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

Alpha () Decay:

Alpha decay occurs when the nucleus spontaneously ejects an   particle. An   particle is really 2 protons and 2 neutrons, or an He nucleus. So when an atom undergoes   decay, its atomic number decreases by 2 and its atomic mass decreases by 4.   particles do not penetrate much material, for they can be stopped by paper. An example of   decay is the following:
Pu239   U235 +   particle (He-4 nucleus)

There is a difference in mass between the original nucleus and the sum of the mass of the   particle and resulting nucleus. This lost mass is converted into energy using the formula E = mc2; the energy would equal the kinetic energy of the   particle and the recoil energy of the resulting nucleus.
particles are usually mono-energetic, but they can have different energies, as in the case of 226 Ra. This isotope of radium has a small percentage of   particles that don't have their full energy; instead the nucleus is left excited and emits gamma rays. Some of these rays will transfer energy to an orbital electron in the process internal conversion.

Beta(�)- Decay:

There are two types of � decay; �+ and �- decay. An excess of neutrons in an atom's nucleus will make it unstable, and a neutron is converted into a proton to change this ratio. During this process, a � particle is released, and it has the same mass and charge as an electron. The resulting atom and the � particle have a total mass which is less than the mass of the original atom, and one would think that the � particles should have the energy equivalent to the mass lost (E = mc2). But � particles aren't mono-energetic, and have a broad energy spectrum from zero to the maximum energy predicted. So the � particle is accompanied by virtually massless and chargeless particles called...