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Nuclear Physics

The Alpha Scattering Experiment

Procedure: In this experiment, a beam of alpha (α)-particles was aimed at a thin piece of gold foil. A zinc sulfide screen mounted on a rotatable microscope was used to detect the alpha (α)-particles. The experiment was conducted in a darkened room so that a very small flash of light could be seen whenever an alpha (α)-particle struck the zinc sulfide screen.

After performing his experiment he made observations:

  • Almost all the alpha particles did pass through the foil but some alpha particles were deflected off at different angles as observed at the screen of the detector.

  • Very few of the alpha particles (one or two) even bounced backwards after hitting the gold foil.

    On the basis of these observations Rutherford made the following conclusions:

  • Since most of the alpha particles passed straight through the gold foil without any deflection, most of the space within the atoms is empty.

  • Since some of the alpha particles (which are big in size) were deflected by large angles or bounced backwards, they must have approached some positively charged region responsible for the deflection. This positively charged region is now called the nucleus.

  • As very few alpha particles undergone the deflection, it was concluded that the volume occupied by the central region (nucleus) is very small.

  • Since the relatively denser alpha particles, were deflected by the central volume of charge, it can be concluded that, most the complete mass of the atom must be within the central volume.

  • High central charge was concentrated into a very small volume in comparison to the rest of the atom and

  • This central volume also contained most of the atomic mass of the atom. This region was named the "nucleus" of the atom in later years.

    Rutherford was able to calculate that the radius of his gold central charge from purely energetic considerations of how far particles of known speed would be able to penetrate toward a central charge of 100e. He founded that it would need to be less (how much less could not be told) than 3.4 x 10-14m. This was in a gold atom known to be 1010 metres or so in radius— a very surprising finding since it implied a strong central charge less than (1/3000)th of the diameter of the atom

    They discovered eventually that atoms have a positively-charged nucleus (with an exact atomic number of charges) in the centre which has radius of about 1.2× 1015× [Atomic Mass Number]1/3 meters. Since electrons were found to be even smaller, this concluded that the atom consists of mostly empty space.

    Following are the main points of Rutherford’s theory:

  • Most of the part of an atom is empty.

  • Approximately all the mass of the atom is concentrated at the center

    of atom which is now called nucleus.

  • In the central region of atom, the positively charged particles are


  • The charge on the nucleus of an atom is positive and is equal to Z.e

    where Z is charge number, e is charge of proton.

  • The negatively charged particles i.e. electrons revolve around the

    central positive portion in different circular orbits.

  • Central region (nucleus) is very small in size if compared to the size

    of atom.