Who first split the atom's nucleus in 1917-1919 experiments?

Rutherford, who fired alpha particles into nitrogen gas and detected hydrogen nuclei coming out - the first deliberate transformation of one element into another, which is alchemy achieved by a physicist who thought chemistry was stamp collecting.

He had already found the nucleus itself in 1909, by firing alpha particles at gold foil and expecting them all to pass through. A tiny fraction bounced straight back, which he described as being as incredible as if you had fired a fifteen-inch shell at a sheet of tissue paper and it had come back and hit you - and the only explanation was that the atom's mass and charge sat in something very small and dense at the centre.

He also declared, in 1933, that anyone who talked about power from atomic transformation was talking moonshine. Leo Szilard read the report the next morning, was irritated by it, and worked out the idea of a neutron chain reaction while waiting to cross a road in Bloomsbury - which is a fair reminder that the people who understand a field best are not reliably the best at predicting what it will do. Rutherford's Nobel Prize was in chemistry, which he found funny.