Atomic masses are very small. To express atomic masses a unit called atomic mass unit (amu; symbol, u) is commonly used. The atomic mass unit is defined as follows:

The mass equal to 1/12th of the mass of 126 C atoms is called one atomic mass unit.

1 atomic mass unit = Mass of a126 C atom/ 12

Absolute mass of a 126 C atom is 1.9924× 1023 g. Therefore,


1 atomic mass unit = 1.9924×1023 g /12

1 atomic mass unit = 1.66 ×1024 g = 1.66×1027 kg

The atomic mass unit is not a practical unit, because it is equivalent to a very small mass. 1 atomic mass unit is equal to 1.66×1027 kg or 1.66 ×1024 g. Such small masses cannot be measured only any balance. According to IUPAC, the atomic and molecular masses are defined as follows: