial nuclear transmutations • 14 N + 4 He 7 O + 1 H Target nucleus ( bombarding particle, ejected particle ) product nucleus • 14 N ( p) 17 O • Write the balanced nuclear equations summarized as followed: • 16 O ( p, • 27 Al (n, a
Slide47NuclearChemistry Measuring Radioactivity • One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. • The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
Slide48NuclearChemistry Mass defect • The mass of the nucleus is always smaller than the masses of the individual particles added up. • The difference is the mass defect. • That small amount translate to huge amounts of energy E = ( m ) c 2 • That energy is the Binding energy of the nucleus, and is the energy needed to separate the nucleus.
Slide49NuclearChemistry Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is − 0.0046 g. The change in energy, E , is then E = ( m ) c 2 E = ( − 4.6 10 − 6 kg)(3.00 10 8 m/s) 2 E = − 4.1 10 11 J This amount is 50,000 times greater than the combustion of 1 mol of CH 4
Slide50NuclearChemistry Types of nuclear reactions fission and fusion • The larger the binding energies, the more stable the nucleus is toward decomposition. • Heavy nuclei gain stability (and give off energy) if they are fragmented into smaller nuclei. (FISSION)
Slide51NuclearChemistry • Even greater amounts of energy are released if very light nuclei are combined or fused together. (FUSION)
Slide52NuclearChemistry Nuclear Fission • How does one tap all that energy? • Nuclear fission is the type of reaction carried out in nuclear reactors.
Slide53NuclearChemistry Nuclear Fission • Bombardment of the radioactive nuclide with a neutron starts the process. • Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.
Slide54NuclearChemistry Nuclear Fission This process continues in what we call a nuclear chain reaction .
Slide55NuclearChemistry Nuclear Fission If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.
Slide56NuclearChemistry Nuclear Fission Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass .
Slide57NuclearChemistry Controlled vs Uncontrolled nuclear reaction • Controlled reactions: inside a nuclear power plant • Uncontrolled reaction: nuclear bomb
Slide58NuclearChemistry Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.
Slide59NuclearChemistry Nuclear Reactors • The reaction is kept in check by the use of control rods. • These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.
Slide60NuclearChemistry FUSION • Combining small nucleii to form a larger one. • Require millions of K of temperature
Slide61NuclearChemistry Fusion • 1 H + 1 H 2 H + 1 e + energy • 1 H + 2 H 3 He + energy • 3 He + 3 He 4 He + 2 1 H + energy • Reaction that occurs in the sun • Temperature 10 7 K • Heavier elements are synthesized in hotter stars 10 8 K using Carbon as fuel
Slide62NuclearChemistry Nuclear Fusion • Fusion would be a superior method of generating power. The good news is that the products of the reaction are not radioactive. The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.
Slide63NuclearChemistry Nuclear Fusion (thermonuclear reactions) • Tokamak apparati like the one shown at the right show promise for carrying out these reactions. • They use magnetic fields to heat the material. • 3 million K degrees were reached inside but is not enough to begin fusion which requires 40 million K
Slide64NuclearChemistry Rates of radioactive decay rate = k N N is the number of radioactive nuclei • Activity: rate at which a sample decays. Expressed in disintegrations per unit time. • Becquerel (Bq) SI unit : one nuclear disintegration per second. • Curie (Ci) 3.7x10 10 disintegrations per second, the rate of decay of 1g of Ra
Slide65NuclearChemistry RADIOACTIVE DECAY • As a radioactive sample decays, the amount of radiation emanating for the sample decays as well. • After one half life, half of the emanations!
Slide66NuclearChemistry Half-Life • Half-life is defined as the time required for one-half of a reactant to react. • Because [A] at t 1/2 is one-half of the original [A], [A] t = 0.5 [A] 0 .
Slide67NuclearChemistry RADIOACTIVE DECAY Is a first order process. Its rate is proportional to the number of radioactive nuclei N in the sample rate= k N N 0 N t ln = kt Time elapsed =t k is the decay constant N 0 is the original amount N t is the amount of sample at time t 0.693 = kt 1/2
Slide68NuclearChemistry Half life • The half life of a reaction is useful to describe how fast it occurs. • For a first order reaction (like nuclear decay!) it does not depend on the initial concentration of the reactants. • HALF LIFE IS CONSTANT FOR A FIRST ORDER REACTION
Slide69NuclearChemistry Half Life Decay of 10.0 g sample of Sr-90 t 1/2 = 28.8 y
Slide70NuclearChemistry Problem 1 • The half life of 210 Pb= 25 y 1) How much left of a sample of 50 mg will remain after 100 y? • 2) Find number of half lives • 3) Find fraction left
Slide71NuclearChemistry • 1- 6.25 g • 2- 4 half lives • 3- 1/16
Slide72NuclearChemistry Problem 2 • How many years will take for 50mg of 210 Pb to decay to 5 mg? • Half life of 210 Pb= 25 y
Slide73NuclearChemistry • 83 years
Slide74NuclearChemistry Problem 3 • 90 % of a radioisotope disintegrates in 36 hs. What is the half life? •
Slide75NuclearChemistry Problem 4 • .953 g of Sr-90 remains after 2 y from a 1.000g sample. • a) find the half life • b) how much will remain after 5 y? •
Slide76NuclearChemistry • Half life = 28.8 years • Amount left (No) = 0.89 g
Slide77NuclearChemistry Radioactive Dating • A rock contains .257 mg of Pb-206 for every mg of U-238. • T 1/2 = 4.5 x 10 9 y • How old is the rock?
Slide78NuclearChemistry • N o = we will assume that all the Pb-206 that is now present will come from the original U, plus the U that is still present • (check the answer in textbook!)
Slide79NuclearChemistry Calculating half life: • If 87.5 % of a sample of I-131 decays in 24 days, what is the half life of the I-131?