The atomic structure for certain atoms is unstable; these unstable atoms are called radioactive atoms or radionuclides. Radioactivity is the natural and spontaneous process by which radionuclides transform or decay to a different state, and emit or radiate excess energy in the form of particles or waves called radiation. Depending on how the radionuclide releases this excess energy, either a lower energy atom of the same form results, or a completely different atom will be formed.  

This process of large unstable atoms becoming more stable by emitting radiation, is called radioactive decay. This radiation can be emitted in the form of a positively charged alpha particle, a negatively charged beta particle, or gamma rays or x-rays. 

Alpha Decay

In alpha decay the nucleus emits a helium nucleus, an alpha particle. Alpha decay occurs most often in massive nuclei that have too large a proton to neutron ratio. An alpha particle, with its two protons and two neutrons, is a very stable configuration of particles. Alpha radiation reduces the ratio of protons to neutrons in the parent nucleus, bringing it to a more stable configuration. Many nuclei more massive than lead decay by this method. 

Beta Decay

Beta decay occurs when, in a nucleus with too many protons or too many neutrons, one of the protons or neutrons is transformed into the other. 

These mechanisms are completely insensitive to the environment: the rate of decay is constant and invariant. 

Gamma Decay

In gamma decay a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element. In the gamma decay of a nucleus, the emitted photon and recoiling nucleus each have a well-defined energy after the decay. The characteristic energy is divided between only two particles.