Question

Givr one relevant reason why scientists choose 1/6th of mass of an atom of naturally occurring oxygen as atomic mass unit.

Answer 1

In the 20th century, until the 1960s chemists and physicists used two different atomic-mass scales. The chemists used a "atomic mass unit" (amu) scale such that the natural mixture of oxygen isotopes had an atomic mass 16, while the physicists assigned the same number 16 to only the atomic mass of the most common oxygen isotope (O-16, containing eight protons and eight neutrons). However, because oxygen-17 and oxygen-18 are also present in natural oxygen this led to two different tables of atomic mass. The unified scale based on carbon-12, 12CX12X2212C, met the physicists' need to base the scale on a pure isotope, while being numerically close to the chemists' scale.