At the time of the discovery of radioactivity physicists believed that the atom was the ultimate, indivisible building block of matter. Then alpha and beta particles were recognized to be discrete units of matter and radioactivity to be a process in which atoms are transformed by the emission of one or the other of these particles into new kinds of atoms possessing new chemical properties. This brought with it the realization that atoms themselves must have structure and that they are not the ultimate, fundamental particles of nature. The nuclear hypothesis has since grown into an elaborate theory of atomic structure, in terms of which the entire phenomenon of radioactivity can be explained. Briefly, the atom has been found to consist of a dense central nucleus surrounded by a cloud of electrons.
Neutrons are electrically neutral and of approximately the same mass as protons.
An alpha particle, or doubly charged helium ion, consists of two neutrons and two protons, and hence can be emitted only from the nucleus of an atom. An atom of the uranium isotope of mass number 238, upon emitting an alpha particle, becomes an atom of another element of mass number 234. Each of the two protons that form part of the alpha particle possesses a unit of positive electric charge. The number of positive charges in the nucleus, balanced by the same number of negative electrons in the orbits outside the nucleus, determines the chemical nature of the atom. Because the charge on the uranium-238 nucleus decreases by two units as a result of alpha emission, the atomic number of the resultant atom is 2 less than that of the original, which was 92.