What does the term "half-life" refer to?

The term "half-life" refers specifically to the time required for half of the radioactive nuclei in a sample to decay. This is a fundamental concept in nuclear chemistry and physics, representing a characteristic property of radioactive substances. During one half-life, only half of the original quantity of a radioactive isotope will have transformed into a different element or a stable form; the other half remains unchanged. This concept is crucial for understanding the stability of isotopes and predicting the behavior of radioactive materials over time.

The other options do not accurately represent the definition of half-life. The time it takes for a chemical reaction to complete pertains to reaction kinetics and is not related to radioactivity. Similarly, stating that it is the time required for all radioactive nuclei in a sample to decay would incorrectly imply that decay occurs completely in a finite timeframe, which is not the case, as the decay process continues over successive half-lives. The notion of a substance losing half its mass may confuse the context of mass and may not apply uniformly across different substances, particularly in nuclear chemistry, where mass changes are not the sole concern of half-life.