Half-life is the time it takes for half of a substance to decay. It varies widely and is constant for a given substance. It’s used in dating methods, pharmacology, and toxicology.
A half-life is the amount of time it takes for half of a given substance to decay. This measurement is applied to materials that undergo exponential decay, meaning that the rate of decay is directly proportional to the amount of the substance, slowing down as the substance is depleted. The half-life varies widely: it can be a few seconds or millions of years, depending on the stability of the substance. The concept has applications throughout the world of science.
A given substance always has the same half-life up to very small amounts, even if the rate of decay slows down over time. Imagine an element has a half-life of five minutes, for example. If we start with 20 ounces of it, five minutes later we will have 10 ounces remaining and in another five minutes only 5 ounces remaining. The decay rate is slowed from 2 ounces per minute to 1 ounce per minute, but the half-life is constant at five minutes.
Half-life is perhaps best known in the context of radioactivity. Radioactive “parent” elements transform into stable or nonradioactive “child” elements and emit radiation as they decay. Knowing the child element and the half-life of a radioactive element, therefore, allows us to date a partially decayed sample by comparing the ratio between the parent element and the child element. This dating method is commonly used to determine the age of various entities in our universe, from fossils to meteorites. Carbon-14, a radioactive element found in living materials on Earth, begins to decay upon death and can be used to date once-living materials such as bone and wood.
This measurement is also important in biology, where it indicates the amount of time that half of a given substance is metabolized and/or eliminated from the body. In this sense, the half-life is used in pharmacology to determine appropriate amounts and dosage ranges. It can also be applied in toxicology to determine the effects of toxins in the body over time.
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