Radioactive Decay

What percentage of radioactive atoms will be left after four half lives?

• 25.0%

• 12.5%

• 6.25%

• 3.125%

If the half-life of carbon is 5,700 years, 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 40,000 years ago?

• carbon-14

• potassium-40

• uranium-238

• rubidium-87

A fossil formed 11,400 years ago. Which percentage of the original amount of carbon-14 remains in the fossil? Remember, the half-life of carbon-14 is 5,700 years.

• 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

• decay into organic substances

• decay at a predictable rate

How much of an 800-gram sample of potassium-40 will remain after 3 half-lives has passed?

• 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

If a sample of a radioactive substance is crushed the half-life of the substance will

• decrease

• increase

• remain the same

An archeologist found an ancient skeleton estimated to be 10,000 to 25,000 years old. Which radioactive isotope would be most useful for finding the age of the skeleton?

• carbon-14

• potassium-40

• uranium-238

• rubidium-87