Question

which definition describes a half-life?
○ the time required for half of a radioactive sample to decay
○ the time required for half of a radioactive sample to become unstable
○ the time required for all of a radioactive sample to change because of heat
○ the time required for all of a sample to break down due to pressure change

Explanation:

• For the first option: By definition, the half - life of a radioactive substance is the time it takes for half of the radioactive nuclei in a sample to undergo radioactive decay.
• For the second option: Radioactive samples are already unstable (that's why they decay), so this option misrepresents the concept.
• For the third option: Half - life is about decay of a radioactive sample, not all of it changing due to heat (heat - related changes are different from radioactive decay).
• For the fourth option: Half - life has nothing to do with pressure - induced breakdown of a sample.

Answer:

A. The time required for half of a radioactive sample to decay