Bond dissociation energies depend on both bond valence and bond character. For a given bond valence, bond energies can vary by hundreds of kJ/mol depending on whether the bond is ionic, covalent, or metallic in character. Covalent bonds less than 2 valence units are particularly affected by the lone pair bond weakening effect, where lone pair electron density on one atom repels bonding electron pairs, weakening the bond. The researchers analyzed bond dissociation energy data for many molecules and crystals, relating the energies to bond valence and character. Mathematical models relating these factors can help parameterize new valence-based force fields and rationalize crystal-chemical trends.