|Isotope||Atomic mass (Da)||Isotopic abundance (amount fraction)|
|175Lu||174.940 78(2)||0.974 01(13)|
|176Lu||175.942 69(2)||0.025 99(13)|
In 1961, the Commission recommended Ar(Lu) = 174.97 based on new mass-spectrometric determinations. The Commission also noted the close agreement with the chemical determinations Ar(Lu). In 1977, it took note of a new isotope-abundance determination, which was deemed more accurate but, like its predecessors, was not calibrated. Nevertheless, as a result, the more precise Ar(Lu) = 174.967(3) was recommended. Moreover, in 1981 the Commission was able to reduce the quoted uncertainty from 0.003 to 0.001. This change was justified because another abundance determination by agreed very well with earlier measurements. The Commission last revised the atomic weight of lutetium in 2007.
The minor isotope, 176Lu, is radioactive with a half-life of 3.57(14)×1010 a. In consequence, Ar(Lu) will change comparably with the current uncertainty (0.0001) in only about 5×108 a. At Oklo, the n(176Lu)/n(175Lu) ratio has been used as a sensitive measure of the average equilibrium temperature of the neutrons at the time of the nuclear reactions. The occurrence at this site of almost pure (99.7 %) isotope 175Lu justifies the annotation "g". The 176Lu–176Hf decay system is used as a geochronometer.
© IUPAC 2003