Rutherford’s Atomic Model – Model, Observations and Conclusion
Ernest Rutherford’s carried out an experiment to verify various models of atom proposed by Dalton and Thomson. He carried out Alpha particle scattering experiments in 1909. The alpha particle is a helium atom from which 2 electrons are removed.
Radioactive source of the alpha particle was placed in the cavity of the lead block. The 𝛂 particle emitted from this source were passed through lead slit system to give a fine beam of alpha particles
This beam of particles was bombarded on a thin gold foil of thickness 100 nm. The gold foil was surrounded by a fluorescent screen of ZnS. The screen was there for detection of alpha particles.
|Alpha (𝛂) particle scattering experiments in 1909|
- Most of the alpha particles passed through the gold foil without deflection
- Some of the alpha particles i.e. out of about 8000, one 𝛂 particle was deflected through a small angle, less than 90 degree
- A very few numbers of alpha particles i.e. out of about 30000, i.e. 𝛂 particle was deflected by a large angle greater than 90 degree
- A very very few, i.e. one alpha particle out of ten million 𝛂 particles, was deflected through an angle of 180 degree
- The atom consists of largely empty space.
- The positively charged alpha particles get repelled and deflected by the positive charges in the atom. He called this positively charged particle as a nucleus
- The volume occupied by the nucleus is negligibly small as compared to the total volume of the atom.
- The radius of the atom is about 10⌃-10 m called as Angstrom unit and that of the nucleus is 10⌃-15 m called as Fermi unit
On the basis of these conclusions, Rutherford proposed his model of the atom as follows
- An atom consists of the tiny positively charged nucleus at its center. The nucleus contains the positively charged particle.
- Almost entire mass of an atom and positive charge is concentrated in the nucleus
- Number of negatively charged electrons, equal to the number of positive charges on the nucleus, revolve in various orbits around the nucleus
- The number of negatively charged electrons, equal to the number of positive charges on the nucleus so an atom is electrically neutral.
- The electrons and nucleus are held together by an electrostatic force of attraction.