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
Isotopes of roentgenium Isotopes of roentgenium Roentgenium (Rg) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was Rg in 1994, which is also the only directly synthesized isotope; all others are decay products of nihonium, moscovium, and tennessine, and possibly copernicium, flerovium, and livermorium. There are 7 known radioisotopes from Rg to Rg. The longest-lived isotope is Rg with a half-life of 2.1 minutes, although the unconfirmed Rg and Rg may have a longer half-life of about 5.1 minutes and 10.7 minutes respectively.