English used in this article or section may not be easy for everybody to understand. (August 2010)
chemical elements are stable: If they are not part of a
chemical reaction, they do not change. Chemical elements are made of
atoms. In stable elements, the atom stays the same. In a
chemical reaction, the atoms will form
chemical bonds, with other atoms. Even if the bonds change during a reaction, the atoms themselves do not.
In the 19th century,
Henri Becquerel discovered that some chemical elements have atoms that change: During their change they send out a particle. In 1898,
Pierre Curie called this phenomenon radioactive decay.
 Becquerel and the Curies were awarded the
Nobel Prize in Physics for this discovery, in 1903.