Thanks so much for your explanation. But could i say that my above attachment is only correct when both the carbon-12 isotope and oxygen-16 isotope are under the same condition, for example both are in isolated form, same temperature and all other possible factors that affects the mass? And one more question, since you said that mass of the atoms are different when in different forms, because of this the unified atomic mass units is apparently a constant but precisely it is a value that keeps on fluctuating at but only at the position of the uncertainties. the same things for the relative isotopic mass of every isotope. They are always fluctuating but only with an extremely minute deficit, is it true??