What is defined as the half-life of a radioactive substance?

The half-life of a radioactive substance is defined as the time required for half of the radioactive atoms in a sample to undergo decay. This concept is critical in understanding the behavior of radioactive materials, as it provides a measure of the stability and longevity of the substance.

When discussing radioactivity, it is important to note that during each half-life, only half of the remaining radioactive atoms decay. For example, if you start with a sample of 100 atoms, after one half-life, you would expect 50 atoms to remain; after another half-life, 25 would remain, and so on. This exponential decay process is a foundational principle in both nuclear science and applications such as nuclear medicine, radiation safety, and environmental studies.

In contrast, the other options misunderstand or misrepresent the concept of half-life. For instance, the idea of 'complete decay' inaccurately suggests a singular event rather than the ongoing process of decay that characterizes radioactive materials. Additionally, monitoring radioactivity is relevant to safety practices but does not directly pertain to the mathematical definition of half-life, further distinguishing the correct answer.