Radiometric dating using isotopes
The recognition that the rate of decay of any radioactive parent atom is proportional to the number of atoms ( Converting this proportion to an equation incorporates the additional observation that different radioisotopes have different disintegration rates even when the same number of atoms are observed undergoing decay.
In other words, each radioisotope has its own Stated in words, this equation says that the rate at which a certain radioisotope disintegrates depends not only on how many atoms of that isotope are present but also on an intrinsic property of that isotope represented by λ, the so-called decay constant.
Most elements exist in different atomic forms that are identical in their chemical properties but differ in the number of neutral particles—i.e., neutrons—in the nucleus.
For a single element, these atoms are called isotopes.
Because isotopes differ in mass, their relative abundance can be determined if the masses are separated in a mass spectrometer ( Radioactive decay can be observed in the laboratory by either of two means: (1) a radiation counter (e.g., a Geiger counter), which detects the number of high-energy particles emitted by the disintegration of radioactive atoms in a sample of geologic material, or (2) a parent atoms.
The particles given off during the decay process are part of a profound fundamental change in the nucleus.
Pursuing this analogy further, one would expect that a new basket of apples would have no oranges but that an older one would have many.Fossils are generally found in sedimentary rock not igneous rock.Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.This follows because, as each parent atom loses its identity with time, it reappears as a daughter atom. In short, one need only measure the ratio of the number of radioactive parent and daughter atoms present, and the time elapsed since the mineral or rock formed can be calculated, provided of course that the decay rate is known.
Likewise, the conditions that must be met to make the calculated age precise and meaningful are in themselves simple:isochron methods that make use of the rubidium-strontium or samarium-neodymium decay schemes, a series of rocks or minerals are chosen that can be assumed to have the same age and identical abundances of their initial isotopic ratios.
Values of λ vary widely—from 10) rather than through the decay constant λ.