(455) Geiger-Marsden-Rutherford experiment [IB Physics SL/HL]

Introduction to the Guer Marsten Rutherford Experiment

  • Focused on the discovery of the atomic nucleus.

  • Set in the late 1890s when knowledge about electrons was limited.

Plum Pudding Model

  • Model proposed that the atom is a "blob" of positive charge with electrons dispersed throughout.

  • Scientists at the time believed that the atom was overall positive with electrons scattered evenly.

  • This model was reasonable based on their observations but was flawed.

Alpha Particles

  • Alpha particles are helium-4 atoms with a positive charge.

  • Scientists expected alpha particles to pass through a thin sheet of gold without deflection due to the evenly distributed charge.

The Experiment

  • Conducted in 1909, researchers fired alpha particles at a thin gold foil.

  • Gold (Au) has an atomic number of 79 and a mass number of 197, which implies 118 neutrons.

  • The expectation was for most alpha particles to pass through undeflected.

Experimental Results

  • Results were surprising: While most alpha particles did pass through, many were deflected at various angles, and some even bounced back.

  • This was likened to firing a bullet at a paper and having it come back.

Conclusions from the Experiment

  • The deflection of alpha particles indicated the presence of a nucleus: a small, dense, positively charged center in the atom.

  • Led to the conclusion that the atom is mostly empty space.

  • This experiment provided crucial evidence for the existence of the atomic nucleus.