Rutherford's Atomic Model (1911)

Rutherford performed an experiment in 1911 to find the internal structure of the atom.

The Experiment:

1. He used a very thin gold foil (0.00004 cm thick).
2. He fired alpha (α) particles from radioactive polonium at the gold foil.
3. A ZnS fluorescent screen was placed around the foil to detect the particles.
4. Observations:
5. 19,990 out of 20,000 particles passed straight through → Most of the atom is empty space.
6. 8 particles were deflected at small angles → Some positive charge exists inside.
7. 2 particles bounced straight back → A dense positive nucleus exists at the center.

Main Points of Rutherford's Model:

1. The atom has a nucleus at the center — it is positively charged and contains protons and neutrons.
2. Electrons revolve around the nucleus at very high speed — like planets around the sun. This is called the Planetary Model.
3. The nucleus is very small compared to the whole atom.
4. Most of the volume of an atom is empty space.
5. The atom is electrically neutral — number of electrons equals number of protons.
6. The nucleus is responsible for the mass and energy of the atom.

Defects / Drawbacks of Rutherford's Model:

1. His model was based on laws for neutral bodies, not for charged particles.
2. According to Maxwell's theory, revolving electrons should lose energy and fall into the nucleus — but this does not actually happen.
3. His model predicted a continuous spectrum, but in reality a line spectrum is observed.
4. It gave no explanation about the chemical properties of elements.