Scattering

If we want to investigate an unknown object we can study the outcomes and changes from an interaction with an object we know a lot about. For example we have a ball bearing and we know its mass, shape and its velocity in certain conditions. We can use the ball bearing determine the shape and size of an object hidden from our view. After the ball bearing collides with the unknown object we can measure its speed and trajectory. After recording the results from a number of collisions from different angles we could draw some conclusions about the size, shape and mass of the unknown object.

This process of finding information about an unknown object by studying interactions with known objects is called scattering. Scattering is often used to investigate objects such as atoms, which we cannot see. When we consider interactions between atomic particles we have to consider changes in the system due to charged particles as well as collisions.

Example of charge interactions
What would you expect to happen if a particle with mass 1 kg, charge +1 and velocity 10 m/s interacted with a particle of mass 1000 kg and charge +1 that was initially at rest?

What would you expect to happen if the large particle had a charge of -1 instead?

Rutherford's Experiment

In the late 1800's and early 1900's physicists knew relatively little about the structure of an atom. In 1897, J.J. Thompson at the Cavendish Laboratories, Cambridge University, discovered the electron. The electron was a negatively charged particle that appeared to be contained in all types of atoms.

Thompson's graduate student and colleague Ernest Rutherford conducted an experiment in 1911 to investigate the structure of gold atoms. It was a scattering experiment. The object he wanted to investigate was a gold atom, and his known object was an alpha particle. An alpha particle was a particle with a known mass, energy and charge of +1. Alpha particles are emitted from radioactive Radon.

The generally accepted model for the atom prior to 1911 was the so-called "plum pudding" model. A plum pudding is a common English dessert that is roughly spherical in shape with plums distributed throughout the dough. In the atomic model an atom consisted of spherical 'dough' of positively charged material, with the negatively charged electrons distributed throughout. In a Northern American context, the atom would be like a spherical chocolate chip cookie with the chocolate chips being the electrons.

Below is a simplified sketch of the experimental set up for Rutherford's experiment:

Rutherford counted the number of alpha particles detected at various scattering angles (q). Given that a gold atom is approximately 50 times the mass of an alpha particle, can you describe the scattering pattern expected from a plum pudding atom?

The graph below summarizes the results Rutherford obtained from his experiment.

Are these results consistent or inconsistent with a plum pudding model of the atom? Please explain why.

If you feel the results are inconsistent with a plum pudding model, can you propose new atomic model?