Chemical changes are caused by rearrangements of outer electrons while nuclei retain their identity. Consequently the identity of atoms remains the same.
In the beginning of 20th century, another kind of transformation, which involves changes in the atomic nucleus was observed by scientists. Certain elements were found to undergo spontaneous changes in their nuclei, resulting in the formation of atoms of different elements. These transformations are called nuclear reactions.The nuclear reactions have become tremendous source of energy all over the world, both for peaceful and destructive purposes. Nuclear chemistry is the branch of chemistry, which deals with the study of atomic nucleus and nuclear changes.
The knowledge gained in the fields of nuclear and radiochemistry has provided valuable information about the structure of atom, it has unlocked the secret to the process of transforming one element into another, and has gifted mankind a new source of energy - the nuclear energy.