Half-Life

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

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

• 25.0%

• 12.5%

• 6.25%

• 3.125%

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?

• 0.3 x 10^9 years

• 1.3 x 10^9 years

• 2.6 x 10^9 years

• 3.9 x 10^9 years

The solid black sections of the bars on the graph represent the percentages of

• carbon-14 from the original sample that has not decayed

• lead-206 decay product resulting from the radioactive decay

• nitrogen-14 decay product resulting from the radioactive decay

• uranium-238 from the original sample that has not decayed

After 22,800 years, approximately what percentage of the original carbon-14 remains?

• 15%

• 12.5%

• 6.25%

• 3.125%

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 × 10^9 years of radioactive decay?

• 50 grams

• 100 grams

• 200 grams

• 400 grams

Which diagram best represents the percentage of this radioactive isotope sample that will remain after 2 half-lives?

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A sample of wood that originally contained 100 grams of carbon-14 now contains only 25 grams of carbon-14. Approximately how many years ago was this sample part of a living tree?

• 2,850 years

• 5,700 years

• 11,400 years

• 17,100 years

A bone sample contains only 1/4 of its original radioactive C¹⁴ content. How old is the bone sample?

• 1 C¹⁴ half-life

• 2 C¹⁴ half-lives

• 3 C¹⁴ half-lives

• 4 C¹⁴ half-lives