Answer:
The atomic number
Explanation:
Transmutation refers to the conversion of one chemical element into another. A transmutation usually involves a change in the structure of atomic nuclei and this may be induced by a nuclear reaction.
Transmutation was first achieved in 1919 by Lord Rutherford when he successfully changed a nitrogen nucleus to an oxygen-18 isotope, producing a proton in the process.
Since transmutation often involves changing one atom into another, the atomic number or mass number of the original element always changes (to form a new element) in the process.
