Br81

Br81 usually denotes bromine-81, one of the two stable isotopes of bromine. Bromine has atomic number 35, so Br-81 represents a nucleus with 35 protons and 46 neutrons, giving a mass number of 81.

Natural bromine is a mixture of Br-79 and Br-81, with roughly equal abundances: Br-79 about 50.7% and Br-81 about 49.3% of the element in terrestrial sources.

Br-81 is a stable, non-radioactive isotope and has a nuclear spin of 3/2, making it a quadrupolar nucleus. This property leads to broad lines in standard nuclear magnetic resonance spectroscopy, which can complicate measurement of bromine-containing samples.

In analytical chemistry and geochemistry, Br-81 is used in isotopic ratio studies and mass spectrometry to determine isotopic composition of bromine in samples, trace bromine sources, and study environmental bromine cycling. Because it is stable, it does not decay and does not pose radiological risk.

Because of its nearly equal natural abundance, isotopic measurements typically compare Br-81 to Br-79 to detect subtle fractionation effects during chemical, biological, or geological processes.