Chemical reactions involve outer or valence electrons. Atoms rearrange, bonds are broken and formed, energy is exchanged with the surroundings, and new substances result. But the constituent atoms remain the same. This is characteristic of the behavior we call "chemical".
There is another category of processes which involve the atom that differs fundamentally from chemical behavior. In it, atoms become different atoms, often being broken apart into smaller pieces. Sub-atomic particles are ejected as massive amounts of energy are released to the surroundings. Mass "disappears". These describe just some of the characteristics of behaviors which are nuclear in nature, involving the inner part of the atom (the nucleus) instead of the outer electrons which we have studied throughout the year.
Because the nucleus is involved in these changes it is important to distinguish among different isotopes of an element which will not have the same behaviors.