<P> 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 . Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus . A particular isotope of a particular element is called a nuclide . Some nuclides are inherently unstable . That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide . This transformation may be accomplished in a number of different ways, including alpha decay (emission of alpha particles) and beta decay (electron emission, positron emission, or electron capture). Another possibility is spontaneous fission into two or more nuclides . </P> <P> While the moment in time at which a particular nucleus decays is unpredictable, 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 nuclide in question will have decayed into a "daughter" nuclide or decay product . In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain, eventually ending with the formation of a stable (nonradioactive) daughter nuclide; each step in such a chain is characterized by a distinct half - life . In these cases, usually the half - life of interest in radiometric dating is the longest one in the chain, which is the rate - limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter . Isotopic systems that have been exploited for radiometric dating have half - lives ranging from only about 10 years (e.g., tritium) to over 100 billion years (e.g., samarium - 147). </P> <P> For most radioactive nuclides, the half - life depends solely on nuclear properties and is essentially a constant . It is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field . The only exceptions are nuclides that decay by the process of electron capture, such as beryllium - 7, strontium - 85, and zirconium - 89, whose decay rate may be affected by local electron density . For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time . This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present . </P> <P> The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation . The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created . It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration . Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body . Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron . This can reduce the problem of contamination . In uranium - lead dating, the concordia diagram is used which also decreases the problem of nuclide loss . Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample . For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3.6 ± 0.05 million years ago (MA) using uranium - lead dating and 3.56 ± 0.10 Ma using lead - lead dating, results that are consistent with each other . </P>

When are relative and absolute/radiometric dating used to date fossils