Tips on Dating

Why Date?

• Different methods of dating will help determine the actual age of a layer of rock or a fossil

• Scientists look at how much radioactive isotopes are left after years of decay

• Radioactive isotopes are isotopes that have have a different number of neutrons compared to the original element and give off radiation

• Eventually, they break down into more stable isotopes through radioactive decay

Parents vs. Daughters• The radioactive isotope is

also called the parent and is unstable, giving off radiation

• The parent decays and changes into a more stable isotope called the daughter

• To figure out the age of a rock or fossil, scientists compare the amount of parent and daughter

• The more daughter there is, the older the rock or fossil is!

Half-Life• By knowing the rate of decay of a radioactive isotope, the age

can be determined. This is called its half-life, which is the time it takes for half of the radioactive isotope to decay into the daughter

• With each half-life, the amount of parent remaining is cut in half • The three most common radioactive isotopes are uranium-238,

potassium-40, and carbon-14

Common Radioactive Isotopes & Their

Daughters• Uranium-238 decays into Lead-206.

Uranium’s half-life is 4.5 billion years, which means it takes 4.5 billion years for half of the uranium to decay!

• Potassium-40 decays into Argon-40. Potassium’s half-life is 1.3 billion years.

• Carbon-14 decays into Nitrogen-14. Carbon’s half-life is 5,730 years. Since all living things contain carbon, this method is used most often for organisms that lived within the last 50,000 years.

More About Half-Lives

• After 1 half-life, there is 50% (1/2) parent, 50% daughter

• After 2 half-lives, there is 25% (1/4) parent, 75% daughter

• After 3 half-lives, there is 12.5% (1/8) parent, 87.5% daughter

• After 4 half-lives, there is 6.25% (1/16) parent, 93.75% daughter