Electron shell

In chemistry and atomic physics, an electron shell, also called a principal energy level may be thought of as an orbit followed by electrons around an atom's nucleus.

Quotes

 * We know through observation that eight electrons (an electron octet) in an atom’s outermost shell, or valence shell, impart special stability to the noble gas elements in group 8A of the periodic table.
 * John McMurry, Organic Chemistry 8th ed. (2012), Ch. 1 : Structure and Bonding