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Isotopes

Osmium has seven naturally occurring isotopes, five of which are stable: Os, Os, Os, Os, and (most abundant) Os. At least 37 artificial radioisotopes and 20 nuclear isomers exist, with mass numbers ranging from 160 to 203; the most stable of these is Os with a half-life of 6 years.

Os undergoes alpha decay with such a long half-life (2.0 ± 1.1) × 10 years, approximately 140 000 times the age of the universe, that for practical purposes it can be considered stable. Os is also known to undergo alpha decay with a half-life of (1.12 ± 0.23) × 10 years. Alpha decay is predicted for all the other naturally occurring isotopes, but this has never been observed, presumably due to very long half-lives. It is predicted that Os and Os can undergo double beta decay, but this radioactivity has not been observed yet.

Os has a spin of 5/2 but Os has a nuclear spin 1/2. Its low natural abundance (1.64%) and low nuclear magnetic moment means that it is one of the most difficult natural abundance isotopes for NMR spectroscopy.

Os is the descendant of Re (half-life 4.56 × 10 years) and is used extensively in dating terrestrial as well as meteoric rocks (see Rhenium–osmium dating). It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons. This decay is a reason why rhenium-rich minerals are abnormally rich in Os. However, the most notable application of osmium isotopes in geology has been in conjunction with the abundance of iridium, to characterise the layer of shocked quartz along the Cretaceous–Paleogene boundary that marks the extinction of the non-avian dinosaurs 65 million years ago.