What is meant by atomic mass, gram atomic mass of an element? Why is the mass have different expressions i.e., ‘u’ and ‘g’?

The atoms are very tiny and their individual mass cannot be calculated as it is negligible. Hence, the mass of atoms is expressed in units with respect to a fixed standard. Initially, a hydrogen atom with mass 1 was taken as a standard unit by Dalton. Later, it was replaced by an oxygen atom (0=16). But due to the isotopes, the masses were found in fractions instead of the whole number. Hence, carbon (C=12) isotope was taken as the standard unit and was universally accepted.

Gram atomic mass: When the atomic mass of an element is expressed in grams, it is called the gram atomic mass of the element.

The mass of atoms and molecules are expressed in ‘u’ and the mass of moles i.e., the molar mass is expressed in g.