What term describes the average time required for half of a radioactive substance to decay?

The term that describes the average time required for half of a radioactive substance to decay is known as half-life. This concept is fundamental in nuclear physics and chemistry as it provides a measure of how quickly a radioactive isotope will lose its radioactivity over time. Each radioactive substance has a unique half-life, which is characteristic of its decay process.

In practical terms, after one half-life, only half of the original amount of the substance remains; after two half-lives, a quarter remains, and so forth. This predictable decay pattern makes half-life an essential concept in various applications, such as radiometric dating, nuclear medicine, and understanding atomic behavior.

The other terms refer to related, but distinct, concepts. For example, decay rate typically describes the frequency at which decay events occur, while the decay constant quantifies the probability of decay of a single atom per unit time. An isochron is used in geochronology to determine the age of rocks and minerals, often utilizing multiple isotopes to provide a more accurate time frame, but it is not directly related to the decay process of a single substance.