Isotopes of chromium

Naturally occurring chromium is composed of four stable isotopes: ⁵⁰Cr (4.345%), ⁵²Cr (83.789%), ⁵³Cr (9.501%), and ⁵⁴Cr (2.365%).¹ The relative abundances are remarkably constant in terrestrial materials, but variations in ⁵³Cr/⁵²Cr ratios (up to ~0.7‰) have been observed in some meteorites and in high-temperature igneous rocks, reflecting mass-dependent fractionation during planetary differentiation.² Because ⁵³Cr is the decay product of extinct ⁵³Mn (half-life 3.7 million years), the ⁵³Cr/⁵²Cr ratio is used as a chronometer for early solar system events, such as the formation of chondrules and the differentiation of planetesimals.³

The longest-lived radioisotope is ⁵¹Cr, which decays by electron capture to ⁵¹V with a half-life of 27.7 days and emits gamma rays at 320 keV — a property exploited in medical imaging to label red blood cells and in industrial tracer studies.⁴ Other notable radioisotopes include ⁴⁹Cr (42.3 min), ⁵⁵Cr (3.5 min), and ⁵⁶Cr (5.9 min), all decaying via beta emission or positron emission. The neutron-rich isotopes ⁵⁷Cr to ⁷⁰Cr have been synthesized in particle accelerators and exhibit half-lives ranging from milliseconds to seconds; their masses and decay energies provide constraints on nuclear shell structure far from stability.⁵ Two metastable isomers, ^45mCr and ^49mCr, have been identified, with the latter decaying by internal transition with a half-life of about 13 minutes.

Stable chromium isotopes serve as tracers in environmental and geological studies. The ⁵³Cr/⁵²Cr ratio is used to monitor chromium contamination and to trace redox processes in groundwater, because reduction of Cr(VI) to Cr(III) preferentially removes lighter isotopes, leaving the residual Cr(VI) enriched in ⁵³Cr.⁶ In nuclear medicine, ⁵¹Cr-labeled red blood cells are used to measure red cell volume and survival time (the classic ⁵¹Cr method).⁴ In astrophysics, the ⁵³Mn–⁵³Cr system provides a high-resolution chronometer for early solar system materials, complementing other short-lived radionuclide chronometers.³

One of the least-known isotopes is ⁴⁸Cr, which is proton-rich and decays by positron emission with a half-life of 21.6 hours; it is produced in small quantities in supernova nucleosynthesis and has been studied for its role in the astrophysical p-process.⁵ Another curiosity is ⁵⁴Cr, which is theoretically unstable to double beta decay but has never been observed to decay; experiments set a lower limit on its half-life of >1.8×10¹⁹ years.⁷ The isotope ⁵⁰Cr is also potentially unstable, but its half-life is similarly unmeasured. In nuclear physics, the mass of ⁵²Cr serves as a reference for atomic mass evaluations, and its precise value is crucial for calibrating mass spectrometers.¹

Isotope

Variant of a chemical element with the same number of protons but a different number of neutrons.

Half-life

Time required for half of a radioactive substance to decay.

Electron capture

A decay mode in which an atomic electron is captured by the nucleus, converting a proton to a neutron.

Metastable isomer

An excited nuclear state with a measurable half-life before decaying to the ground state.