Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Evidence for the hollow atom

The main and first evidence for the hollow atom came from Rutherford's alpha scattering experiment. However, the first evidence students see for a hollow-atom often comes from cloud-chamber photographs. Although this may be historically back to front, it is reasonable to use the cloud chamber photographs as the first indication that atoms are mainly empty. 
Chronology of evidence 
Rutherford had devised his model of a nuclear atom by 1910 - before alpha particle tracks were photographed in cloud chambers (c1911). However, Rutherford and Wilson worked in the same laboratory so it is likely that Rutherford had seen tracks in cloud chambers. 
The evidence provided by cloud chamber photographs and the inferences that can be made are extremely useful whether you present them as preparation for the Rutherford model or follow-up support for it. 
Evidence from cloud chambers 
Most of the time there is just a straight track produced when an alpha particle passes through the cloud-chamber, producing ions. Mostly, these ions are produced by inelastic collisions with electrons in neutral particles. An alpha particle will have around 100,000 inelastic collisions before it runs out of kinetic energy. The number of collisions shows that electrons are easily removed. 
The straightness of the tracks shows that: 

  • an electron has a mass that is much smaller than the mass of an alpha particle (now known to be about 7000 times smaller). 
  • the atom is hollow: each straight track represents about 100,000 collisions without any noticeable deviation. All of these collisions missed anything with significant mass. During a session, the class might observe 1000 tracks between them – all of which are straight. 

Therefore, in all of these 100 million collisions with atoms, the alpha particles never hit anything with significant mass. So most of the atom is empty. 
However, students will see photographs that show large deflections of alpha particles. These are rare events (requiring thousands of photographs to be taken). They show that: 

  • there is something in an atom that has a mass that is similar to the mass of an alpha particle; only a target with a comparable mass could cause a large deviation. 
  • this mass is very concentrated; the rareness of the forked tracks shows that most alpha particles miss this massive target. 

Evidence from alpha particle scattering 
The hollowness of the atom is treated more quantitatively in the Rutherford scattering experiment. In this, 99.99% of the alpha particles are undeflected. This gives an indication of how tightly the positive charge of the nucleus is packed together.