and then collected on aÛuorescent
screen. Both the surface structure
and changes that occur after
chemisorption and surface reactions
can be investigated in this way. It is
necessary for the surface to be care-
fully cleaned and kept at ultrahigh
vacuum pressure. Although many
surfaces are altered by the electron
beam and therefore cannot be stud-
ied using this method, there are
enough surfaces and surface
processes that can be studied using
LEED to make it a very useful tech-
nique. DifÜculties in interpreting
LEED patterns arise as multiple-
scattering theory, rather than single-
scattering theory (as in X-ray or neu-
tron scattering), is required. See also
electron diffraction.Lennard-Jones potential A po-
tential used to give an approximate
description of the potential energy
interaction, V, of molecules as a func-
tion of intermolecular distance r. The
general form of the Lennard-Jones
potential is
V = Cn/rn– C 6 /r^6 ,
where Cnand C 6 are coefÜcients that
depend on the speciÜc molecules and
n is greater than 6 so that at small
separations the repulsion term domi-
nates the interaction, the r–6term
being attractive. The value n = 12 is
frequently chosen. In this case the
Lennard-Jones potential is given by:
V = 4W[(r 0 /r)^12 – (r 0 /r)^6 ],
where W is the depth of the potential
well and r 0 is the separation at which
V = 0. The minimum value of the
well occurs at the separation re=
2 1/6r 0. The representation of the re-
pulsive part of the interaction by a
1/r^12 term is not realistic; a much
more realistic term is the exponen-
tial term, exp(–r/r 0 ), as it is closer to
the exponential decay of the wavefunctions and thus of their overlap,
which describes the repulsion.leucineSee amino acid.leuco formSee dyes.
leucomalachite green test A
*presumptive test for blood. The
reagent is the dye leucomalachite
green dissolved in water along with
sodium perborate (NaBO 3 ). A blue-
green colour indicates a positive re-
sult.lever ruleA rule enabling the rela-
tive amounts of two phases a and b,
which are in equilibrium, to be
found by a construction in a phase
diagram. (For example, a can be gas
and b can be liquid.) The distances la
and lbalong the horizontal *tie line
of the phase diagram are measured.
The lever rule states that nala= nblb,
where nais the amount of phase a
and nbis the amount of phase b. The
rule takes its name from the similar
form of the rule, mala= mblb, relating
the moments of two masses maand
mbabout a pivot in a lever.Lewis, Gilbert Newton (1875–
1946) US physical chemist who spent
most of his career at Berkeley, Cali-
fornia. His ideas on chemical bond-
ing were extremely inÛuential, and
he introduced the idea of a stable
octet of electrons and of a covalent
bond being a shared pair of elec-
trons. He also introduced the concept
of Lewis acids and bases (see acid).Lewis acid and baseSee acid.Liebermann’s reaction A method
of testing for phenols. A small sam-
ple of the test substance and a crystal
of sodium nitrite are dissolved in
warm sulphuric acid. The solution is
then poured into excess aqueous al-
kali, when the formation of a blue-
green colour indicates the presence
of a phenol.Lennard-Jones potential 324l