The atomic mass of an element obtained from different sources changes slightly due to the variation in relative isotope concentration from one source to another. This is an example of uncertainty arising from a systematic error. Here, the distribution of atomic masses is rectangular, unlike the Gaussian distribution observed in random errors. This means there is an approximately equal probability of finding values between the lowest and highest values. The standard deviation or standard uncertainty calculated from the plot serves as a measure of uncertainty. The individual uncertainties from different atoms that make up a molecule contribute to the net uncertainty in the molecular mass. The propagation of systematic uncertainty in the molecular mass of a heteroatomic molecule is estimated by multiplying the number of identical atoms with their standard uncertainties, followed by summing up the squares of total standard uncertainties from all elements present in the molecule, and finally, finding the square root.