A "Shell Closure" refers to a specific arrangement of electrons in an atom that leads to increased stability. This occurs when the outermost electron shell is completely filled, resulting in a configuration that is less reactive. Elements with shell closures are often found in the noble gases group of the periodic table, such as helium and neon. In addition to noble gases, shell closures can also be observed in certain isotopes of nuclei, where a complete set of protons and neutrons contributes to stability. This concept is important in understanding both chemical reactivity and nuclear stability in various elements.