Question

Why oxygen -16 was replaced by carbon-12 as a reference atom

Answer 1

this was because the scientist did get the molecular mass in whole no. and it made calculations diff. so carbon-12 atom was taken as refrence atom.

Answer 2

Earlier Oxygen was used as the reference for mass measurements. Then the O17, O18 isotopes of Oxygen were also discovered being present in the natural air/oxygen. Carbon was being increasingly used as a reference in mass spectroscopy.  Choosing pure C-12 isotope as the reference made the changes in the masses of elements and compounds to be small. Choosing oxygen needed making bigger changes. It was preference of Physicists and chemists.