The unit used to describe the mass of an atom is the unified atomic mass unit, symbolized as amu or μ.
The standard used for measuring the mass of an atom is a 12C atom. Carbon-12 (12C) is an isotope of Carbon with 6 protons, 6 neutrons, and 6 electrons.
The unified atomic mass unit is defined as exactly 1/12 th the mass of one Carbon-12 atom.
The mass of one Carbon-12 atom is 1.994 x 10-23 gm, then one unified atomic mass unit (1 amu) is:
1 amu = 1/12 mass of one C-12 atom
= 1/12 x 1.994 x 10-23 gm= 0.1661 x 10-23 gm= 1.661 x 10-24 gm
Therefore, 1 amu is significantly smaller than the mass of one Carbon atom.
In 1993, IUPAC, an international body developing nomenclature and terminology, suggested a shorter name, dalton (symbol Da), for the unified atomic mass unit.
Therefore,
1 amu= 1 Da