Question

why oxygen was considered as relevant atom to calculate relative atomic Mass of other elements
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Answer 1

Answer:

The actual masses of individual oxygen atoms are not whole numbers even though their mass numbers are whole numbers. When protons and neutrons combine to form the oxygen nucleus, a very small amount of the total mass is converted to nuclear binding energy. However, we cannot predict the amount of nuclear binding energy just by counting the numbers of protons and neutrons. It must be experimentally determined using a mass spectrometer. For example, since the mass of one atom of oxygen-16 is 1.33291 times the mass of one carbon-12 atom, the mass of one oxygen-16 atom should be equal to 1.33291 x 12 = 15.99492 atomic mass units.