In 1896 the French scientist Henri Becquerel was studying uranium salts. He wrapped a photographic plate in black paper and left some uranium on top. Even without any sunlight, the plate became fogged. Becquerel realised the uranium was giving off invisible rays all by itself. This was the accidental discovery of radioactivity.
The work of the Curies
Marie Curie and her husband Pierre Curie studied these rays further. Marie Curie gave the phenomenon the name "radioactivity". Working with the mineral pitchblende, the Curies discovered two new radioactive elements, polonium and radium. Marie Curie won two Nobel Prizes for this work, one in physics and one in chemistry. Her careful measurements helped scientists understand that the rays carried energy and came from inside the atom itself.
Key idea
Radioactivity is the spontaneous emission of radiation from the unstable nucleus of an atom. It happens by itself, without any outside help.
Properties of radioactivity
Radioactive decay is a random and spontaneous process. It cannot be switched on or off, and it is not affected by physical conditions such as temperature or pressure, nor by chemical reactions. The radiation comes from changes inside the nucleus, not from the electrons around it. This discovery opened up a whole new area of physics and later led to important uses in medicine and energy. At the time, the early scientists did not yet know that the rays could be harmful, so they worked without proper protection.
Remember
- Becquerel discovered radioactivity in 1896 using uranium.
- Marie Curie named it and discovered radium and polonium.
- Radioactivity is spontaneous and comes from the nucleus.