(a) Atomic mass unit can be defined as one twelfth (1/12) of the mass of an unbound atom of carbon-12. The atomic masses of all the elements are determined by comparing the mass of their atom with that of the carbon-12 atom.
Atomic mass unit = 1/12 the mass of a carbon-12 atom
(b) The symbol of atomic mass unit is “u”.
(c) Atomic mass of an element refers to the relative mass of its atom compared to the mass of a carbon-12 atom. (Mass of carbon-12 atom is taken to be 12 u).
(d) By saying that the atomic mass of oxygen is 16, we mean that the atom of oxygen is 16 times heavier compared to 1/12th the mass of a carbon-12 atom.