In the context of
atomic orbitals, an
open shell is a
valence shell which is not completely filled with
electrons or that has not given all of its valence electrons through
chemical bonds with other
atoms or
molecules during a chemical reaction. Atoms generally reach a
noble gas configuration in a molecule. The noble gases (
He,
Ne,
Ar,
Kr,
Xe,
Rn) are less reactive and have configurations 1s
2 (He), 1s
22s
22p
6 (Ne), 1s
22s
22p
63s
23p
6 (Ar), etc.