Radioactive carbon dating wiki
A quantity of carbon-14 will decay to half of its original amount (on average) after 5,730 years, regardless of how big or small the original quantity was.After another 5,730 years, one-quarter of the original will remain.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%.A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radiological activity.
Perhaps a puddle of a certain size will evaporate down to half its original volume in one day.
The decay of many physical quantities is not exponential—for example, the evaporation of water from a puddle, or (often) the chemical reaction of a molecule.
In such cases, the half-life is defined the same way as before: as the time elapsed before half of the original quantity has decayed.
However, unlike in an exponential decay, the half-life depends on the initial quantity, and the prospective half-life will change over time as the quantity decays.
As an example, the radioactive decay of carbon-14 is exponential with a half-life of 5,730 years.