Atomic mass in 12 grams of the carbon isotope C-12. The number of atoms in a mole is called Avogadro's number, the value of which is approximately 6.022 × 10. One mole of a substance always contains almost exactly the "standard atomic weight" or "molar mass" of that substance; however, this may or may not be true for the "atomic mass," depending on whether or not the element exists naturally in more than one isotope. For example, the standard atomic weight of iron is 55.847 g/mol, and therefore one mole of iron as commonly found on earth has a mass of 55.847
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