|Isotope||Atomic mass (Da)||Isotopic abundance (amount fraction)|
|138La||137.907 12(3)||0.000 8881(71)|
|139La||138.906 36(2)||0.999 1119(71)|
In 1961, the Commission recommended a value of Ar(La) = 138.91, based on the recalculation of chemical determinations and mass-spectrometric determinations. In 1969, the Commission recommended Ar(La) = 138.9055(3) based upon the same determinations, taking into account that two additional digits were justified in view of the small influence any mass discrimination could have on the atomic weight of this nearly monoisotopic element. In effect, the chemical determinations were no longer considered significant. In 1985, the Commission re-examined the available data and decided to revise Ar(La) to 138.9055(2), and then to 138.905 47(7) in 2005.
The minor isotope 138La is radioactive with a half-life of 1.06(4)×1011 a, with 138Ba and 138Ce as decay products. The atomic weight of all three elements remains unaffected even over geologic time periods. The "g" annotation arises from the presence of naturally occurring fission products found in fossil reactors at Gabon, south-west Africa.
© IUPAC 2003