1Radioactive Decay and Radiometric DatingExtra credit: chapter 7 in BrysonSee online (link fixed) or moodleRadioactivity and radiometric dating•Atomic nucleus• Radioactivity• Allows us to put numerical ages on geologic eventsAtomic structure• Nucleus—composed of protons and neutrons• Orbiting the nucleus are electrons –negative electrical chargesAtom model Atomic number• Identifying number• Number of protons • Determines the propertiesMass number• Protons + Neutrons • Nucleons• Not the same as Atomic MassPeriodic Table Series in Periodic Table Isotope• Same number of protons• Different number of neutrons• Different mass number than another isotope of the same element• Variant atom of the same element• Say “Gold 188” for Au-1882RadioactivitySpontaneous breaking apart (decay) of atomic nucleiNucleus• Very small—10-15to 10-14m radius• “Strong interaction” binds nucleons• Protons repel one another– Neutrons counteract this– More neutrons than protons in larger atomsNuclear forces• Very strong at small distances (10-15m)• Weakens at 10X that distance (10-14m)• Elements at. # 82 + are unstable, because they are big• “Radioactive”Radioactive decay• Parent atom– an unstable isotope• Daughter products – Formed from the decay of a parent atoms– Different element because of nuclear changesTypes of radioactive decay• Alpha emission α• Beta emission β• Gamma radiation γAlpha emission α•2 N + 2 P+• Positive charge• LargeÆ non-penetratingBeta emission β• Negative charge—electron•NÆ P++ e-• Small, but low energy—minor penetrationGamma radiation γ• Penetrating, energetic photons• Lower energy of excited daughter nucleus• No charge, less mass than electronNuclear Decay EquationAlpha Decay•Ra-226ÆRn-222+αTotal nucleons stays same, but on daughter (product) sideAlpha decay α removes 2Noand 2P+3Nuclear Decay EquationBeta Decay•H-3ÆHe-3 + βTotal nucleons stays sameBeta decay β NoÆ P++ e-Balancing Nuclear Equations•P+(protons) + e-(electrons) = No(neutrons)• Keep track of protons: elemental symbol• α decay decreases by two by releasing them with two neutrons…so atomic number goes down by 2, atomic mass goes down by 4• β decay increases protons by releasing electron…so atomic number goes up by 1, atomic mass stays the sameNAMES--Pa: protactiniumTh: thoriumRa: radiumRn: radonPo: poloniumAt: astatinePb: leadBi: bismuthTl: thalliumDecay of U-238Measuring Radioactivity• Radioactive decay strips electrons from atoms• Ions created • Geiger counter--charged wire, results in ‘clicks’ of counter• Others rely on visual reactions of ionsHalf LifeDecay is random for any radioactive atomBUT: Predictable for mass of material• One half of unstable parent material Ædaughter product: HALF LIFE:L1/2• Decay rate constant, unaffected by external conditionsParent to daughter ratios of H-3Half Life equationAmount remaining can be calculated bynIR21=R remaining I initial n number of half-livesCalculate amount from half-life• Start with 400 mg of Co-60• Half life is 5.25 years• How much is left after 15.75 years?• To use equation at right,• First calculate how many half lives3 half-livesnIR21=halflifeyryears25.575.15 =÷yrhalflifeyearshalflifeyryears25.575.1525.575.15 =⋅=÷Calculate amount from half-life• 400 mg of Co-60• Half life is 5.25 years• How much is left after 3 half-lives?• Calculate amount mgR21)400(3=⋅=mgmgR
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