After one half-life has elapsed, one half of the atoms of the substance in question will have decayed.
Many radioactive substances decay from one nuclide into a final, stable decay product (or "daughter") through a series of steps known as a decay chain.
Various methods exist differing in accuracy, cost and applicable time scale.
All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus.
Although decay can be accelerated by radioactive bombardment, such bombardment tends to leave evidence of its occurrence.
In most cases, the half-life of a nuclide depends solely on its nuclear properties; it is not affected by temperature, chemical environment, magnetic and electric fields, or any other external factors.The half-life of any nuclide is also believed to be constant through time.Radiometric dating is a technique used to date materials based on a knowledge of the decay rates of naturally occurring isotopes, and the current abundances.It is our principal source of information about the age of the Earth and a significant source of information about rates of evolutionary change.
In this case, usually the half-life reported is the dominant (longest) for the entire chain, rather than just one step in the chain.Nuclides useful for radiometric dating have half-lives ranging from a few thousand to a few billion years.