sion process for the evolution of vari-
ables in a system. The Fokker–Planck
equation, which can be used to
analyse such problems as Brownian
movement, is soluble using statistical
methods and the theory of probabil-
ity. It is named after the Dutch physi-
cist Adriaan Fokker (1887–1968) and
Max Planck.
folacin See folic acid.
folic acid (folacin)A vitamin of the
vitamin B complex. In its active
form, tetrahydrofolic acid, it is a
coenzyme in various reactions in-
volved in the metabolism of amino
acids, purines, and pyrimidines. It is
synthesized by intestinal bacteria
and is widespread in food, especially
green leafy vegetables. DeÜciency
causes poor growth and nutritional
anaemia.
food additiveA substance added
to a food during its manufacture or
processing in order to improve its
keeping qualities, texture, appear-
ance, or stability or to enhance its
taste or colour. Additives are usually
present in minute quantities; they in-
clude colouring materials, sweeten-
ers, preservatives, *antioxidants,
emulsiÜers, and stabilizers. In most
countries the additives used must be
selected from an approved list of
such compounds, which have been
tested for safety, and they must be
listed on the food labels of individual
products.
fool’s gold See pyrite.
forbidden bandSee energy
bands.
forbidden transitions Transitions
between energy levels in a quantum-
mechanical system that are not al-
lowed to take place because of
*selection rules. In practice, forbid-
den transitions can occur, but they
do so with much lower probability
than allowed transitions. There are
three reasons why forbidden transi-
tions may occur:
(1) the selection rule that is violated
is only an approximate rule. An ex-
ample is provided by those selection
rules that are only exact in the ab-
sence of *spin–orbit coupling. When
spin–orbit coupling is taken into ac-
count, the forbidden transitions be-
come allowed – their strength
increasing with the size of the
spin–orbit coupling;
(2) the selection rule is valid for di-
pole radiation, i.e. in the interaction
between a quantum-mechanical sys-
tem, such as an atom, and an electro-
magneticÜeld, only the (variable)
electric dipole moment is considered.
Actual transitions may involve mag-
netic dipole radiation or quadrupole
radiation;
(3) the selection rule only applies for
an atom, molecule, etc., in isolation
and does not necessarily apply if ex-
ternalÜelds, collisions, etc., are taken
into account.force constant A constant charac-
terizing the strength of the bond in a
diatomic molecule. Near the equilib-
rium position, Re, of the potential en-
ergy curve of a diatomic molecule,
the potential energy V is accurately
represented by a parabola of the
form V = k/2(R – Re)^2 , where R is the
internuclear distance and k is the
force constant. The greater the force
constant, the stronger is the bond be-
tween the atoms, since the walls of
the potential curve become steeper.
Regarding the molecular vibrations
as simple harmonic motion, the
force constant occurs in the analysis
of the vibrational energy levels.
forced convection See convec-
tion.formaldehydeSee methanal.formalinA colourless solution of231 formalin
f