By: HULK And Bruce Banner 7.2 Half-Lives By: 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 20𝑔 × 1 2 × 1 2 =5𝑔

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

Half-Life of Parent (y) Effective Dating Range (y) Common Isotope Pairs Isotope Half-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 Thorium-235 Lead-208 14 billion Rubidium-87 Strontium-87 47 billion

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 Which two isotopes get compared in radiocarbon dating? What is used to measure radioactive decay rate? Does a half-life change for a given radioisotope? What is a decay curve? What is the daughter isotope of uranium-235? What happens to the amount of argon-40 as the amount of potassium-40 decreases?

Answers Carbon-12 and carbon-14 Half-life No A graph of the decay of a radioisotope Lead-207 The amount of argon-40 increases