Rutherford Atomic Theory

Rutherford, a student of J. J. Thomson modified the atomic structure with the discovery of another subatomic particle called “Nucleus”. His atomic model is based on the Alpha ray scattering experiment.

Alpha Ray Scattering Experiment

Construction:

• A very thin gold foil of 1000 atoms thick is taken.
• Alpha rays (doubly charged Helium He²⁺) were made to bombard the gold foil.
• Zn S screen is placed behind the gold foil.

Observations:

• Most of the rays just went through the gold foil making scintillations (bright spots) in the ZnS screen.
• A few rays got reflected after hitting the gold foil.
• One in 1000 rays got reflected by an angle of 180° (retraced path) after hitting the gold foil.

Conclusions:

• Since most rays passed through, Rutherford concluded that most of the space inside the atom is empty.
• Few rays got reflected because of the repulsion of its positive with some other positive charge inside the atom.
• 1/1000th of rays got strongly deflected because of a very strong positive charge in the center of the atom. He called this strong positive charge as “nucleus”.
• He said most of the charge and mass of the atom resides in the Nucleus

Rutherford’s Structure of Atom

Based on the above observations and conclusions, Rutherford proposed his own atomic structure which is as follows.

• The nucleus is at the center of an atom, where most of the charge and mass are concentrated.
• Atomic structure is spherical.
• Electrons revolve around the nucleus in a circular orbit, similar to the way planets orbit the sun.

Limitations of Rutherford Atomic Model

• If electrons have to revolve around the nucleus, they will spend energy and that too against the strong force of attraction from the nucleus, a lot of energy will be spent by the electrons and eventually, they will lose all their energy and will fall into the nucleus so the stability of atom is not explained.
• If electrons continuously revolve around the ‘nucleus, the type of spectrum expected is a continuous spectrum. But in reality, what we see is a line spectrum.