James Chadwick · The neutron
Is there a particle with no charge?
That time, that place
Taking the nucleus to be made of protons did not add up.
A helium nucleus carries the charge of two protons and the mass of four. Where the other two units of mass came from could not be explained.
Why this question
In Germany, Bothe and Becker found that firing alpha particles at beryllium produced a highly penetrating radiation. In France, the Joliot-Curies interpreted it as gamma rays.
But something was off. The radiation knocked protons out of paraffin, and for gamma rays to knock out protons like that would require an impossible amount of energy.
What was found
Chadwick read it differently. What if this is not light but a particle of roughly a proton's mass carrying no charge?
He fired the radiation into various materials and measured the speeds of the nuclei it knocked out. As with colliding billiard balls, knowing the recoil speeds lets you compute the mass of what struck them.
The computed mass was almost exactly a proton's. The neutron.
The mass problem was solved — and something more consequential followed. Having no charge, a neutron feels no electrical repulsion from the nucleus. Here was a tool that could walk straight in.
The nucleus was made of protons alone
A mass back-calculated from the recoil speeds of nuclei in different materials
Six years later that tool led to the discovery of fission, and a few years after that to the atomic bomb