<Dd> A (Si) = (7001279769300000000 ♠ 27.976 93 × 0.922297) + (7001289764900000000 ♠ 28.976 49 × 0.046832) + (7001299737700000000 ♠ 29.973 77 × 0.030872) = 28.0854 </Dd> <P> The estimation of the uncertainty is complicated, especially as the sample distribution is not necessarily symmetrical: the IUPAC standard relative atomic masses are quoted with estimated symmetrical uncertainties, and the value for silicon is 28.0855 (3). The relative standard uncertainty in this value is 1 × 10 or 10 ppm . </P> <P> Apart from this uncertainty by measurement, some elements have variation over sources . That is, different sources (ocean water, rocks) have a different radioactive history, and so different isotopic composition . To reflect this natural variability, in 2010 IUPAC made the decision to list the standard relative atomic masses of 12 elements as an interval rather than a fixed number . </P>

The standard on which the atomic mass unit is based is the mass of a (1 point)