one twelveth mass of C-12 isotope is taken to be 1 amu
earlier scientists took the mass of protium isotope of hydrogen as a reference to measure the mass of other atom
And it is also given that when they did so with hydrogen as reference they got fractional atomic mass for many atoms
but what is the difference between the earlier one and C-12
both are same and = 1 amu only so why did they get fraction with hydrogen and integer with carbon
pleae explain
All these choices are arbitrary. Nobody can do justice by summarizing 200 years of atomic weights (now atomic masses) history. Atomic reference scales have changed, earlier hydrogen was taken as 1 (exact) and sometimes oxygen was taken as 16 (exact). Other numbers have been chosen as well. When a mass spectrometer was invented, it started to change the story. It allowed physicists to measure "masses" from pure electricity and magnetism principles without relying on chemical reactions. They started their own scale. So chemists had one set of atomic masses, and physicists had another set of atomic masses. All of them were close but slightly different. This issue affected trade, international business and of course chemical industries. In order to overcome this issue, C-12 was suggested as compromise between chemist's and physicist's scale.
Must read for interested readers: The carbon-12 scale of atomic masses by Abbas Labbauf
earlier scientists took the mass of protium isotope of hydrogen as a reference to measure the mass of other atom And it is also given that when they did so with hydrogen as reference they got fractional atomic mass for many atoms but what is the difference between the earlier one and C-12 both are same and = 1 amu only so why did they get fraction with hydrogen and integer with carbon
Prout's (incorrect) hypothesis was that every element's atomic mass must be an integer multiple of hydrogen's atomic mass. Who stated that if we choose C-12 as the reference, then the atomic masses of other elements become integers?
