Definitions for "Electron Shell"
a grouping of electrons surrounding the nucleus of an atom; "the chemical properties of an atom are determined by the outermost electron shell"
a region of space around an atom's nucleus that electrons are allowed to exist in, corresponding to the stable "standing wave" patterns of de Broglie and Bohr
Electrons do not orbit the nucleus of an atom in a random manner. Instead they are arranged in specific orbits at particular distances from the nucleus. These are the electron shells.