Radioactive Decay and Radiometric Dating Radioactivity and radiometric datingAtomic structure reviewedAtom modelAtomic numberMass number Periodic TableIsotopeNuclear NotationIn class activity # 1RadioactivityRadioactive decayNucleusNuclear forcesTypes of radioactive decayAlpha emission aBeta emission bGamma radiation gNuclear Decay EquationNuclear Decay EquationIn class activity #2In Class Activity # 3Balancing Nuclear EquationsUranium-238Measuring RadioactivityHalf LifeParent to daughter ratiosHalf Life equationIn class activity # 4Radiometric datingCarbon-14 datingFormation of Carbon-14Decay of Carbon-14C-14 dating cross and tree ring chronologyImportance of radiometric datingGeologic time scaleGeologic Time Scale SubdivisionsEonEras of the Phanerozoic eonIn Class Activity # 3In Class Activity # 3In Class Activity # 3In class activity # 4In class activity # 4In class activity # 4In class activity # 4In class activity # 41Radioactive Decay and Radiometric DatingRadioactivity and radiometric dating• Atomic structure• Radioactivity• Allows us to put numerical ages on geologic eventsAtomic structure reviewed• Nucleus—composed of protons and neutrons• Orbiting the nucleus are electrons –negative electrical chargesAtom modelAtomic number• Identifying number• Number of protons • Determines the propertiesMass number• Protons + Neutrons • Nucleons• Not the same as Atomic Weight2Periodic 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 Au18879Nuclear NotationCl3517Atomic mass Number(neutrons+protons)Atomic number (protons)In class activity # 1• State the number of protons and neutronsU23592Cl3717RadioactivitySpontaneous breaking apart (decay) of atomic nucleiRadioactive decay• Parent atom– an unstable isotope• Daughter products – Formed from the decay of a parent atoms– Different element because of nuclear changes3Nucleus• 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”Types 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 protons• Lower energy of excited daughter nucleus• No charge, less mass than electron4Nuclear Decay Equation• Nucleons same on both sides• Alpha decay α removes Noand P+RaÆ22688RnHe+2228642Nuclear Decay Equation• Nucleons same on both sides• Beta decay β NoÆ P++ e-HÆ31Hee-+320-1In class activity #2• What type of radioactive decay is occurring in this nuclear decay equation?• Beta Decay−−+→ eNiCo0160286027In Class Activity # 3• Write a balanced nuclear decay equation for Fermium-250 that undergoes alpha decayFm250100Balancing Nuclear Equations• Number of nucleons the same on each side of equation•P+(protons) + e-(electrons) = No(neutrons)• Keep track of protons Æ elemental symbolUranium-238Note the atomic number decreases to right5Measuring Radioactivity• Radioactive decay strips electrons • Ions created • Geiger counter--charged wire, results in ‘clicks’ of counter• Others rely on visual reactions ionsHalf Life• Decay is random for any radioactive atom• BUT: Predictable for mass of material• One half of unstable parent material Ædaughter product: HALF LIFE• Decay rate constant, unaffected by external conditionsParent to daughter ratios Half Life equationAmount remaining can be calculated bynIR21=In class activity # 4• 400 mg of Co-60• Half life is 5.25 years• How much is left after 15.75 years?Radiometric dating• Known Half-life• Closed system• Cross-checked for accuracy • Yields numerical dates6Carbon-14 dating• Half-life 5760 years• Recent events• C-14 produced in upper atmosphere– Incorporated into carbon dioxide– Absorbed by living organisms• Useful tool for anthropologists, archeologists, historians, and geologists who study very recent Earth historyFormation of Carbon-14 Decay of Carbon-14C-14 dating cross and tree ring chronologyImportance of radiometric dating• Confirms the idea that geologic time is immense• Rocks from several localities have been dated at more than 3 billion years• Radiometric dating is a complex procedure that requires precise measurement7Geologic time scale• Divides geologic history into units• Originally created using relative dates• Bracket events and arrive at agesGeologic Time ScaleSubdivisions•Eon–Era•Period–EpochEonGreatest expanse of time• Four eons –Phanerozoic ("visible life") – the most recent eon–Proterozoic–Archean–Hadean – the oldest eonEras of the Phanerozoic eon• Cenozoic ("recent life")• Mesozoic ("middle life")• Paleozoic ("ancient
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