|Isotope||Atomic mass (Da)||Isotopic abundance (amount fraction)|
The atomic weight of lead is quite variable in nature because the three heaviest isotopes are the stable end-products of the radioactive decay of uranium (238U to 206Pb and 235U to 207Pb) and thorium (232Th to 208Pb). In fact, the variability of Ar(Pb) had been incontrovertibly shown before the discovery of isotopes and the isotopic composition of common lead must now be regarded as a variable mixture of primeval and radiogenic components.
Recognizing this, in 1961, the Commission recommended an atomic weight of 207.19 that was based on the chemical measurements, and stated that "...it quite well represented the lead most likely to be encountered in normal laboratory work". However, the Commission's policy now aims for the implied range of the standard atomic weights to cover all normal sources of an element. In the 1969 report, the Commission considered natural variations in the atomic weight of lead ranging from 207.184 to 207.293 and recommended the value of Ar(Pb) = 207.2(1). These circumstances justify the annotation "r". In addition, the annotation "g" warns of the existence of abnormal sources outside the implied range.
The decay of uranium and thorium to lead permits geological age determinations to be made of minerals containing the heavy radioactive elements. Extensive use of lead over the history of mankind has led to widespread pollution, and the isotope-abundance variations reflected in the atomic weights enable historical and modern sources to be identified.
© IUPAC 2003