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 1s² (He), 1s²2s²2p⁶ (Ne), 1s²2s²2p⁶3s²3p⁶ (Ar), etc.