Tips on Dating

1. Tips on Dating

2. 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

3. 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 daughterthere is, the older the rock or fossil is!

4. 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

5. 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.

6. 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