radiocarbon dating used to calculate the age of plant and animal remains while organism alive...
TRANSCRIPT
7.2 Half-LivesBy: HULK
And Bruce Banner
Radiocarbon Dating
• Used to calculate the age of plant and animal remains
• While organism alive carbon-14 to carbon-12 ratio stays same
• When organism dies, carbon-14 decays without being replenished
• Puny scientists like Bruce Banner measure ratio of carbon-14 to carbon-12 to find how old organism is
Rate of Radioactive Decay• Radioactive decay is measured using
half-lives• Half-life is constant for any
radioisotope• Half-life is the time it takes for half a
sample of a radioisotope to decay• Ex: Strontium-90 has a half-life of 29
years• If you had 100g of strontium, after 29
years there would be 50g left
Decay Curve
• Graph of decay of radioisotope• After each half-life, amount of
radioisotope drops by half
Decay Curve
• For example:• Iodine-131 is used for treating thyroid
cancer• It has a half-life of 8 days• Suppose you have 20g of iodine-131• You can find out how much will remain
after 16 days:• 16 days = 2 half-lives
Common Isotope Pairs
• Isotope that decays called parent isotope
• Stable product(s) of parent isotope's decay called daughter isotope(s)
• On next slide, some common isotope pairs that can be used for radioisotope dating
Common Isotope Pairs
IsotopeHalf-Life of Parent (y)
Effective Dating Range
(y)Parent Daughter
Carbon-14 Nitrogen-14 5730 Up to 50 000
Uranium-235 Lead-207 710 million > 10 million
Potassium-40 Argon-40 1.3 billion 10 000 to 3 billion
Uranium-238 Lead-206 4.5 billion > 10 million
Thorium-235 Lead-208 14 billion > 10 million
Rubidium-87 Strontium-87 47 billion > 10 million
Potassium-40 Clock• Potassium-40 has a half-life of 1.3 billion years• It’s daughter isotope is argon-40• When rock is created from lava, the argon-40 is
forced out leaving only potassium-40• After time the potassium-40 decays and creates
argon-40 which gets trapped in the rock• When scientists examine the rock they can use
the amount of argon-40 to find the age of the rock
• Since these rocks were probably formed at the beginning of the earth’s life, the age of the rock is close to the age of the earth
Potassium-40 Clock• As the amount of potassium-40 decreases, the amount of
argon-40 increases• Using the graph and the ratio of potassium-40 to argon-40,
scientists can find how old the rock is
Video
http://www.youtube.com/watch?v=KWAsz59F8gA
Quiz
1. Which two isotopes get compared in radiocarbon dating?
2. What is used to measure radioactive decay rate?
3. Does a half-life change for a given radioisotope?
4. What is a decay curve?
5. What is the daughter isotope of uranium-235?
6. What happens to the amount of argon-40 as the amount of potassium-40 decreases?
Answers
1. Carbon-12 and carbon-14
2. Half-life
3. No
4. A graph of the decay of a radioisotope
5. Lead-207
6. The amount of argon-40 increases