What is 1 amu? From where it is derived? Why it is compared with carbon-12? Why it was once compared with hydrogen-1? and now its not? Can anyone please explain me logically? And what would have happened if other elements were used? May be my question is wrong. If it is, can anyone point my mistake out?
Originally, atomic mass had been just relative and without any units. In 1803, John Dalton decided to use hydrogen-1 as a basis for atomic mass, but Wilhelm Ostwald disagreed later on, and said that relative atomic mass should be measured relative to 1/16 of an oxygen-16 atom. This was, however, before elemental isotopes were discovered.
When isotopes of oxygen were discovered in 1923, relative atomic masses had different representations based on the isotopes used. In physics, pure oxygen-16 was given an RAM of 16 amu, whilst chemists gave this value to naturally occurring, isotopically-weighted oxygen.
These different values led to errors in computations, and so in 1961, the reference standard was changed to carbon-12. Carbon-12 was used to stop any more divergence in values for RAM. The new (and current) unit became u, the "unified atomic mass unit".
Therefore, the atomic mass unit is 1/12 of carbon-12, and so the mass of carbon-12 is 12.