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Geologic Time and Absolute Dating

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.

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Geologic Time and Absolute Dating

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  1. Geologic Time and Absolute Dating

  2. 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

  3. Example: Krypton • Krypton’s atomic number is 36, therefore • A neutral krypton atom has 36 protons and 36 electrons • If atomic number ≠ 36 ≠ krypton • The number of neutrons =atomic number-atomic weight (rounded up)=48 • The number of neutrons can vary without altering the identity of an atom—Isotopes

  4. Isotopes • Isotopes are like people—some are stable; some are not • And, like people, it’s the unstable ones that attract our attention the most Stable… Seriously unstable…

  5. Radioactivity • Unstable isotopes are radioactive—their nuclei will decay over time • A any radioactive isotope is called a “parent” isotope • The decay product is called the “daughter” isotope • When an isotope decays, they do so in one of three ways…

  6. Alpha emission • Nucleus emits two protons and two neutrons • Plutonium-240 decays to uranium-236 • Beta emission • Nucleus emits an electron • Radium-228 decays to Actinium-228 • Electron capture • An atom’s nucleus captures an electron which reacts with a proton creating a neutron • Carbon-11 decays to Boron-11 • In a nutshell: When the nucleus decays, a new, more stable isotope is created Electron capture

  7. Radioactive Decay and Popcorn…yummy • Radioactive decay is a spontaneous and irreversible process • Ex. popcorn

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

  9. Half-lives • We can use the half-life of an isotope to figure out the age of a rock • How can we do this? • Half-lives are constant • Actinium-227 always has a half-life of 22 years • As the parent decays the daughter accumulates • Older samples = higher number of daughter isotopes

  10. Example 100 grams of Bob-12 • If 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 years • How old is our sample if only 25 grams of Bob-12 remain? • Our sample is 800 My old 400 My (one half-life) 50 grams Bob-12 50 grams Joe-11 + = 100 g 400 My years (now two half-lives have passed) 25 grams Bob-12 75 grams Joe-11 + = 100 g

  11. Good vs. Bad • Isotopes with long half-lives are good for old rocks • Young materials are best dated by short lived isotopes

  12. Commonly Used Isotopes

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