A tight-binding variable-charge model (SMTB-Q) has been used to calculate bulk, surface and point defect properties in uranium dioxide. It provides us with a better description of the iono-covalent oxides than classical, purely ionic models. A good agreement is found in the structural properties and cohesive energy between the model and experimental data; the charges calculated on the uranium and oxygen ions are Q(U) = 2.804 and Q(O) =- 1.402 respectively. The stability and relaxation of low index surfaces were evaluated: the (111) surface consistently has the lowest surface energy and the smallest surface relaxation, followed by the (110) surface and the (100) surface, in agreement with previous predictions from semi-empirical potentials and from ab initio calculations. The energy ranking of intrinsic defects is oxygen Frenkel pair < Schottky trio < uranium Frenkel pair, which is consistent with literature. The clustering energy of small vacancy clusters has been also calculated. Additionally, the atomic relaxations and the charge transfer at surfaces and around defects have been investigated. All the results obtained in the present work prove the ability of the SMTB-Q model to describe the bulk properties as well as the surface and defect properties in uranium dioxide. Finally, this model provides us with a new fundamental insight into the role played by the charge transfer in UO2 properties.