Pierre and Marie Curie in the laboratory

What is Radioactivity?

  1. A phenomenon of spontaneous disintegration.
  2. First observed in certain naturally occurring heavy elements like- radium, actinium, uranium, thorium, etc. with emission of alpha, beta and Gamma rays.
  3. It is the property of nuclide to disintegrate in which a transformation takes place of a relatively stable nuclide accompanied with the emission of particles or electromagnetic radiation. The nuclide that decay is called radioactive.


  1. Accidentally discovered in 1896 by French physicist Henry de Becquerel.
  2. Later Pierre and Madam Curie showed similar phenomenon in other metals like polonium, Francium, and radium.
  3. Out of all radioactive elements, Radium is the most powerful radioactive element and it was discovered by Madam Curie

Radioactive Particles

Alpha Particles:

  1. Positively charged Helium atom
  2. Very little penetrating power
  3. absorbed by a sheet of paper
  4. stopped by Aluminium foil
  5. deflected by electric and magnetic field
  6. maximum power of ionization through the gases
  7. velocity is less than that of light

Beta Particle:

  1. Negatively charged Light Particle
  2. stream of fast moving electrons
  3. Penetrating power is greater than &alpha
  4. less power of ionization through the gases as compared to &alpha-rays

Gamma Particle:

  1. Electromagnetic radiation of low wavelength, high frequency and high energy
  2. penetrating power is very great
  3. can pass through several centimeter of lead
  4. composed of photons of high energy
  5. velocity is equal to the velocity of light in vacuum

Half life period of a radioactive element

  1. Time during which half of the total number of atoms of the radioactive element disintegrate
  2. Different radioactive elements have different half lives
  3. Independent of all external conditions such as temperature, pressure, mass, etc.
  4. Can be detected by means of its half life period
  5. Smaller the half life period of a radioactive element, larger is its radioactivity and vice versa.

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