(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.