Although, Dalton’s atomic theory remains till today as one of the most brilliant triumphs of scientific vision, but in the light of later developments in science, it has been modified.
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 relative atomic mass is denoted by Ar. The relative atomic mass is a pure number, and hence it has been no unit.
The average mass of an atom of an element is known as its atomic mass. Atomic mass of an element is designated as A. Since, atomic mass is actual mass, hence it has the unit of mass, viz, gram, kilogram, atomic mass unit, etc.