Lutetium

Isotope	Atomic mass (Da)	Isotopic abundance (amount fraction)
¹⁷⁵Lu	174.940 777(8)	0.974 01(13)
¹⁷⁶Lu	175.942 692(8)	0.025 99(13)

In 1961, the Commission recommended A_r(Lu) = 174.97 based on new mass-spectrometric determinations. The Commission also noted the close agreement with the chemical determinations A_r(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 A_r(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, A_r(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.

SOURCE Atomic weights of the elements: Review 2000 by John R de Laeter et al. Pure Appl. Chem. 2003 (75) 683-800
© IUPAC 2003

CIAAW

Lutetium
A_r(Lu) = 174.9668(1) since 2007

The name derives from Lutetia, the ancient name for the city of Paris. The discovery of lutetium is credited to the French chemist Georges Urbain in 1907 although it had been separated earlier and independently by the Austrian chemist Carl Auer (Baron von Welsbach) from an ytterbium sample.
Von Welsbach had named the element cassiopeium after the constellation Cassiopeia. However, because Urbain published his results before Auer, his name for the element was adopted by IUPAC in 1949.