A valence scheme has been proposed for alloys such as CrSe, MnTe, and FeS, with the nickel arsenide structure, which indicates why they may be semiconductors, while chalcogenides of Co and Ni and antimonides and bismuthides of Cr, Mn, Fe, Co, and Ni with a similar structure are metallic. The valence scheme is thus consistent with the known electrical properties (and also magnetic properties) of these phases. It then predicts that FeSe and CrS may well be semiconductors and indicates that all other compounds of Cr, Mn, Fe, Co, and Ni with the NiAs structure should be metallic.