CHEM 106 1nd Edition Lecture 16Outline of Last Lecture I. Elementary Stepsa. ExamplesII. CatalysisOutline of Current Lecture II. Notation reviewIII. Primordial NucleosynthesisIV. Stellar NucleosynthesisV. Beta DecayVI. Radioactive DecayVII. Nuclear Binding EnergyCurrent LectureChapters 2 & 21: Nuclear and RadiochemistryChapter 2, section 2.7 and chapter 21Some review: nuclides, isotopes, and notationXZAx = element z = atomic number (# of protons) A = mass number (protons + neutrons)Examples:C612Fe2656U92235Can also write for smaller nuclei:p11 = protonn01 = neutronβ−10 = electron (negatron)β+10 = electron (positron)-neutrons and protons are made up of quarks (subatomic particles)Neutron: 1 up + 2 down quarksProton: 2 up + 1 down quarkDeuteron: neutron + proton → H12 or d12Alpha particle: He24 or α24Balanced nuclear reactions:Conserve mass + chargeSum of A, superscriptsSum of Z, subscriptsWhere do elements come from?Fig. 2.20 pg. 66Primordial Nucleosynthesis2 d12 → α24fusion reactionsH01 H12He24stable nuclei5 minutes after the big bang, the universe was 75% H and 25% HeIt remained this way for millions of yearsAfter the temperature cooled to < 108 K, fusion slowed significantlyStellar NucleosynthesisCoalescence of He + H, driven in part by gravity Fig. 2.21 pg. 67 Fusion by He24You can get heavier elements at higher T + PFig. 2.22 pg. 68Smaller nuclei fuse to produce heavier nuclei and release energy up to formation ofFe❑56, which is the most stable nucleusSubsequent fusion reactions involving 56Fe consume E:Fe2656 + α24 → ¿2860This reaction would require energy inputAs a star collapses, T & P ↑, reheating to 109 KNuclei disintegrate into free p11 and n01Fe2656+3 n01→ Fe2659neutron captureCan also have transformations within nucleusBeta Decay (β)2 typesNegatron decay: original nucleus is neutron rich, neutron converts to proton−¿+´vX 1ZA→ X 2Z+1A+β¿ Parent daughter negatron antineutrinoPositron decay: original nucleus is proton rich, proton converts to neutron+¿+vX 1ZA→ X 2Z−1A+β¿ Parent daughter positron neutrinoEx: isotope of Kr (z=36) undergoes β decay. What element is produced?*assume it is referring to negatron decay↑ proton # by 1 z=37 RbNeutron capture and β decay continue as star collapses in on itself, until it eventually explodes (supernova)Chapter 21: Nuclear Chemistry21.11 Radioactive Decay1st order kinetics, see notes from February 12 + 14Fig. 21.1 pg. 997No = # nuclei at t=0 n = # half-livesNt = No e−ktNt = # nuclei at t n = tt 1/2Radioactive Decay rate or half-life not dependent on initial coneEx: t ½ 60Co is 5.3 years, how much of a 1.000 mg sample of 60Co after 15.9 years?15.9 years = 3 half-lives (5.3 * 3)NtNo = 12¿¿(12)n;¿ = 18so, only 1/8 of the original quantity remains after 3 half-lives0.125 mg remain21.1 Hydrogen Fusion, limitless E?Fusion reactions: p11+ n01→ d122d12→ α24In our sum: +¿4 p11→ α24+2 β+10 ¿Nuclear Binding EnergyChange in mass when 2 nuclei fuse ΔmMass is energy: e=m c2Δm = mass defect or difference = (mass of free nuclear particles)-(mass of nucleus)Binding energy (BE): energy released when nucleons form nucleus21.3 Nuclear Binding EnergiesStrong nuclear force holds protons together in nucleusEx of mass defect: α24Masses of free particles: use table 21.1 pg. 10012 neutrons 2 x 1.67493 x 10-272 protons 2 x 1.67262 x 10-27=6.695 x 10-27 kgMass of α24: 6.644 65 e-27 kgΔm = 5.045 x 10-29 kg mass differenceBE = Δmc2 = (5.045 x 10-29 kg) (2.998 x 108 m/s)2 = 4.534 e-12 kg m2/s2 for 1 atomFor 1 mole of α24 atoms: (multiply by Avogadro’s number)E = 2.73 x 1012 JBinding energy per nucleon – fig. 21.3Isotopes lighter than 56Fe – fusionIsotopes heavier than 56Fe – fissionFig. 21.4 & Fig. 21.5 – shows stable and known radioactive isotopesChart of