For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the human body. The original term, half-life period, dating to Ernest Rutherford's discovery of the principle in 1907, was shortened to half-life in the early 1950s.
Rutherford applied the principle of a radioactive element's half-life to studies of age determination of rocks by measuring the decay period of radium to lead-206.
Instead, the half-life is defined in terms of probability: "Half-life is the time required for exactly half of the entities to decay on average".
In other words, the probability of a radioactive atom decaying within its half-life is 50%.
It is incorporated into plants through photosynthesis, and then into animals when they consume plants.
The carbon-14 undergoes radioactive decay once the plant or animal dies, and measuring the amount of carbon-14 in a sample conveys information about when the plant or animal died.half life = [ time • ln (2) ] ÷ ln (beginning amount ÷ ending amount) half life = [ 11 • .69315 ] ÷ ln (326.04 ÷ 126) half life = [ 15.870 ] ÷ ln (2.5876) half life = 7.6247 ÷ .95073 half life = 8.0198 days A typical "half-life problem" might be worded: Tungsten 181 has a "k" value of -0.005723757/days. Half-Life = ln(.5) ÷ k Half-Life = -.693147 ÷ -0.005723757 Half-Life = 121.1 days Scroll down for 4 more half-life problems.) is the time required for a quantity to reduce to half of its initial value.The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo, or how long stable atoms survive, radioactive decay.The term is also used more generally to characterize any type of exponential or non-exponential decay.For example, the image on the right is a simulation of many identical atoms undergoing radioactive decay.Note that after one half-life there are not exactly one-half of the atoms remaining, only approximately, because of the random variation in the process.While a radioactive isotope decays almost perfectly according to so-called "first order kinetics" where the rate constant is a fixed number, the elimination of a substance from a living organism usually follows more complex chemical kinetics.For example, the biological half-life of water in a human being is about 9 to 10 days, though this can be altered by behavior and various other conditions.In that case, it does not work to use the definition that states "half-life is the time required for exactly half of the entities to decay".For example, if there is just one radioactive atom, and its half-life is one second, there will not be "half of an atom" left after one second.