To the end of the 19th century, Henri Becquerel found that the pechblende ore produces invisible rays, the radioactive rays.
At the beginning of the 20th century, Pierre and Marie Curie found where the radioactivity of pechblende comes from: This ore contains in very small quantities
Two new radioactive elements : Radium (Ra) and polonium (Po)
The Curie isolated those elements.
Exposing radioactive rays to a magnetic field, the physicists saw that
Radioactive rays are of three types: - β (beta) rays are strongly deviated particles, therefore lightweight. They are negatively charged electrons. - α (alpha) rays are particles which are slightly deviated to the opposite direction of electrons, therefore they are charged positively and much heavier than electrons. Lateron it was found that they gave helium atoms when associated with electrons. - γ (gamma) rays are not deviated. They are of the type of electromagnetic radiation like light or X-rays, but much more intensive.