Final answer:
For a radioactive sample of 500 grams, after two half-lives, 125 grams would remain, with the rest having decayed into daughter elements.
Step-by-step explanation:
When dealing with radioactive decay, after each half-life period, half of the original mass of the radioactive sample remains. Given a starting mass of 500 grams of a radioactive sample, after one half-life, 250 grams would remain. After two half-lives, only 125 grams would remain, as each half-life period reduces the remaining sample by half. The mass of the radioactive sample does not disappear; it is transformed into decay products, also known as daughter elements, while the original radioactive atoms are referred to as parent atoms.