1 atomic mass unit is eqal to the mass of one-twelth of mass of one atom of carbon atom. one atomic mass unit is exactly equal to one-twelfth the mass of carbon isotope-12. it is taken with reference to atomic mass of one twelfth of carbon isotope 12. One amu (or dalton) is exactly one twelfth of the mass of an atom of the most common form of carbon. 1 amu = 1.0078...