Why are the Elemental Nonmetals (F2 , Cl2 , Br2 , I2 , S8 , P4 ) of so Many Hues or of Any Hues and Where is the Chromophore? Insight into Intera-X-X Bonds.

A unique approach is used to relate the HOMO-LUMO energy difference to the difference between the ionization potential (IP) and electron affinity (EA) to assist in deducing not only the colors, but also chromophores in elemental nonmetals. Our analysis focuses on compounds with lone pair electrons and σ electrons, namely X2 (X = F, Cl, Br, I), S8 and P4 . For the dihalogens, the [IP - EA] energies are found to be: F2 (12.58 eV), Cl2 (8.98 eV), Br2 (7.90 eV), I2 (6.78 eV). We suggest that the interahalogen X-X bond itself is the chromophore for these dihalogens, in which the light absorbed by the F2 , Cl2 , Br2 , I2 leads to longer wavelengths in the visible by a π → σ* transition. Trace impurities are a likely case of cyclic S8 which contains amounts of selenium leading to a yellow color, where the [IP - EA] energy of S8 is found to be 7.02 eV. Elemental P4 with an [IP - EA] energy of 9.09 eV contains a tetrahedral and σ aromatic structure. In future work, refinement of the analysis will be required for compounds with π electrons and σ electrons, such as polycyclic aromatic hydrocarbons (PAHs).