The effective nuclear charges (Z.,ff), which are empirical parameters in an approximate spin-orbit Hamiltonian, are determined for main group elements in the second to fifth periods by using experimental results for the fine structure splittings (FSS) in II states of diatomic hydrides. All calculations use full valence multiconfiguration self-consistent field (MCSCF) wave functions with the effective core potential (ECP) basis sets proposed by Stevens et al., augmented by one set of polarization functions. These effective nuclear charges are tested by predicting the FSS in many diatomic molecules and are then applied to evaluate the relativistic potential energy curves of the methylene analogs AHz (A = C, Si, Ge, and Sn), as well as XHX and NaX (X = Br and 1).