When a nucleus is unstable it either splits apart spontaneously into fragments or else ejects small particles. The end result of this "decay" is either a more stable nucleus or a completely stable (non-radioactive) nucleus.
The emission of the small
particles and any energy that accompanies them is called radioactivity. The phenomenon was first described by
the French scientist Henri Becquerel and the general
characteristics of the small particles given off during decay
were determined through experiment by Rutherford. 
The common pathways for nuclear decay include the following:
| type of decay | alpha | beta | positron | electron capture | gamma |
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none |
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