Re: What is Radioactivity and who discovered it?

Radioactivity is the property of a material that describes its tendency to
emit subatomic particles (called "radiation").  Certain elements,
particularly heavy ones, have unstable nuclei that "decay" at a certain
rate, emitting particles (usually neutrons) so that they change into
lighter, more stable elements.  Other types of radioactive decay are
possible, such as beta decay, where an electron is emitted, gamma decay,
where an x-ray (a very high-energy form of light) is emitted, and alpha
decay, where a helium nucleus (two protons and two neutrons all stuck
together, also known as an alpha particle) is emitted.

Radioactivity was discovered by a French scientist named Henri Becquerel in
1896, when he observed a photographic plate turning black after being
exposed to uranium salts in the dark.  Since no light was getting in to
expose the plate, he knew something must be emitting from the uranium. 
More details about what was being emitted in the different kinds of
radioactive decay were discovered by the Curies and Ernest Rutherford.

For more information, look at the wikipedia article on Radioactive decay,
found at http://en.wikipedia.org/wiki/Radioactivity.
 The article is quite
accurate.