Relative Atomic Mass

Atom is an extremely small particle. It is not possible to weigh it directly. Therefore we take its mass by comparing with slected Standard Atomic mass of C-12 (the  stable light isotope of carbon)
Atomic mass is defined as the mass of one atom of the element compared with the mass of one atom of C-12.

It is a number which tells how heavy or light an atom is as compared to the mass of another atom.
The atomic mass of "Mg" is 24 which mean each atom of Mg is twice as heavy as Carbon atom.

A Swedish chemist J. Berzelius (1779-1848) determined the atomic masses of elements.
Atomic is the average of the masses of all the isotopes of an element with respect to their relative abundance.
It is determined by multiplying the masses of all the isotopes which relative abundance and then add them up and finaly divide them by 100.
For example:- Average Atomic Mass of Cu =63.55 a.m.u

Mass spectrometer is a technique by means of which the relative abundance and atomic mass of the element can be determined by Mass Spectrometer.

 Unit of Atomic Mass:-
Average mass of an atom is not absolute mass it is a relative mass hence has no unit. It is unit less quantity.

Atomic Mass Unit (a.m.u):-Generally atomic masses are expressed in atomic mass units (a.m.u). This is unit that describes the mass of a single atom. One single atom of Carbon has a mass of 12 a.m.u. This is very small. It may be defined as 1/12th part of the mass of one atom of C-12.

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