# In carbon dating which isotope of carbon is used

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.

Nevertheless, when there are many identical atoms decaying (right boxes), the law of large numbers suggests that it is a very good approximation to say that half of the atoms remain after one half-life.

) is the time required for a quantity to reduce to half 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.

In a medical context, the half-life may also describe the time that it takes for the concentration of a substance in blood plasma to reach one-half of its steady-state value (the "plasma half-life").

The relationship between the biological and plasma half-lives of a substance can be complex, due to factors including accumulation in tissues, active metabolites, and receptor interactions.

The term is also used more generally to characterize any type of exponential or non-exponential decay.

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.

Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a characteristic unit for the exponential decay equation.

Perhaps a puddle of a certain size will evaporate down to half its original volume in one day.

But on the second day, there is no reason to expect that one-quarter of the puddle will remain; in fact, it will probably be much less than that.

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.

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".