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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
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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
• 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
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…
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…
• 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
Radioactive Decay and Popcorn…yummy
• Radioactive decay is a spontaneous and irreversible process• Ex. popcorn
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
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
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)
Good vs. Bad
• Isotopes with long half-lives are good for old rocks
• Young materials are best dated by short lived isotopes
Commonly Used Isotopes