geologic time and absolute dating. review: atomic structure atom – basic unit of an element –...
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Geologic Time and Absolute Dating
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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
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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
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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…
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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…
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• 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
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Radioactive Decay and Popcorn…yummy
• Radioactive decay is a spontaneous and irreversible process• Ex. popcorn
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If the Decay of an Atom Occurs Randomly, How is it Useful to Us?
Sampleof actinium
Even 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 time
Actinium-227 has a half-life of 22 years
After 22 years, exactly half of the Atoms have decayed to thorium-227
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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
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Example
• 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
100 grams of Bob-12
400 My(one half-life)
50 grams Bob-12
50 gramsJoe-11+
= 100 g
25 gramsBob-12 +
75 grams Joe-11 = 100 g
400 My years(now twohalf-lives have passed)
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Good vs. Bad
• Isotopes with long half-lives are good for old rocks
• Young materials are best dated by short lived isotopes
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Commonly Used Isotopes