Question

How did the Conservation of Momentum allow Chadwick to prove the existence of the neutron when it was believed that the combination of protons and electrons alone made atoms?

Answer 1

According to the conservation of linear momentum, if two particles of equal masses collide elastically, they completely exchange their velocities. This means that if one particle of mass 'm' moving with speed 'v' strikes another particle of same mass at rest, then, after collision, the first particle will come to rest and the second particle will move with the speed 'v'.

In the experiment done by Bothe and Becker, it as observed that when beryllium foil is bombarded with alpha particles, it ejects an uncharged radiation. Later, it was found that when this uncharged radiation is bombarded on paraffin slab, protons are ejected from it. These protons energy is around 5.6 MeV. People thought that the uncharged radiation is actually gamma rays. But to eject protons, gamma rays energy had to be at least 55 MeV, way more higher than the protons' energy.

Chadwick used the conservation of linear momentum argument that the uncharged radiation is neutral particle (neutron) whose mass is almost equal to the mass of proton. He argued that the neurton strikes protons at rest and because their masses are same, neutron transfers its speed to the proton and itseld comes to rest.