Radioactive Decay

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

• 25.0%

• 12.5%

• 6.25%

• 3.125%

How many half-lives have passed if a sample contains 25% of its original carbon-14?

• 1 half-life

• 2 half-lives

• 3 half-lives

• 4 half-lives

Radioactive carbon-14 is often useful in determining the absolute age of geologic samples because radioactive isotopes

• decay at a regular rate

• become less stable during decay

• remain unchanged over time

• stabilize after four half-lives

Scientists infer that early North American humans hunted the mastodon. Carbon-14 dating of the rib bone indicates that 2.4 half-lives have passed since the mastodon was killed. Approximately how many years ago did the mastodon die?

• 5700

• 11,400

• 13,700

• 17,100

Which radioactive isotope takes the greatest amount of time to disintegrate?

• potassium-40

• uranium-235

• uranium-238

• rubidium-87

Radioactive decay of ⁴⁰K atoms in an igneous rock has resulted in a ratio of 25 percent ⁴⁰K atoms to 75 percent. ⁴⁰Ar and ⁴⁰Ca atoms. How many years old is this rock?

A) B) C) D)

• 0.3 X 10⁹ years

• 1.3 X 10⁹ years

• 2.6 X 10⁹ years

• 3.9X 10⁹ years