Rutherford's Gold Foil Experiment (HL IB Physics)

• Evidence for the structure of the atom was discovered by Ernest Rutherford at the beginning of the 20th century from the study of alpha particle scattering

• The experiment consisted of beams of high-energy alpha particles fired at thin gold foil and a detector on the other side to determine
  • The different angles of deflection of the alpha particles
  • The number of alpha particles that were deflected at each angle

Apparatus for the Rutherford Scattering Experiment

• The setup for the scattering experiment consisted of:
  • A source of alpha particles in a lead container
  • A thin sheet of gold foil
  • A movable detector 
  • An evacuated chamber

Experimental set up for α-particle scattering

Purpose of the lead container

• Alpha particles are emitted in all directions, so the source was placed in a lead container
• This was to produce a collimated beam of alpha particles
• This is because alpha particles are absorbed by lead, so a long narrow hole at the front allowed a concentrated beam of alpha particles to escape and be directed as needed

Purpose of the thin sheet of gold foil

• The target material needed to be extremely thin, about 10⁻⁶ m thick
• This is because a thicker foil would stop the alpha particles completely
• Gold was chosen due to its malleability, meaning it was easy to hammer into thin sheets

Purpose of the evacuated chamber

• Alpha particles are highly ionising, meaning they only travel about 5 cm before interacting with molecules of air
• So, the apparatus was placed in an evacuated chamber
• This was to ensure that the alpha particles did not collide with any particles on their way to the foil target

Findings from the Rutherford Scattering Experiment

• An alpha (α) particle is the nucleus of a helium atom, so it has a positive charge

When α-particles are fired at thin gold foil, most of them go straight through but a small number bounce straight back

• The observations from Rutherford's experiment were:

A. The majority of α-particles passed straight through the foil undeflected

• • This suggests the atom is mostly empty space

B. Some α-particles deflected through small angles of <10°

• • This suggests there is a positive nucleus at the centre (since two positive charges would repel)

C. Only a small number of α-particles deflected straight back at angles of >90°

• • This suggests the nucleus is extremely small and is where most of the mass and charge of the atom are concentrated
  • This led to the conclusion that atoms consist of small, dense positively charged nuclei surrounded by negatively charged electrons

An atom: a small positive nucleus, surrounded by negative electrons

• (Note: The atom is around 100,000 times larger than the nucleus!)