Rutherford’s Atomic Model
Rutherford,1911 performed an experiment in order to find the arrangement of electrons and protons in an atom.
Rutherford’s Gold Foil Experiment
Rutherford bombarded a very thin gold foil about 0.00004 cm thickness with alpha particles.
Alpha particles were obtained from the disintegration of polonium.
Alpha particles are helium nuclei. i.e. He ++ is doubly positively charged.
What do we observe?
1. Most of the alpha particles pass straight through the gold foil without any deviation from their original path.
2. Only a few particles are slightly deflected.
3. A very few particles, one in one million deviate through an angle greater than 90 0 from their original path.
Rutherford performed a series of experiments using thin foils of other elements and observed similar results from these experiments.
- Since most of the alpha particles pass straight through the gold foil without any deviation from their original path shows that there is a lot of empty space in an atom.
- Only a few particles have slightly deviated showing that massive alpha particles are not deflected by electrons.
- A very few particles, one in one million have deviated through an angle greater than 90 0 from their original path showing the electrostatic repulsion between positively charged alpha particles and the positively charged part of an atom (nucleus).
Planetary Model of an atom
On the basis of conclusions drawn from these experiments, Rutherford proposed a new model called the Planetary model for an atom ( similar to the solar system).
- An atom is a neutral particle.
- The mass of an atom is concentrated in a very small dense positively charged area. He named this area as Nucleus.
- The centre of an atom is a positively charged region, and electrons revolve around the nucleus in circular orbits. These circular paths are called orbits.
- The centripetal force due to the rotation of electrons balances the electrostatic force of attraction between the nucleus and electrons.
Defects in Rutherford Atomic Model
Rutherford’s model of an atom resembles our solar system.
Rutherford’s atomic model has the following defects:
Stability of Atom:
Classical physics shows that electrons, which are charged particles, continuously release energy as they revolve around the nucleus. Therefore, the orbital of the spin electron gets smaller and smaller until it falls into the nucleus. This destroys the atomic structure. So it cannot explain the stability of atoms.
If a revolving electron emits energy continuously, it should form a continuous spectrum of an atom, but instead, get a line spectrum.
When white light is passed through a prism it is scattered into 7 colours.
- Continuous spectrum:
- In a continuous spectrum, different colours are diffused into one another and not separated by dark spaces.
- Line Spectrum:
- In the line spectrum, different colours are not diffused into one another and are separated by dark spaces.