Final answer:
After two half-life decays, 25% of parent radioactive nuclei would remain. The process is exponential decay, where half of the remaining parent nuclei decays after each half-life.
Step-by-step explanation:
If a radioactive isotope goes through two half-life decays, the amount of parent nuclei remaining would be 25% of the original amount. After the first half-life, 50% of the initial material remains, as each nucleus has a 50-50 chance of decaying within this time.
When the second half-life period has passed, this remaining 50% is again halved, which leaves us with 25%. It's an example of exponential decay, where with each half-life that passes, half of the remaining radioactive nuclei decay into their daughter elements.
For example, let's consider a substance with a half-life of 5 years. Starting with 100 grams of this isotope, after 5 years we would have 50 grams remaining. After another 5 years (making a total of 10 years and thus two half-lives), we would have 25 grams left. The decay is calculated as 100 x (1/2)2 (since two half-lives have passed), which equals 25 grams.