When there is nuclear instability, particles are emitted from that same nuclei, and this is referred to as radioactivity. How did it begin? Or we may ask how it was discovered?

In 1896, radioactivity was accidentally discovered by Antoine Henri Becquerel. Becquerel was a French physicist who was known to be the first to discover evidences of radioactivity. He was about to test and prove that some phosphorescent chemicals might emit weak X-rays when exposed to sunlight. He experimented by putting phosphorescent crystals of different chemical compounds on top of photographic plates which are wrapped in black paper, and expose it to sunlight so that the chemicals would fluoresce.  He then developed the plates to see if there was any sigh of X-rays that penetrated the black paper. In one of his experiments, he used a compound containing uranium. Unfortunately, during this experiment, the weather seems cloudy, and so he was not able to expose the plate into the sun, and just kept it in a drawer. After how many days, he developed the plate that was kept in the drawer, and to his surprise, it was strongly exposed. He then concluded that the uranium emits radiation that made the plates exposed even if it was not exposed to sunlight. And this led to the discovery of radioactivity.

Few months later, Marie Curie chose the field of radioactivity for her graduate thesis. She investigated the uranium compounds and found out that the amount of radiation depends only on the amount of the uranium content of the compounds and not on the nature or amount of any other elements present in the compound, or on the shape or temperature or on any properties of the compounds. This discovery led to the generalization that the radioactivity of an element is determined by something inside the atom and not affected by any external factors, even if it is combined with any other elements.

Further in her investigation, she examined the ores to which uranium is obtained, these ores were pitchblende and chalcolite. In her experiment, she found out that the ores were more radioactive than the pure uranium and this made her suspect that the ores might contain any other radioactive elements.

After undergoing several processes, her experiment on pitchblende made her discover a new radioactive element which was found to be 300 times more radioactive than uranium. She named the element, Polonium, after her birthplace, Poland. She also found out that the residue left from her experiment was also highly radioactive.

After six months of working, Marie, together with her husband, Pierre Curie discovered another radioactive element, still found in pitchblende. They named it radium, and is found to be 900 times as radioactive as uranium. They announced this discovery in December 1898.

For their investigation on radioactivity, Becquerel, Marie and Pierre were jointly awarded a Nobel prize in physics in 1903. For her discovery of polonium and radium, Marie was also awarded another Nobel prize in 1911. She was then the first woman to win a Nobel prize and the only person ever to win two Nobel prizes for science.

Later on, radium is found to be an effective treatment of cancer, for its ability to kill cancer cells.

Both Becquerel and the Curies also observed that some radioactive radiation could be deflected by a magnetic field, and that emitted radiation were deflected in the same way as the cathode rays. Radiation which are emitted in the same direction as the magnetic field were negative and those which are deflected in the opposite direction were positive.

Pierre Villard also discovered a third and more penetrating radiation that was not affected by magnetic or electric field.

In 1889, Ernest Rutherford studied radioactivity. He named the three types of radioactive emission as: alpha (\alpha) particles, beta (\Beta) particles, and gamma (\gamma) rays. He was able to discover some properties of these emissions by putting a small amount of radium in a small lead box with an opening at the top. Some of these properties are as follows:

Alpha Particles

  • Positive particles (actually, nuclei of helium atom)
  • Ejected at a high speed, but have a range of only a few centimeters in air.
  • Stopped by an ordinary sheet of thin aluminum foil.


Beta Particles

  • Stream of high-energy electrons.
  • Ejected at various speeds, sometimes approaching the speed of light.
  • Some can penetrate several millimeters of aluminum.


Gamma Rays


These three radioactive emissions including X-rays are also called ionizing radiation for the reason that they tend to ionize nearby atoms and molecules.