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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 (see below Use of mass spectrometers).

Half-life is defined as the time period that must elapse in order to halve the initial number of radioactive atoms.

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In fact, one would expect that the ratio of oranges to apples would change in a very specific way over the time elapsed, since the process continues until all the apples are converted. A particular rock or mineral that contains a radioactive isotope (or radio-isotope) is analyzed to determine the number of parent and daughter isotopes present, whereby the time since that mineral or rock formed is calculated.Of course, one must select geologic materials that contain elements with long half-lives— those for which some parent atoms would remain.To compensate for the loss of mass (and energy), the radioactive atom undergoes internal transformation and in most cases simply becomes an atom of a different chemical element.In terms of the numbers of atoms present, it is as if apples changed spontaneously into oranges at a fixed and known rate.Given below is the simple mathematical relationship that allows the time elapsed to be calculated from the measured parent/daughter ratio.

The age calculated is only as good as the existing knowledge of the decay rate and is valid only if this rate is constant over the time that elapsed.

Radioactive decay can be observed in the laboratory by either of two means: (1) a radiation counter (parent atoms.

The particles given off during the decay process are part of a profound fundamental change in the nucleus.

The importance of internal checks as well as interlaboratory comparisons becomes all the more apparent when one realizes that geochronology laboratories are limited in number.

Because of the expensive equipment necessary and the combination of geologic, chemical, and laboratory skills required, geochronology is usually carried out by teams of experts.

Fortunately for geochronology the study of radioactivity has been the subject of extensive theoretical and laboratory investigation by physicists for almost a century.