The new thorium isotope 207Th has been produced in the 5 n evaporation channel of the fusion reaction 36Ar+ 176Hf. It was separated in flight by the gas-filled recoil separator SHANS and identified on the basis of a correlated α -decay chain. The α decay of 207Th, measured with an α -particle energy of 8167(21) keV and a half-life of 9 .7−4.4+46.6 ms, is assigned to originate from ground state. By combining with existing data, we find that the α -decay energies of nuclei with Z >82 and N <126 show a regular and distinct odd-even staggering (OES) rather than the commonly supposed smooth pattern. A theoretical analysis has been performed within relativistic Hartree-Fock-Bogoliubov and large-scale shell-model approaches. It is found that the OES originates from both pairing correlations and blocking of particular orbitals by unpaired nucleons. Of particular importance is that pairing correlations result in the OES not only through the contribution of pairing energy to binding energy, but also by configuration mixing induced by scattering nucleons to orbitals away from Fermi levels.