The halflife of an isotope is the time taken for exactly half of the isotope to transform, by radioactive decay, into another element. This is exponential decay. In one halflife interval, half of the original isotope has transformed itself into an isotope of another element. For each individual atom, this is a purely random event, with a particular probability of occurrence. After an interval of ten halflives, there will be just 1/1024th of that isotope left. Some isotopes, like tellurium130 which decays by double beta decay, have a half life of 2.4×10^{21} years, longer than the present age of the Universe at 13.4×10^{10} yrs, whilst others have halflives measured in fractions of a second.
The halflives of metastable nuclides are shown as solid cyan coloured histogram bars for metastable states that decay by other than internal transition (e.g. beta or inverse beta decay), and those for internal transition isomers as hollow cyan bars. Thus in the above halflives of Uranium example, the isotope with the longest halflife is U238, whilst the shortest halflive is possessed by that of U220. U235 has a metastable state, with a shorter halflife (shown in cyan) that the ground state (shown in blue). There are two isotopes with fertile metastable states, with halflives (much shorter that 1 microsecond) shown in magenta. Carbon14, with a halflife of 5730 years, is useful in radiocarbon dating once living organisms, as, once dead, the ratio of stable ^{12}C to radioactive ^{14}C changes exponentially with time. Several other longer lived isotope pairs are useful in dating rocks and minerals, ground water and polar icecores, like ^{16}O/^{18}O, ^{87}Rb/^{86}Sr and ^{87}Sr/^{86}Sr ratios. See RadioMetric Dating.
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