Nuclear transformation occurs when the atoms of one element change to become atoms of another element. This is most commonly seen with isotopes.
The nuclear transformation of isotopes occurs when an isotope of one element loses one or more neutrons. This causes the atom to lose mass. The number of protons remains the same, so it is still considered the same element. This is called nuclear decay. An atom will continue to decay until it becomes relatively stable. Nuclear transformations can also occur when the atoms of one element are bombarded with the atoms from another element, causing a shift in the atom's proton/electron/neutron balance.