Chemical bond properties of YBa2Cu3O7 were studied by using the average band-gap model. The calculated results show that the covalency of Cu(1)-O bond is 0.406, and one of Cu(2)-O is 0.276. Mssbauer isomer shifts of 57Fe in Y-123 were calculated by the chemical surrounding factor hv defined by covalency and electronic polarizability. The charge-state and site of Fe were determined. The relation between the coupling constant of electron-phonon interaction and covalency is employed to explain that the Cu(2)-O plane is more important than the Cu(1)-O chain on the superconductivity in the Y-123 compounds.