Why was hydrogen replaced by oxygen-16 as a reference Atom on atomic mass scale?
Firstly, the hydrogen atom was used as a standard of mass and set its atomic weight at 1. Others chemist accepted the idea of using a specific atom as a standard of mass, but they preferred a heavy element in order to reduce experimental error. So chemists used oxygen which have atomic weight 16. Oxygen was chosen because it forms chemical compounds with many other elements, simplifying determination of their atomic weights.