A nuclear change is an alteration to the structure of an atom's nucleus. This can happen with the addition of particles (as with fusion), the removal of particles (as with fission), or the spontaneous decay of the atom from one isotope to another.
In fusion, two atoms of a relatively light element collide with enough kinetic energy that their nuclei merge, or fuse, into a new, relatively heavy element. Fission is the opposite process. In fission, a heavy nucleus splits and emits energy, forming two lighter atoms in the process.
Another type of nuclear change involves the spontaneous alteration of a proton into a neutron or of a neutron into a proton. This process can change the isotope number of a single element, or it can convert an atom of one element into an atom of another element.
Nuclear changes are not to be confused with chemical changes. Chemical changes occur when the electrons of atoms interact with each other in new ways. During chemical changes, the nuclei of the atoms involved are not altered in any way. For nuclear changes, however, the electrons are essentially disregarded, as they play little to no role in the processes taking place in the heart of the atom.