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Naturally occurring chromium is composed of four stable isotopes; Cr, Cr, Cr, and Cr with Cr being the most abundant. Cr is suspected of decaying by ββ to Ti with a half-life of 1.8×10 years. Twenty-two radioisotopes, all of which are entirely synthetic, have been characterized with the most stable being Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute, the least stable being Cr with a half-life of 10 milliseconds. This element also has 2 meta states, Cr, the more stable one, and Cr, the least stable isotope or isomer.
Cr is the radiogenic decay product of Mn. Chromium isotopic contents are typically combined with manganese isotopic contents and have found application in isotope geology. Mn-Cr isotope ratios reinforce the evidence from Al and Pd for the early history of the Solar System. Variations in Cr/Cr and Mn/Cr ratios from several meteorites indicate an initial Mn/Mn ratio that suggests Mn-Cr isotope systematics must result from in-situ decay of Mn in differentiated planetary bodies. Hence Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the Solar System. The same isotope is preferentially involved in certain leaching reactions, thereby allowing its abundance in seawater sediments to be used as a proxy for atmospheric oxygen concentrations.
The isotopes of chromium range from Cr to Cr. The primary decay mode before the most abundant stable isotope, Cr, is electron capture and the primary mode after is beta decay.