Chapter 16: Radioactivity

Rutherford’s Scattering Experiment


Hans Geiger and Ernest Marsden worked with Ernest Rutherford in his Manchester laboratories in 1909. They fried alpha particle of few MeV into a thin piece of gold foil. This was dine in an evacuated chamber connected to a vacuum pump. When the alpha particles passed through the gold foil they hit a zinc sulphide screen which emits light whenever an alpha particle strikes it. This screen was observed using a moving microscope in a dark room. At that time the accepted structure of the atom was like a plum pudding.

As a result they found about 1 in every 8000 was reflected back or scattered through an angle of more than 900. If the plum pudding model was the structure of the atom this would be like firing a bullet at a piece of toilet paper and it bouncing back which is not possible.

The Nuclear Model

We know from Rutherford’s experiment that the structure of an atom consists of positively charged protons and neutral neutrons in one place called nucleus. The nucleus sits in the center and has negatively charged electrons orbiting it. The nucleus must be positively charged. Almost all the mass of the atom in the tiny nucleus which takes up practically no space when compared to the size of atom.


Isotopes are different forms of an element. They always have the same number of protons but have a different number of neutrons. Since they have the same number of protons and electrons they behave in the same way chemically. Take neon as an example;

1020 Ne          1021Ne         1022Ne

Radiation from Radioactivity

  • α and β- radiation are particles of matter. Γ-radiation is a photon of electromagnetic radiation, similar to an X-ray.
  • An α- particle consists of two protons and two neutrons. The mass of α- particle is nearly 10000 times that of an electron.
  • β-radiation is absorbed by few millimeters of metal.
  • γ-radiation is never completely absorbed but a few centimeters of lead greatly reduces the intensity.

The Electronvolt (eV)

One electron volt is the energy transferred when an electron travels through a potential difference of one volt.