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
Atomic nucleus fm. The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm. Nuclei can be spherical, rugby ball-shaped (prolate deformation), discus-shaped (oblate deformation), triaxial (a combination of oblate and prolate deformation) or pear-shaped. Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is a minor residuum of the strong interaction which binds quarks together to form protons and neutrons. This force is much weaker "between" neutrons and protons because it is mostly neutralized within them, in the same way that electromagnetic forces "between" neutral atoms (such