Atomic Mass Unit (AMU)
The symbol for this is the lower case letter ' u '.
1 atomic mass unit, unified = 1 u = 1.6605655 x 10-27 kilograms.
It is 1/12 of the mass of a carbon-12 atom.
Hydrogen has a mass of 1.007825 atomic mass units
Uranium-238 has a mass of 238.0508 u.