[dleviwing]

Muthu;
Atomic weight is a function on the periodic table that references all elements to hydrogen. At the time, it was not known that hydrogen had an isotope (deuterium). You shouldn’t confuse “atomic weight” with mass value even though it is basically a count of proton/electron equivalence. This is a chemistry thing and not a pure physics approach to mass. The notion that the atom would not change its mass nor its atomic weight is still true regardless of where in the space of the universe you wish to put it. I do agree that the numbers do not actually represent a quantitative measure of fundamental substance.
Regards;
David