Neutron was discovered in 1932....then how did Mendeleev calculated the mass of the elements without knowing the neutrons in each element????
Answers
Mendeleev's Atomic masses were derived from experimental chemical measurements of combining weights of elements, not from atomic physics (experiment or theory).
Basically the atomic mass of any element was the mass of that element that combines chemically with 1 g of hydrogen. In the case that the element does not combine with hydrogen, or not conveniently, you can still get it by the mass that combines with that mass of some other element that combines with 1g of hydrogen.
(Gay-Lussac was mentioned. His work added clarification - It is very helpful to know that molecules of hydrogen gas are made of two atoms. But I believe that Mendeleev could still have produced his table without knowing this.)
Bad teaching has also been mentioned. Well it is true that your question comes from a hindsight definitive modern version of the system of the world. Which makes a fairly simple story – though not one that anyone would have invented out of their heads. It may well be that chemistry is often taught these days in this hindsight version. Reason for that could be that combining weights weights always used to be a basic of chemistry teaching (which is difficult to do, so it is not surprising that it can be criticised) but this involved a lot of boring exercises which students often found terribly difficult, For that reason the hindsight version maybe preferred.
I know why they found (find) it difficult. It involved an arithmetical principle called 'simple proportions'. The students usually managed to master this several years before starting chemistry. But then it was called 'Arithmetic', and was taught in arithmetic lessons. The students usually mastered this at the time. A few years later they meet the same thing again but because it is called 'chemistry' they can't do this arithmetic any more. Add to that that the chemical formulae look like a weird and arbitrary kind of algebra. I suggest to any teacher who is up against this, or any student for that matter, to picture on paper the various species of atom as little balls, white ball for a hydrogen atom, red ball for an oxygen atom, green ball for a chlorine atom, etc., Each atom with a number attached - its atomic mass. ( and maybe sticks for valencies.) You will then more easily see what mass of something combines with what mass of something else producing what mass of something else again.