ATOMIC MASS OF AN ELEMENT
History:

The atomic weight scale has traditionally been a relative scale, that is without an explicit unit, with the first atomic weight basis suggested by John Dalton in 1803 as ^{1}H.

Despite the initial mass of ^{1}H being used as the natural unit for atomic weight, It was suggested by Wilhelm Oswald that atomic weights would be best expressed in terms in units of 1/16 weight of oxygen.

The discovery of isotopic oxygen in 1929 led to a divergence in atomic weight representation, which could result in errors in computations.

The reference was changed to carbon12 in 1961.

Carbon12 atom has been assigned an atomic mass of exactly 12 atomic mass units.

Earlier abbreviated as ‘amu’, a new symbol ‘u’ was recommended by IUPAC.

The current unit is referred to as the "unified atomic mass unit" u.

One atomic mass unit is a mass unit equals to exactly (1/12^{th}) the mass of one atom of Carbon12.

The relative atomic masses of all elements have been found with respect to an atom of carbon12.

The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12^{th} the mass of one carbon12 atom.

It has been found experimentally that mass of carbon12 atom is 1.9926 × 10^{–23} g.

Unified atomic mass unit is 1/12 of mass of carbon12 atom, If this value is divided by 12, the absolute value comes to be 1.6605 × 10^{–24} g.
Atomic masses of few elements