Conversion factor between atomic mass units and grams

                                            

Conversion factor between atomic mass units and grams

                        The standard scientific unit for dealing with atoms in macroscopic quantities is the mole (mol), which is defined arbitrarily as the amount of a substance with as many atoms or other units as there are in 12 grams of the carbon isotope C-12. The number of atoms in a mole is calledAvogadro's number, the value of which is approximately 6.022 × 10²³ mol⁻¹. One mole of a substance always contains almost exactly therelative atomic mass or molar mass of that substance (which is the concept of molar mass), expressed in grams; however, this is almost never true for the atomic mass. 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 grams. The atomic mass of an ⁵⁶Fe isotope is 55.935 u and one mole of ⁵⁶Fe will in theory weigh 55.935g, but such amounts of pure ⁵⁶Fe have never been found on Earth.

The formulaic conversion between atomic mass units and SI mass in grams for a single atom is:

where M_u is the Molar mass constant and N_A is the Avogadro constant.