CBSE Class 9 Answered

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. Later, 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. Chemists assigned natural oxygen -- which is a mixture of 16O, 17O and 18O -- a mass of exactly 16. On the other hand, physicists measuring the masses of individual atoms or molecules, assigned a mass of exactly 16 to 16O itself. This led to discrepencies in the third or fourth significant figure, and eventually the conflict was resolved by adopting a compromise in 1961. 12C would be assigned a mass of exactly 12. This placed the new scale approximately midway between the two old, oxygen-based scales. Hence, carbon-12 is used as the standard reference for measuring atomic mass.