The words “hard” and “soft” are commonly used to describe two broad classes of Lewis acids and bases (See “Chemical Reactions”). Hard acids and bases typically have small atomic (or ionic) radii, high oxidation states, high electronegativity (for bases), and are not very polarizable. Soft acids and bases tend to be the opposite in that they have relatively large atomic (or ionic) radii, low oxidation states, low electronegativity, and are highly polarizable. As it turns out, hard acids tend to react more rapidly and form stronger bonds with hard bases, and the same is true for soft acids paired with soft bases. This pattern is what makes the theory of hard and soft acids and bases useful for predicting and understanding reactivity in inorganic complexes.