I don’t know the history of the discovery of isotopes. I wouldn’t be surprised if atomic theory started with weights assigned by the most common isotopes, and further checking led to “Hey, what we thought was just one sort of atom for each element needs to be more sophisticated because we were almost cutting reality at the joints but not quite”.
That’s how it was. Atomic weights were known to be not in whole number ratios, although sometimes tantalisingly close to them, and a lot of effort went into determining them precisely. There was a certain amount of chagrin when scientists realised that these numbers had no fundamental significance, but were just the average weights of the distribution of the different isotopes.
I don’t know the history of the discovery of isotopes. I wouldn’t be surprised if atomic theory started with weights assigned by the most common isotopes, and further checking led to “Hey, what we thought was just one sort of atom for each element needs to be more sophisticated because we were almost cutting reality at the joints but not quite”.
That’s how it was. Atomic weights were known to be not in whole number ratios, although sometimes tantalisingly close to them, and a lot of effort went into determining them precisely. There was a certain amount of chagrin when scientists realised that these numbers had no fundamental significance, but were just the average weights of the distribution of the different isotopes.