Question

select the correct answer.
the unstable isotope nickel - 59 can be used to date rocks that contain it. nickel - 59 decays into the more stable cobalt - 59. the nickel isotope has a half - life of 76,000 years. which statement about nickel - 59 is true?
a. after 76,000 years, all the nickel - 59 in a rock decays into cobalt - 59.
b. nickel - 59 can only be used to date rocks that are younger than 76,000 years.
c. all the nickel - 59 stabilizes after it completes one half - life period of 76,000 years.
d. the amounts of nickel - 59 and cobalt - 59 remain the same over a half - life period.
e. the amounts of nickel - 59 and cobalt - 59 tell how long ago the rock cooled to closure temperature.

Explanation:

• Option A: After one half - life (76,000 years), half of the nickel - 59 decays, not all. So this option is wrong.
• Option B: Nickel - 59 can be used to date rocks of various ages. The ratio of nickel - 59 to cobalt - 59 changes over time, and we can use the half - life formula \(N = N_0(\frac{1}{2})^{\frac{t}{T}}\) (where \(N\) is the amount of the radioactive substance at time \(t\), \(N_0\) is the initial amount, and \(T\) is the half - life) to date rocks of different ages. So this option is wrong.
• Option C: After one half - life, half of the nickel - 59 decays, and the other half remains. It does not all stabilize. So this option is wrong.
• Option D: Over a half - life period, the amount of nickel - 59 decreases by half, and the amount of cobalt - 59 (the decay product) increases. So this option is wrong.
• Option E: When a rock cools to closure temperature, the system (the ratio of nickel - 59 to cobalt - 59) is “closed” to exchange. The ratio of nickel - 59 (parent isotope) to cobalt - 59 (daughter isotope) can be used in radiometric dating to determine how long ago the rock cooled.

Answer:

E. The amounts of nickel - 59 and cobalt - 59 tell how long ago the rock cooled to closure temperature.