Atomic mass unit (amu) is defined as the mass unit equal to exactly 1/12th of the mass of one atom of C-12 isotope.
Atom may exists independently in free state (e.g. He, Ne and Ar) or in combined state. The combined form of atom may have similar kind of atoms (H2, N2, 02 etc.) or different kind of atoms (H20, C02,H2S etc).