WHY WAS THE ATOMIC MASS OF 1 ATOM CHANGED FROM 1ATOM OF OXYGEN TO 1/12 THE ATOM OF CARBON?

The reference was basically changed due to existence of other isotopes of oxygen. When it was decided to use oxygen as reference, it was not known that isotopes of oxygen also exist. Natural oxygen also contains small percentage of other isotopes as well. So, it was found that the atomic mass of 16 actually represents the average of atomic masses of 3 isotopic forms of oxygen. As a result, a second scale was devised in which the atomic mass of principal isotope of oxygen (and not mixture) was taken as 16. This scale, known as physical scale, was preferred by physicists. Thus while chemists used the previous scale, the second scale was being used by physicists. 

In other words, at the same time two different scales were being used, which let to ambiguity in measurements. This led to carbon being chosen as a standard for calculating the atomic masses of different elements. 

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because oxygen was found in many forms in our atmostphere

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because oxygen has atomic mass16u and carbon has12u which is lesser than carbon.

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