Face Centered Cubic Array: A close packed array of spheres which give a structure with spheres at each of the corners and in the center of each of the faces.

Fast Ion Conductor: This is an ionic compound in which the conductivity is discontinuous, with it suddenly increasing due to a phase transition which results in the melting of either the cationic or anionic sublattices.

Fermi Level: This is the highest occupied energy level in a band at T = 0 K.

Fermi-Dirac Distribution: This describes the population of levels above the Fermi level, which become occupied at temperatures above absolute zero.

Force Constant: A property of a bond which determines the steepness of the potential energy parabola confining the two atoms. The larger the constant, the stiffer the bond.

Formal Charge: In the Lewis representation of molecular structure, the formal charge, FC, on an atom may be defined as FC = N(electrons on parent atom) -N(unshared electrons) -0.5N(shared electrons).

Free Rotation: Free rotation about a bond means that the energy barrier that must be surmounted to rotate is much less than kT, hence practically all molecules possess enough energy to rotate about a single bond. Double bonds are very fixed and hence rotation about these is fairly impossible, and certainly not ‘free’.

Frenkel Defect: This is a defect where an ion is moved off its lattice site and into an interstitial site.

Frontier Orbitals: These are the set of HOMO and LUMO for a given species.

Frost Diagram: This shows the reduction potential for the conversion of a species of a given oxidation state to the zero oxidation state, and weighted by the number of electrons transferred in that conversion. It represents an effective plot of free energy of an oxidation state against the oxidation state.

Fugacity: An effective pressure, adjusted to take into account the non-ideality of a real gas.

Fullerene: An allotrope of carbon containing 5- and 6-membered rings, whose structure is based on that of C60, Buckminster Fullerene.

Functional Group: A functional group is part of a molecule with a characteristic reaction or property. Hence molecules with the same functional groups can be expected to have similarities in their properties.

Fundamental transition: The dominant transition in most vibrational spectra, from the v = 0 to the v = 1 level.

Furan: A hetero-aromatic. See also THF.