History of Life:Origins of LifeChapter 14-3

Age of Earth• The earth is about 4.5 billion years old• How did we measure that?• Radiometric Dating = calculating the age

of an object by measuring proportions of radioactive isotopes

Radiometric Dating• Radioactive Isotope = an unstable form of an element,

decays into stable element, gives off energy (radiation)• Ex. Carbon-14 decays into Nitrogen • Ex. Potassium-40 decays into Argon-40

Radiometric Dating• Half-life = the time it

takes for half of a radioactive isotope to decay• Ex. K-40 half life is 1.3

billion years

Practice• You are determining the age of an organic object

using carbon-14 dating. You know that the half-life of carbon-14 is 5,730 years. If only 25% of the original amount of carbon-14 is left in the object, approximately how old is the object?

Carbon-14 half life = 5,730 yearsWhen ½ is left then 5,730 years have gone byWhen ¼ is left then an additional 5,730 years have gone byTherefore the object is 11,460 (5,730 + 5,730) years old

Your Turn To Try

• You know that the half-life of K-40 is 1.3 billion years. If only 12.5% of the original amount of K-40 is left in the object, approximately how old is the object?• 3.9 billion years old

• If the half-life of a radioactive isotope is 4,000 years, how much of the radioactive isotope in a specimen will be left after 12,000 years?• 1/8 or 12.5%