A model of covalent and ionic bond strength is proposed in terms of the tensile unbinding force by introducing the concept of the effectively bonded valence electron (EBVE) number of a chemical bond. Bond strength proves to be exclusively dependent on two microscopic parameters: bond length and EBVE number. This model allows us to determine bond strength for a variety of crystals and accounts for the observation that a low-coordination number of binding atoms has a tendency to higher bond strength. For crystals of simple structures, we propose linking bond strength to the theoretical tensile strength of a crystal; the latter reproduces the results of first-principles calculations. The model also allows for the assessment of the theoretical tensile strength of graphene and single-walled nanotubes constructed with typical material systems.