Isotope …, "m" are the atomic masses of each individual isotope, and "x", …, "x" are the relative abundances of these isotopes. Several applications exist that capitalize on properties of the various isotopes of a given element. Isotope separation is a significant technological challenge, particularly with heavy elements such as uranium or plutonium. Lighter elements such as lithium, carbon, nitrogen, and oxygen are commonly separated by gas diffusion of their compounds such as CO and NO. The separation of hydrogen and deuterium is unusual because it is based on chemical rather than physical properties, for example in the Girdler sulfide process.
Standard atomic weight "atomic" either, as it does not correspond to the property of any individual atom. The same argument could be made against "relative atomic mass" used in this sense. IUPAC publishes one formal value for each stable element, called the "standard atomic weight". Any updates are published biannually (in uneven years). The last change was published in 2015, setting a new value for ytterbium In 2017, no changes were published. The value published can have and uncertainty be an "interval" like for neon: , or can be an interval, like for boron: [10.806, 10.821]. Next to these 84 values, IUPAC also