CBSE Class 9 Answered

Dear student

 

From about 1900 until 1961, oxygen was used as the reference standard, with an assigned value of 16.

The unit of atomic mass was thereby defined as 1/16 the mass of an oxygen atom.

In 1929 it was discovered that natural oxygen contains small amounts of two isotopes slightly heavier than the most abundant one and that the number 16 represented a weighted average of the three isotopic forms of oxygen as they occur in nature. This situation was considered undesirable for several reasons, and, since it is possible to determine the relative masses of the atoms of individual isotopic species, a second scale was soon established with 16 as the value of the principal isotope of oxygen rather than the value of the natural mixture. This second scale came to be known as the physical scale, and the earlier scale continued in use as the chemical scale, favoured by chemists, who generally worked with the natural isotopic mixtures rather than the pure isotopes.

Although the two scales differed only slightly, the ratio between them could not be fixed exactly, because of the slight variations in the isotopic composition of natural oxygen from different sources. It was also considered undesirable to have two different but closely related scales dealing with the same quantities.

 

 We hope that clarifies your query.