George de Hevesy

This was something that was kind of known by chemists at that time.

And in fact, Bohr found out about it from a Hungarian chemist called George de Hevesy, who was studying also with Rutherford in Manchester.

And so what Hevesy said was actually there seem to be more kind of different elements or forms of elements than there are places for them in the periodic table.

And so Bohr suggested that maybe there are different forms of elements with somehow a kind of different structure for their nucleus, but they have the same structure for their electron shells so they're you know have the same chemical properties and these were called isotopes and you know no one really understood exactly what the differences were between isotopes at that point because that required the discovery of the neutron in the 1930s.