 While physical modeling has been important historically, and is still a central part of chemical education and some investigations in stereochemistry, contemporary chemical models are almost always mathematical. Families of partially overlapping, partially incompatible models such as the valence bond, molecular orbital, and semi-empirical models are used to explain and predict molecular structure and reactivity. Molecular mechanical models are used to explain some aspects of reaction kinetics and transport processes. And lattice models are used to explain thermodynamic properties such as phase. These and other mathematical models are ubiquitous in chemistry textbooks and articles, and chemists see them as central to chemical theory.