Now there is a unit that scientists created to deal with very, very small amounts of atoms. Now the AMU, or atomic mass unit is the unit of mass used for elements and compounds. Now by definition we define that one atom, a single atom of carbon. Twelve is composed of twelve AMU and everything else is relative to this amount. For example, an H atom is one AMU and it's relative to this. In other words, this is twelve times that's heavier in terms of AMU than an H atom.