Atomic weights of the elements 2013 (IUPAC Technical Report)

The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 19 elements. The standard atomic weights of four elements have been revised based on recent determinations of isotopic abundances in natural terrestrial materials:

• cadmium to 112.414(4) from 112.411(8),

• molybdenum to 95.95(1) from 95.96(2),

• selenium to 78.971(8) from 78.96(3), and

• thorium to 232.0377(4) from 232.038 06(2).

The Commission on Isotopic Abundances and Atomic Weights (ciaaw.org) also revised the standard atomic weights of fifteen elements based on the 2012 Atomic Mass Evaluation:

• aluminium (aluminum) to 26.981 5385(7) from 26.981 5386(8),

• arsenic to 74.921 595(6) from 74.921 60(2),

• beryllium to 9.012 1831(5) from 9.012 182(3),

• caesium (cesium) to 132.905 451 96(6) from 132.905 4519(2),

• cobalt to 58.933 194(4) from 58.933 195(5),

• fluorine to 18.998 403 163(6) from 18.998 4032(5),

• gold to 196.966 569(5) from 196.966 569(4),

• holmium to 164.930 33(2) from 164.930 32(2),

• manganese to 54.938 044(3) from 54.938 045(5),

• niobium to 92.906 37(2) from 92.906 38(2),

• phosphorus to 30.973 761 998(5) from 30.973 762(2),

• praseodymium to 140.907 66(2) from 140.907 65(2),

• scandium to 44.955 908(5) from 44.955 912(6),

• thulium to 168.934 22(2) from 168.934 21(2), and

• yttrium to 88.905 84(2) from 88.905 85(2).

The Commission also recommends the standard value for the natural terrestrial uranium isotope ratio, N(²³⁸U)/N(²³⁵U)=137.8(1).