outershell

Outer shell, often called the valence shell, is the electron shell that lies farthest from the nucleus in an atom and contains the valence electrons responsible for chemical behavior. In many texts it is written as outer shell, but some sources use outershell as a single term. The outer shell is defined by the highest principal quantum number among occupied shells and its electron count largely governs an element’s reactivity and bonding patterns.

The capacity of a shell follows the rule 2n^2, where n is the shell’s principal quantum number. For main-group elements, the outer shell typically contains up to eight electrons, following the octet rule. The specific arrangement of electrons in the outer shell determines an element’s valence and its tendency to gain, lose, or share electrons to achieve a more stable configuration.

Bonding and chemical properties are driven largely by outer-shell electrons. Ionic bonding involves transfer of valence electrons, covalent bonding involves sharing them, and metallic bonding involves a delocalized pool of valence electrons. The ease with which outer-shell electrons are removed or added is described by ionization energy and electron affinity, respectively, and these properties influence periodic trends such as reactivity, electronegativity, and bonding type.

In spectroscopy, transitions involving outer-shell electrons produce characteristic absorption or emission lines, providing insights into an element’s electronic structure. In materials science and catalysis, the behavior of outer-shell electrons affects surface bonding, conductivity, and catalytic activity.