The absolute masses of the individual atoms of substances are extremely small. The calculation of amu in chemistry is used in order to avoid chemical calculations with the very small numerical values of the absolute atomic masses, a ratio was introduced – the relative atomic mass – and the absolute masses are compared with this unit of mass.
The relative atomic mass indicates how many times larger the mass of an atom is than the atomic mass unit.
Formula symbol: Ar
Unit: 1 (ratio)
The relative atomic mass of an element (numerically identical to the atomic weight) can be read from the periodic table of the elements.
The mass of individual atoms of an element is so small that it cannot be determined directly. However, one can determine mass relations between the atoms of different elements.
In the electrolysis of water, hydrogen and oxygen are obtained in a mass ratio of 1: 7.936, so the oxygen atom is 15.872 times as heavy as a hydrogen atom. Similarly, one can determine relations for the other elements through quantitative analysis of compounds and then create a mass scale of the elements using an arbitrarily chosen mass unit.
One could have established the mass of the hydrogen atom as the lightest element as a unit of mass. However, since almost all elements form oxygen compounds, for practical reasons the mass of 1/16 of the mass of oxygen was initially chosen as the unit of mass, so that the mass of hydrogen is approximately 1.
However, when it turned out that oxygen consists of an isotope mixture with 99.762% O-16, 0.038% O-17 and 0.200% O-18, this definition was no longer clear, because in physics one referred to 1/16 of the isotope O-16, but in chemistry to 1/16 of the isotope mixture of oxygen. Both values differ by a factor of 1,000275.
Therefore, from 1962, the new standard for the atomic mass unit 1/12 of the mass of the carbon isotope 12C was established. The designation is 1 amu (amu = atomic mass unit).
The absolute value of the unit of mass is obtained by dividing the unit by the Avogadro constant NA.
1 / 6.0221367 * 1023 = 1.6605402 * 10-24 g = 1.6605402 * 10-27 kg
The relative mass number of an element thus indicates how many times heavier the atoms of the element are on average than the atomic mass unit.
Non-integer mass numbers of elements mostly result from the existence of several isotopes of the element. The element chlorine with the mass number 35.453 has 2 isotopes, chlorine-35 with a share of 75.53% and chlorine-37 with 24.47%. The relative mass number of the element thus represents the weighted average.
For more example based explanation please read following article,