The diamagnetic susceptibility and the optical properties, reflectivity and refractive index, are powerful probes of chemical bonding in semiconductors. The reflecitivity and its pressure derivative have been measured in the photon energy range 0.5 eV to 4.5 eV. The magnetic susceptibility and its temperature derivative are consistent with a model which describes the bonding in a‐As as p‐like. Measurements of expansion coefficient and the temperature derivative of refractive index are also reported.