Periodic table found in nature. Elements 99 to 118 have only been synthesized in laboratories or nuclear reactors. The synthesis of elements having higher atomic numbers is currently being pursued: these elements would begin an eighth row, and theoretical work has been done to suggest possible candidates for this extension. Numerous synthetic radionuclides of naturally occurring elements have also been produced in laboratories. Each chemical element has a unique atomic number ("Z") representing the number of protons in its nucleus. Most elements have differing numbers of neutrons among different atoms, with these variants being referred to as isotopes. For example, carbon has
Mirror nuclei Mirror nuclei Mirror nuclei are nuclei where the number of protons of element one (Z) equals the number of neutrons of element two (N) and the number of protons of element two (Z) equals the number of neutrons in element one (N), such that the mass number is the same (A = N + Z = N + Z). As that Z = N and Z = N, A = N + N = Z + Z. By making the substitution Z = Z and Z = Z − 1, the mass number can be rewritten in the form 2Z