Radioactivity is the spontaneous disintegration of atomic nuclei. The nucleus emits   particles, ß particles, or electromagnetic rays during this process.

Alpha decay occurs when the nucleus spontaneously ejects an α particle. An α particle is really 2 protons and 2 neutrons, or an He nucleus. So when an atom undergoes α   decay, its atomic number decreases by 2 and its atomic mass decreases by 4. α   particles do not penetrate much material, for they can be stopped by paper. An example of  α  decay is the following:

Pu239  U235 + α   particle (He-4 nucleus)