Nuclear reactions involve a change in the nucleus. Under certain arrangements of protons and neutrons the electrostatic force can cause instability in the nucleus causing it to decay. It will continue to decay until it reaches a stable combination. This graph shows the stable nuclei in red . The more protons in the nuclei, the more neutrons are needed for stability.
Stability is favored by even numbers of protons and even numbers of neutrons.
Unstable nuclei, called radioactive isotopes , will undergo nuclear decay as it becomes more stable. There are only certain types of nuclear decay which means that most isotopes can't jump directly from being unstable to being stable. It often takes several decays to eventually become a stable nuclei. When unstable nuclei decay, the reactions generally involve the emission of a particle and or energy. Below is a table describing the types of nuclear decay.