All the atomic masses of the elements are determined relative to 12C. The Mass of a neutral atom of a nuclide - also called "atomic weight." The atomic weight of an element is the weighted average of each isotope. Click on Term to Read More unit (AMU) is a precise number defined as exactly 1/12th the mass of a 12C (Element commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*. Click on Term to Read More) atom (1.9944235 × 10-23 g / 12 = 1.66057 x 10-24 g). Atomic mass units allow for determination of relative atomic masses of atoms for different elements. So, the AMU of Carbon is precisely 12 while that of 16O is 15.99491 AMU.