structural:scholar_edition
Isotopes

Naturally occurring chromium is composed of four stable isotopes; Cr, Cr, Cr and Cr, with Cr being the most abundant (83.789% natural abundance). Cr is observationally stable, as it is theoretically capable of decaying to Ti via double electron capture with a half-life of no less than 1.3 × 10 years. Twenty-five radioisotopes have been characterized, ranging from Cr to Cr; the most stable radioisotope is 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 less than 1 minute. Chromium also has two metastable nuclear isomers. The primary decay mode before the most abundant stable isotope, Cr, is electron capture and the primary mode after is beta decay.

Cr is the radiogenic decay product of Mn (half-life 3.74 million years). Chromium isotopes are typically collocated (and compounded) with manganese isotopes. This circumstance is useful in isotope geology. Manganese-chromium isotope ratios reinforce the evidence from Al and Pd concerning 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 isotopic composition 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. Cr has been posited as a proxy for atmospheric oxygen concentration.