If an isotope has a half-life of 4 million years and a fossil is 16 million years old, how much of the original isotope will be found in the fossil?
A) one-half of the original amount
B) one-quarter of the original amount
C) one-eighth of the original amount
D) one-sixteenth of the original amount

Question

Quizplus · Accepted Answer

Since the isotope has a half-life of 4 million years, after 4 million years, half of the original amount will remain. After an additional 4 million years (total of 8 million), half of that remaining amount will remain. Therefore, after 16 million years, which is four half-lives, the original amount will be divided by 2 four times, resulting in 1/2 × 1/2 × 1/2 × 1/2 = 1/16 of the original amount remaining. Therefore, the correct answer is D) one-sixteenth of the original amount.

If an Isotope Has a Half-Life of 4 Million Years