Ch. 22 – Nuclear Chemistry Patterns of Stability • • Like charges repel. Why does the nucleus stay together? • “Strong Nuclear Force” • The neutrons play a role. • Patterns of Stability Stable nuclei tend to have “stable ratios of protons to neutrons. • We see this in the “belt of stability.” Patterns of Stability • Zr – 90 Patterns of Stability 40 / 50 1.25 : 1.00 • Sn – 120 50/70 • Hg – 200 • These ratios are quite stable. 80/120 1.4 : 1.0 1.5 : 1.0 Patterns of Stability • Belt of stability ends at #83 • All elements #84 and above are radioactive. • All isotopes of U are radioactive. Patterns of Stability • 3 situations can now occur. • 1) Nuclei above the belt of stabilitycan lower their atomic number via Beta emission. • 2) Nuclei below can increase at.no. vie positron emission of electron capture. Patterns of Stability • 3) Nuclei with at. No. ≥ 84 : • Undergo alpha emission. Decrease both at. No and at. Mass to move toward the belt of stability. Patterns of Stability • Magic Numbers. • • 2, 8, 20, 28, 50, 82 protons 2, 8, 20, 28, 50, 126 neutrons • Are more stable than nuclei which do not contain these numbers. Patterns of Stability • Even Numbers • Nuclei with even number of protons and neutrons are more stable than those with odd numbers. Sample 21.4 as to how this works. • Radioactivity The nucleus undergoes a spontaneous decay emitting a ray or particle. • Nuclear Chemistry is the study of nuclear reactions and their uses with chemistry. • Many misunderstandings Radioactivity • • • • Nucleons – protons and neutrons. Mass Numbers Isotopes – A AT.Mass ZX. At No.X • • U-233 233 92U U-235 235 92U U-238 238 92U • Radioactivity Nuclide refers to a specific Nucleus. • Radionuclide refers to radioactive nuclei. • Radioisotopes refer to atoms containing the above. Nuclear Equations • U-238 undergoes alpha decay • 238 • Radioactive Decay • 238 • • Mass Must balance 238 = 234 + 4 • • Charge must balance 92 = 90 + 2 • Alpha Decay – 4 2He or 4 2á • Beta Decay - 0 -1e or 0 • Gamma Ray - 0 92U Æ 23490Th + 4 + 4 2He Nuclear Equations 92U Æ 23490Th 2He Types of Decay -1â 0ã Types of Decay • • I-131 undergoes Beta Decay 131 131 0 53I Æ 54Xe + -1â • Beta is equivalent to conversion of a neutron to a proton. • 1 0n Æ 11p + 0 -1 â • Types of Decay Gamma Radiation consists of High Energy Photons. • 0 • See table 22-6 0ã Other Types of Decay Positron Emission 11 6C Æ 115B + 0 1e Equivalent to converting a proton to a neutron. 1 1p Æ 10n + 0 1e Other Types of Decay • Electron Capture • 81 • • Equivalent to converting a proton to a neutron. 1 0 1 1p + -1â Æ 0n 37Rb + 0 -1â Æ 81 36Kr Chem.03.04 • Radioactive Series Some nuclei cannot gain stability by a single emission. This leads to a series of emissions. • U-238 Æ Th-234 Æ Pa-234 Æ eventually to Pb-208 Radioactive Series Radioactive Series • Radioactive Series or Nuclear Disintegration Series • Radioactive Daughters. • Transmutations Induced nuclear changes involve a nucleus that is struck by a particle and then undergoes changes • Nuclear Transmutations. • 14 • Here N-14 is changed to O-17. • The Alchemist’s Dream? • Transmutations Commonly list the transmutation in order: • Target nucleus: Bombarding particle: ejected particle: product nucleus • Example 21.5 • Transmutations Sometimes these particles must go very fast to induce radioactivity. • In this case we use particle accelerators. Transmutations 7N + 4 2HE Æ 17 8O + 1 1H Fermilab Fermilab Fermilab Fermilab Fermilab S.L.A.C. S.L.A.C. Using Neutrons • 58 26Fe • • + 59 59 27Co + 1 0n 26Fe 1 0n Æ 59 26Fe Æ 5927Co + Æ 60 27C0 0 -1 e • 238 92U • 239 93Np Æ • 239 94Pu + 1 + 0n Æ 239 4 239 92U Æ 239 0 e 94Pu + Æ 242 2He -1 Transuranium elements 0 93Np + -1 e 96Cm + 1 0n • Half-Life Radioactivity follows classic first order reaction kinetics. • There is only a single reactant and it reacts spontaneously. • It will show a half –life t 1/2 • Half-Life In one t ½ the material diminishes by 50%. • In the next t ½ the remaining material diminishes by 50% • Etc, etc, etc, ………………. Half-Life – Sr-90 Half-Life • • • • • • • • Check the numbers for half-life Time Counts 0 1000 1 500 2 250 3 125 4 62 5 32 • • • • • • • Time 6 7 8 9 10 11 Counts 16 8 4 2 1 50/50 chance % Decay 0 50 75 87.5 94.0 96.9 Half-Life % Decay 98.4 99.2 99.6 99.8 99.9 Half-Life • ALL half-lifes are the same. • What are the differences? • Length of half-life / Intensity of decay / Type of decay • Dating We can use Radioisotopes for dating purposes. • (which for some of you will be the only dating you’ll do this year……….) • Dating C14 undergoes neutron capture in the upper atmosphere. • 14 • 14 7 N + 6 C Æ 1 0 n Æ 14 7 14 N Æ 6 C + 0 -1 1 1 p e Dating • C-14 has a half-life of 5715 Years. • Based on our 10 half-life rule of thumb, that makes about 50,000 yrs worth of dating. • After Death C-12 remains constant while the C-14 levels drop due to Decay. • Dating We can use a variety of isotopes to bridge and leap back in time as the dating process goes on. • See Table 22-2 – for various Half-lives. • • • • Nuclear Rxns and Energy 2 E= mc E = energy; m= mass; c=speed of light (3.0x108 m/s) In nuclear changes it is possible for a tiny amount of mass to convert to energy, which of course is substantial. Mass Loss • 238 • • • 238 • 233.9942 + 4.0015 – 238.0003 = -.0046 92U Æ 234 90Th + 4 2He U = 238.0003amu Th= 233.9942 amu 4 He = 4.0015amu 234 Mass Loss • • If we do this on a gram scale…… Mass loss is = .0046g • E = (-.0046g) x (3.00x108m/s)2 • E = -4.1 x 1011 Kg-m/s2 • E = -4.1 x 1011 J • Binding Energies But what really holds the protons and neutrons together in the nucleus? • Let’s look at the numbers involved. • He-4 • • Two protons 2 x 1.00728 amu • • Two neutrons 2 x 1.00866 amu • Should = 4.03188 amu He - 4 He-4 • Actual mass of He-4 = 4.00150 amu • • • • • Calculated = Actual = “MASS DEFECT” 4.03188 amu 4.00150 amu ---------------0.03038 amu • Mass Defect Mass difference between a nucleus and its constituent nucleons. • E + • U E = c2 Um • • Mass Defect UE = (2.9979x10 m/s) x (.03038amu) x (1g/6.022 x1023) (1kg / 1000g) = • 4.543 x 10 -12 J • Binding Energy per nucleon. 4 2He Æ 2 11 P + 2 10 n 8 2 Binding Energy Finally • The End • Chem. ’03 – ‘04

© Copyright 2020