james Chadwick   

A series of observations in 1931 gave rise to the solution; Walter Bothe and Herbert Becker found that if you bombard beryllium, boron or lithium with alpha-radiation (we’ll talk about what that is later on), the material radiated out something that didn’t react to magnetic fields. Because they thought that all particles either had a positive or negative charge and thus should be affected by the magnetic field, they assumed the outgoing radiation was gamma-radiation. Rutherford and Chadwick weren’t convinced and Chadwick performed a series of experiments showing that the radiation was a stream of neutrally charged particles that had about the same mass as the proton. This discovery of the neutron earned Chadwick the Nobel prize in 1935.

Bohr’s modified model (again)

With the discovery of the neutron, Bohr’s model had to be slightly modified to include neutrons and it is this model that everybody uses to imagine atoms and with which most of the chemistry and physics involving the atoms can be explained. We’ll be using this model in the remainder of this booklet.

The strange reality that is quantum mechanics

Bohr’s modified model is a good tool but the reality is far weirder. When Chadwick discovered the neutron, quantum mechanics was already being developed and it had helped the scientists on quite a few occasions to rule out certain hypotheses and point the experiments in the right direction. The uncertainty principle of Heisenberg showed that nuclear electrons were impossible saving a lot of time looking for these particles. With more and more experiments proving that quantum mechanics was indeed true, the scientific view of the atom had to change. It meant that protons, neutrons and electrons could no longer be seen as little balls at a specific place but as a cloud of places that the particle might be found when you looked. When you look, you’ll find it at one of those places but when you don’t look, it is in none of these places and at all of those places at the same time.

Einstein rejected that idea and got in a heated argument with Bohr about it. Einstein is supposed to have said: “This is nonsense; the moon is there even when I’m not looking!”. Einstein was later to be proven wrong about this. 

Assignments

28.     Who discovered the neutron?

29.     What would have been the difference between a normal electron and a nuclear electron?

30.     How did Bothe and Becker know that the detected radiation had no charge?

Chadwick was able to prove that the particles in this radiation had a mass.

31.     How does this prove that this radiation wasn’t gamma-radiation?

32.     Why did Einstein get in a fight with Bohr?