Question

Assume you began with 10 g of a radioactive parent isotope. How many grams of parent isotope will be present in the sample after 2 half-lives?

Answer 1

2.5g

10÷2=5
5÷2=2.5

everytime it completes a half life, you divide the number by two

Answer 2

2.5 grams

Step-by-step explanation:

In radioactive decay, one half-life is the time it takes for half of the radioactive sample to decay. So if it takes two hours (for example) for a sample to go from 8 grams to 4 grams, then one half-life has passed (since half the sample has decayed), and the half-life is two hours.

Calculation:

2 half-lives = sample halved twice

10 grams / 2 = 5 grams (one half-life has passed)

5 grams / 2 = 2.5 grams (two half-lives have passed)