The mass of a single atom is extremely small, i.e. the mass of a hydrogen atom is 1.6736 × 10-24g. Hence, it is not possible to weigh a single atom.
In the present system, mass of an atom is determined relative to the mass of an atom of carbon-12 as the standard. This was decided in 1961 by international agreement.
The atomic mass of carbon-12 is assigned as 12.00000 atomic mass unit (amu).
The masses of all other elements are determined relative to the mass of an atom of carbon-12 (C-12).
The atomic masses are expressed in amu which is exactly equal to one twelth of the mass of one carbon-12 atom.