bitals in a molecule: the highest oc-
cupied molecular orbital (HOMO) and
the lowest unoccupied molecular
orbital (LUMO). These two molecular
orbitals are usually the most impor-
tant ones in determining chemical
and spectroscopic properties of the
molecule.
frontier-orbital theory A theory
of the reactions of molecules that
emphasizes the energies and symme-
tries of frontier orbitals. Frontier or-
bital theory was developed by the
Japanese chemist Kenichi Fukui
(1919–98) in the 1950s and is an al-
ternative approach to the Wood-
ward–Hoffmann rules. It has been
very successful in explaining such re-
actions as the *Diels–Alder reaction.
Frost diagram A graph showing
how standard electrode potentials
vary with oxidation state for differ-
ent oxidation states of an element.
Frost diagrams can be constructed
from *Latimer diagrams. For an el-
ement M, the standard electrode po-
tential E Šis calculated for the
reaction.
M(N) + Ne–→M(0)where M(N) indicates the species in
oxidation state N and M(0) indicates
the zero oxidation state. The Frost di-
agram is then obtained by plotting
NEŠagainst N for the different
species. NEŠis proportional to the
standard Gibbs free energy of the
particular half reaction.
In a Frost diagram, the lowest
point corresponds to the most stable
oxidation state of the element. Also,
the slope of a line between two
points is the standard potential of
the couple represented by the points.
Like Latimer diagrams, Frost dia-
grams depend on pH.
froth ÛotationA method of sepa-
rating mixtures of solids, used indus-
trially for separating ores from the
unwanted gangue. The mixture is
ground to a powder and water and a
frothing agent added. Air is blown
through the water. With a suitable
frothing agent, the bubbles adhere
only to particles of ore and carry
them to the surface, leaving the
gangue particles at the bottom.
fructose(fruit sugar; laevulose)A
simple sugar, C 6 H 12 O 6 , stereoiso-
meric with glucose (see monosaccha-
ride). (Although natural fructose is
the d-form, it is in fact laevorotatory.)
Fructose occurs in green plants,
fruits, and honey and tastes sweeter
than sucrose (cane sugar), of which it
is a constituent. Derivatives of fruc-
tose are important in the energy me-
tabolism of living organisms. Some
polysaccharide derivatives (fructans)
are carbohydrate energy stores in
certain plants.fructose 1,6-bisphosphateAn in-
termediate formed in the initial stage
of *glycolysis by the phosphorylation
of glucose using ATP.fruit sugarSee fructose.
FT-IR See fourier-transform in-
frared.
5-FUSee 5-fluorouracil.fuelA substance that is oxidized or
otherwise changed in a furnace or
heat engine to release useful heat or
energy. For this purpose wood, veg-
etable oil, and animal products have
largely been replaced by *fossil fuels
since the 18th century.
The limited supply of fossil fuels
and the expense of extracting them
from the earth has encouraged the
development of nuclear fuels to pro-
duce electricity.fuel cellA cell in which the chemi-
cal energy of a fuel is converted di-
rectly into electrical energy. The
simplest fuel cell is one in which hy-
drogen is oxidized to form water237 fuel cell
f