The work of Becquerel and Curie soon led other scientists to suspect that this theory of the atom was untenable.
Radioactive atoms emit three different kinds of radiation.
One kind of radiation is a particle of matter, called the alpha particle. It has a positive electric charge and about four times the mass of a hydrogen atom. Alpha particles exit radioactive atoms with high energies, but they lose this energy as they move though matter. An alpha particle can pass through a thin sheet of aluminum foil, but it is stopped by anything thicker. The beta particles travel at nearly the speed of light and can make their way through half a centimeter of aluminum.