We discuss the structural, electronic and dielectric properties of a model structure of the Si(100)-SiO2 interface that accounts for the amorphous nature of the oxide. After showing that the structural properties of this model are consistent with a variety of experimental data, we consider the variation of the valence and conduction band edges across the interface. Our calculations indicate that the width of the electronic structure transition from the substrate to the oxide is 0.7-0.8 nm. We also address the dielectric properties based on an atomic-scale approach, and provide estimates for the effective dielectric constants of the substrate, the suboxide, and the stoichiometric oxide regions.