How much of a radioactive parent isotope will remain after three half-lives have passed?
A) one-third
B) three-halves
C) one-eighth
D) one-sixth

Question

Quizplus · Accepted Answer

After each half-life, half of the original parent isotope will decay. So after one half-life, there will be half left (1/2). After two half-lives, there will be half of half left (1/2 x 1/2 = 1/4). After three half-lives, there will be half of half of half left (1/2 x 1/2 x 1/2 = 1/8). Therefore, the answer is C, one-eighth.

How Much of a Radioactive Parent Isotope Will Remain After