A nuclear reaction is a reaction that changes the mass of the nucleus. Nuclear reactions occur both in nature and in nuclear reactors. In nuclear reactors the standard nuclear reaction is the decay of uranium-235.
The superheavy elements in the periodic table, that is to say, those with atomic numbers exceeding 83, undergo alpha decay to reduce the number of protons and neutrons in the nucleus of the atom.
Elements with a high neutron to proton ratio undergo beta decay, in which a neutron is changed into a proton and an electron. As the entire process takes place in the nucleus of the atom, and the nucleus can only contain protons and neutrons, the electron that is generated is ejected from the nucleus as a beta particle.
Gamma decay, unlike the other modes of radioactive decay, does not change the number of protons and neutrons in the nucleus of the atom- instead, it lowers the atom's energy level by one.
An example of alpha decay would be the decay of uranium-235 into thorium-231:
#""_92^235 U# + #""_90^231 Th# #rarr# #alpha#
An example of beta decay would be the decay of uranium-235 into neptunium-235:
#""_92^235 U# + #""_93^235 Np# #rarr# #beta#
An example of the gamma decay of technetium-99m into technetium-99:
#""_43^(99m) Tc# + #""_43^99 Tc# #rarr# #gamma#
The 'm' in Tc-99m stands for metastable, which in terms of an atom, ion or atomic nucleus, means that the atom is in an excited state.