Radiometric dating is also used to date archaeological materials, including ancient artifacts.
Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.
A particular isotope of a particular element is called a nuclide. That is, at some random point in time, an atom of such a nuclide will be transformed into a different nuclide by the process known as radioactive decay.
This transformation is accomplished by the emission of particles such as electrons (known as beta decay) or alpha particles.
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While the clocks cannot yield absolute dates for rocks, they can provide relative ages that allow us to compare any two rock units and know which one formed first.
They also allow us to compare rock units in different areas of the world to find which ones formed at the same time.
While the moment in time at which a particular nucleus decays is random, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques.
After one half-life has elapsed, one half of the atoms of the substance in question will have decayed.
Various methods exist differing in accuracy, cost and applicable time scale.