The atomic weights many elements are not invariant but depend on the origin and treatment of the material. The standard values of Aᵣ(E) and the uncertainties (in parentheses, following the last significant figure to which they are attributed) apply to elements of natural terrestrial origin. The footnotes to this Table elaborate the types of variation which may occur for individual elements and which may be larger than the listed uncertainties of values of Aᵣ(E).