What are the observations of Rutherford's ray scattering experiment. Write down the conclusions drawn from it about the structure of an atom. [2.5+2.5]

Observations of Rutherford’s α-Ray Scattering Experiment

Rutherford’s α-ray scattering experiment (also called the gold foil experiment) involved bombarding a thin gold foil with a beam of α-particles (positively charged helium nuclei). The key observations were:

1. Most α-particles passed straight through the foil

   • Nearly all α-particles (about 99.9%) traveled through the gold foil without any deflection, suggesting that most of the atom is empty space.

2. A few α-particles were deflected by small angles

   • Some α-particles were scattered at small angles (less than 90°), indicating that they encountered weak repulsive forces from positively charged regions within the atom.

3. Very few α-particles rebounded or were deflected by large angles (even > 90°)

   • Approximately 1 in 8000 α-particles bounced back toward the source, showing that a tiny fraction encountered a strong repulsive force from a highly concentrated positive charge.

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Conclusions Drawn from the Experiment

Based on these observations, Rutherford proposed the nuclear model of the atom, leading to the following conclusions:

1. Most of the atom is empty space

   • The fact that most α-particles passed undeflected suggested that atoms consist mostly of empty space, with no uniform distribution of positive charge.

2. Presence of a tiny, dense, positively charged nucleus

   • The large-angle deflections (including backscattering) implied that the positive charge and most of the atom’s mass are concentrated in a very small region—the nucleus—at the center of the atom.
   • The nucleus contains protons (later discovered) and accounts for nearly all the atomic mass.

3. Electrons revolve around the nucleus at a distance

   • Since α-particles were not repelled by electrons (due to their negligible mass), Rutherford inferred that electrons must occupy the space outside the nucleus, moving in orbits (though Bohr later refined this with quantized energy levels).

4. The atom is electrically neutral

   • The positive charge of the nucleus balances the negative charge of the orbiting electrons, making the atom overall neutral.

5. The nucleus is extremely small compared to the atom

   • The radius of the nucleus (10⁻¹⁵ m) is about 100,000 times smaller than the atomic radius (10⁻¹⁰ m), meaning the atom is mostly empty space.

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