Parent isotope — Science definition
The radioactive isotope that decays.
Where it is taught
- Radiometric Dating & Half-Life — Earth’s History, Earth & Space Science, Grade 9
Questions using parent isotope
Problem 1
A sample retains 12.5% parent isotope. How many half-lives have elapsed?
Answer and explanation
3 half-lives
100 → 50 → 25 → 12.5.
Problem 2
A closed sample retains 25% of its original parent isotope. Half-life is 8 million years. Find elapsed time. Explain the reasoning in your own words.
Answer and explanation
16 million years under the supplied closed-system assumptions.
100%→50%→25% is two half-lives.
2×8 million=16 million years.
Response criteria
- Use the stated conditions.
- Show the evidence or calculation.
- Explain the conclusion and any stated limitation.
Problem 3
A sample retains 25% parent isotope with a 50-million-year half-life. Find its model age.
Answer and explanation
100 million years
Two half-lives × 50 = 100.
Problem 4
What percentage of parent isotope remains after four half-lives?
Answer and explanation
6.25 %
100 × (1/2)^4 = 6.25.