The Discovery of the Nucleus

• Rutherford fired a narrow beam of alpha particles, all of the same kinetic energy, at a thin gold foil. The scattered alpha particles could be detected by a detector that was able to move round at a constant distance from the point of impact of the beam. Rutherford used a microscope to observe the pinpoints of light emitted when the alpha particles hit the fluorescent screen
• Rutherford's results showed that:

  • Most of the alpha particles passed straight through the gold foil with no deflection
  • About 1 in 2000 alpha particles were deflected by small angles
  • About 1 in 10,000 alpha particles were deflected by more than 90*
• Rutherford concluded that:

  • Most of the atom is empty space with the majority of the atom's mass is concentrated in a small region, the nucleus, at the centre of the atom
  • The nucleus is positively charged (because it repels positively charged alpha particles that approach too closely)
• Particle C rebounds back at 180* because it collides head-on and is repelled back by the electrostatic repulsion of the positively charged nucleus
• Particles A, B and D are deflected through different angles. The closer the initial direction of an a-particle to a 'head-on' direction,

  • The greater its deflection because the electrostatic force of repulsion between the a-particle and the nucleus increases with decreased separation between them
  • The smaller the least distance of approach of the a-particle to the nucleus is
• Particle E is undeflected it does not approach the nucleus closely enough to be significantly deflected