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ConcepTest: What percentage of parent isotopes remain?

ConcepTest questions by David McConnell , David Steer , Walter Borowski, Jeffrey Dick, Annabelle Foos, Jeffrey Knott, Alvin Konigsberg, Michelle Malone, Heidi McGrew, Kathie Owens, and Stephen Van Horn


The half-life of a radioactive isotope is 1,000 million years. Scientists testing a rock sample discover that the ratio of parent and daughter isotopes indicates that the rock is 3,000 million years old. What percentage of parent isotopes remain in the sample?

a. 40
b. 25
c. 12.5
d. 5

Student Responses:

No data yet. The correct response is c.

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References and Notes:


Geoscience:Geology:Historical Geology, Geochemistry:Radioisotopes

Resource Type

Activities:Classroom Activity:Short Activity:ConcepTests

Grade Level

College Lower (13-14):Introductory Level

Ready for Use

Ready to Use

Earth System Topics

Time/Earth History, Solid Earth:Earth Materials


Time/Earth History


Teach the Earth:Teaching Environments:Intro Geoscience, Teach the Earth:Teaching Topics:Early Earth, Teach the Earth:Course Topics:Geochemistry