Radioactive Decay

What percentage of rubidium-87 atoms will be left after four half lives?

• 25.0%

• 12.5%

• 6.25%

• 3.125%

How old is a bone that has 12.5% of the original amount of radioactive carbon-14 remaining?

• 5,700 years

• 11,400 years

• 17,100 years

• 22,800 years

Which radioactive isotope is most often used when determining the age of fossil bones found in sediments deposited during the Holocene Epoch?

• carbon-14

• potassium-40

• uranium-238

• rubidium-87

Due to radioactive decay, an igneous rock sample now contains one-fourth of the amount of potassium-40 that it originally contained. The age, in years, of this rock sample is approximately

• 0.7 x 10⁹ years

• 1.3 x 10⁹ years

• 2.6 x 10⁹ years

• 5.2 x 10⁹ years

A fossil formed 11,400 years ago. Which percentage of the original amount of carbon-14 remains in the fossil?

• 100%

• 50%

• 25%

• 12.5%

Radioactive dating of fossils and rocks is possible because radioactive isotopes

• are found in all fossils and rocks

• are easily collected and measured

• disintegrate into organic substances

• disintegrate at a predictable rate

Which radioactive element is used to determine the absolute age of late Pleistocene animal remains?

• rubidium-87

• uranium-238

• potassium-40

• carbon-14

How much of an 800-gram sample of potassium-40 will remain after 3.9 × 10⁹ years of radioactive decay?

• 50 grams

• 100 grams

• 200 grams

• 400 grams

How old is a fossil that has radioactively decayed through 4 half-lives of carbon-14?

• 5,700 years

• 17,100 years

• 22,800 years

• 28,500 years

A whalebone that originally contained 200 grams of radioactive carbon-14 now contains 25 grams of carbon-14. How many carbon-14 half-lives have passed since this whale was alive?

• 1

• 2

• 3

• 4