Geologic Time and Absolute Dating

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Geologic Time and Absolute Dating. Review: Atomic Structure. Atom Basic unit of an element Composed of protons and neutrons (nucleus) surrounded by electrons The identity of an atom is determined by the number of protons the atom has. Example: Krypton. - PowerPoint PPT Presentation


  • Geologic Time and Absolute Dating

  • Review: Atomic StructureAtomBasic unit of an elementComposed of protons and neutrons (nucleus) surrounded by electronsThe identity of an atom is determined by the number of protons the atom has

  • Example: KryptonKryptons atomic number is 36, thereforeA neutral krypton atom has 36 protons and 36 electronsIf atomic number 36 kryptonThe number of neutrons =atomic number-atomic weight (rounded up)=48The number of neutrons can vary without altering the identity of an atomIsotopes

  • Isotopes

    Isotopes are like peoplesome are stable; some are notAnd, like people, its the unstable ones that attract our attention the mostStableSeriously unstable

  • RadioactivityUnstable isotopes are radioactivetheir nuclei will decay over timeA any radioactive isotope is called a parent isotopeThe decay product is called the daughter isotopeWhen an isotope decays, they do so in one of three ways

  • Alpha emissionNucleus emits two protons and two neutronsPlutonium-240 decays to uranium-236Beta emissionNucleus emits an electronRadium-228 decays to Actinium-228Electron captureAn atoms nucleus captures an electron which reacts with a proton creating a neutronCarbon-11 decays to Boron-11In a nutshell: When the nucleus decays, a new, more stable isotope is created

    Electron capture

  • Radioactive Decay and PopcornyummyRadioactive decay is a spontaneous and irreversible processEx. popcorn

  • If the Decay of an Atom Occurs Randomly, How is it Useful to Us?Sampleof actiniumEven a small sample is composed of billionsof actinium atoms (Ac-227)All isotopes of actinium are unstable and willdecay over time. Since every atom has a certain probability of decaying, on average, half of theatoms in a given sample will decay to a (more) stable daughter isotope over a set period of timeActinium-227 has a half-life of 22 yearsAfter 22 years, exactly half of the Atoms have decayed to thorium-227

  • Half-livesWe can use the half-life of an isotope to figure out the age of a rockHow can we do this?Half-lives are constantActinium-227 always has a half-life of 22 yearsAs the parent decays the daughter accumulatesOlder samples = higher number of daughter isotopes

  • ExampleIf we have a rock with 100 grams of a particular isotope (Bob-12)Bob-12 decays to Joe-11 and has a half-life of 400 Million yearsHow old is our sample if only 25 grams of Bob-12 remain?Our sample is 800 My old100 grams of Bob-12400 My(one half-life)50 grams Bob-1250 gramsJoe-11+= 100 g25 gramsBob-12+75 grams Joe-11= 100 g

    400 My years(now twohalf-lives have passed)

  • Good vs. BadIsotopes with long half-lives are good for old rocksYoung materials are best dated by short lived isotopes

  • Commonly Used Isotopes


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