The important point that I want to now go on to talk about is that pretty much until this time, following Dalton, an atom of an element had a characteristic weight and all the atoms in... of that element... had that weight.

Once we had isotopes this varied and it explained, for example, why chlorine has an atomic weight of 35.5 – that couldn’t be explained on most ordinary theories until isotopes were discovered. And it was that chlorine had an isotope 35 and 37 and it was a mixture of the two of them, so 35.5 meant that there was more 35 than 37, but it was not a whole number. That was creating a problem. The problem of oxygen was there, but much worse, it turned out that as we did – or had – better and better results and measurements, it became clear that for many elements, the atomic weight did, in fact, vary, and it varied for one of several fairly simple reasons.