Atomic mass: The standard atomic weight of an element is the average mass of the element written in atomic mass units (amu). ... We can choose selenium, which has an atomic number of 34, ...

One atomic mass unit, or amu, is equal to 1/12 the mass of a single carbon-12 atom. (To put that in perspective, a single carbon atom is roughly equal to 5.857 × 10^-26 ounces.) ...