The number at the top is how many half-lives have elapsed.

Note the consequence of the law of large numbers: with more atoms, the overall decay is more regular and more predictable.

A half-life usually describes the decay of discrete entities, such as radioactive atoms.

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.

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

The accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed.

Simulation of many identical atoms undergoing radioactive decay, starting with either 4 atoms per box (left) or 400 (right).

For example, if there are 3 radioactive atoms with a half-life of one second, there will not be "1.5 atoms" 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".

In other words, the probability of a radioactive atom decaying within its half-life is 50%.

) 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.