Yttrium one stable isotope, Y, which is also the only isotope that occurs naturally. However, the lanthanide rare earths contain elements of even atomic number and many stable isotopes. Yttrium-89 is thought to be more abundant than it otherwise would be, due in part to the s-process, which allows enough time for isotopes created by other processes to decay by electron emission (neutron → proton). Such a slow process tends to favor isotopes with atomic mass numbers (A = protons + neutrons) around 90, 138 and 208, which have unusually stable atomic nuclei with 50, 82, and 126 neutrons, respectively. Y
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