Breaking with a century-old tradition, the International Union of Pure & Applied Chemistry (IUPAC) is publishing a new table of standard atomic weights in which the values for 10 elements (H, Li, B, C, N, O, Si, S, Cl, Tl) will be given as mass ranges rather than single values. This historic change in format is being instituted to address the natural variation in a material’s isotope abundances, which depend on the sample’s physical, chemical, and nuclear history. For example, boron’s atomic mass until now has been given as 10.811 amu. The new tables will list the value as [10.806; 10.821] to reflect the element’s true atomic weight, which can vary over that range depending on the material’s source. “We’ve grown up thinking that standard atomic weights given in the periodic table are constants of nature,” says Tyler B. Coplen, an isotope specialist at the U.S. Geological Survey, in Reston, Va. Coplen, who is a coauthor of the IUPAC report highlighting these changes (Pure Appl. Chem., 10.1351/PAC-REP-10-09-14 ) and a companion paper in Chemistry International , adds that that fact holds true only for elements with just one stable isotope.