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