The Lanczos method is employed to solve the valence bond model exactly for pi-electronic systems of benzenoid hydrocarbons. The exact valence bond ground-state wave functions have been obtained for benzenoid hydrocarbons of up to 20 pi centers. By defining a set of local indices which can be easily calculated from these wave functions, bond lengths, reactivity, and local aromaticity of these benzenoid hydrocarbons have been discussed in a systematic way. The results are in excellent agreement with the known experimental facts and related predictions from molecular orbital theory.