Kapustinskii Equation: This is a generalized form of the Born-Lande equation, such that an average value of the Madelung constant is taken so that the use of this equation to calculate lattice energies is independent of structure.

Kekulé Structure: An early proposal for the structure of benzene, consisting of alternating double and single bonds in a planar hexagonal structure. It has now been shown to be wrong, but still provides a very useful way of considering he way in which benzene might react.

Ketone: Molecule/functional group of generic type R2C=O, where R is a carbon chain of some sort. See here for a picture.

Kinetic Control: A reaction under kinetic control will produce products in a ratio related to the difference in the energy of the transition states of the products. i.e. the product that forms fastest is the most abundant, not necessarily the most stable one.

Kinetic Isotope Effect: For example, reaction in which a C-H bond must be broken will proceed more slowly if the H atom is replaced with a deuterium. This rate change is known as the kinetic isotope effect. Clearly, in principle, it occurs for all elements and their isotopes, however, the effect is greatest for hydrogen, because this has the largest percentage increase.

Kinetic Product: If multiple products are possible, the kinetic product is the one which forms fastest. It is not necessarily the most stable product. Can be selectively obtained using kinetic control. See also thermodynamic product.

Koopman: This states that the energy of an orbital, atomic or molecular, is the negative of the energy required to ionize an electron from that orbital, ie. εi = -Ii.