and then collected on a Ûuorescent
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 carefully cleaned and kept at ultrahigh
vacuum pressure. Although many
surfaces are altered by the electron
beam and therefore cannot be studied using this method, there are
enough surfaces and surface
processes that can be studied using
LEED to make it a very useful technique. DifÜculties in interpreting
LEED patterns arise as multiplescattering theory, rather than singlescattering theory (as in X-ray or neutron scattering), is required. See also
electron diffraction.
Lennard-Jones potential A potential used to give an approximate
description of the potential energy
interaction, V, of molecules as a function of intermolecular distance r. The
general form of the Lennard-Jones
potential is
V = C n /r
n – C 6 /r
6
,
where C n and C 6 are coefÜcients that
depend on the speciÜc molecules and
n is greater than 6 so that at small
separations the repulsion term dominates the interaction, the r
–6 term
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 r e =
2
1/6
r 0 . The representation of the repulsive part of the interaction by a
1/r
12 term is not realistic; a much
more realistic term is the exponential term, exp(–r/r 0 ), as it is closer to
the exponential decay of the wave
functions and thus of their overlap,
which describes the repulsion.
leucine See amino acid.
leuco form See 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 bluegreen colour indicates a positive result.
lever rule A rule enabling the relative 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 l a
and l b along the horizontal *tie line
of the phase diagram are measured.
The lever rule states that n a l a = n b l b ,
where n a is the amount of phase a
and n b is the amount of phase b. The
rule takes its name from the similar
form of the rule, m a l a = m b l b , relating
the moments of two masses m a and
m b about a pivot in a lever.
Lewis, Gilbert Newton (1875–
1946) US physical chemist who spent
most of his career at Berkeley, California. His ideas on chemical bonding were extremely inÛuential, and
he introduced the idea of a stable
octet of electrons and of a covalent
bond being a shared pair of electrons. He also introduced the concept
of Lewis acids and bases (see acid).
Lewis acid and base See acid.
Liebermann’s reaction A method
of testing for phenols. A small sample of the test substance and a crystal
of sodium nitrite are dissolved in
warm sulphuric acid. The solution is
then poured into excess aqueous alkali, when the formation of a bluegreen colour indicates the presence
of a phenol.
Lennard-Jones potential
324
l
www.AzShimi.ir www.AzShimi.com
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 carefully cleaned and kept at ultrahigh
vacuum pressure. Although many
surfaces are altered by the electron
beam and therefore cannot be studied using this method, there are
enough surfaces and surface
processes that can be studied using
LEED to make it a very useful technique. DifÜculties in interpreting
LEED patterns arise as multiplescattering theory, rather than singlescattering theory (as in X-ray or neutron scattering), is required. See also
electron diffraction.
Lennard-Jones potential A potential used to give an approximate
description of the potential energy
interaction, V, of molecules as a function of intermolecular distance r. The
general form of the Lennard-Jones
potential is
V = C n /r
n – C 6 /r
6
,
where C n and C 6 are coefÜcients that
depend on the speciÜc molecules and
n is greater than 6 so that at small
separations the repulsion term dominates the interaction, the r
–6 term
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 r e =
2
1/6
r 0 . The representation of the repulsive part of the interaction by a
1/r
12 term is not realistic; a much
more realistic term is the exponential term, exp(–r/r 0 ), as it is closer to
the exponential decay of the wave
functions and thus of their overlap,
which describes the repulsion.
leucine See amino acid.
leuco form See 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 bluegreen colour indicates a positive result.
lever rule A rule enabling the relative 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 l a
and l b along the horizontal *tie line
of the phase diagram are measured.
The lever rule states that n a l a = n b l b ,
where n a is the amount of phase a
and n b is the amount of phase b. The
rule takes its name from the similar
form of the rule, m a l a = m b l b , relating
the moments of two masses m a and
m b about a pivot in a lever.
Lewis, Gilbert Newton (1875–
1946) US physical chemist who spent
most of his career at Berkeley, California. His ideas on chemical bonding were extremely inÛuential, and
he introduced the idea of a stable
octet of electrons and of a covalent
bond being a shared pair of electrons. He also introduced the concept
of Lewis acids and bases (see acid).
Lewis acid and base See acid.
Liebermann’s reaction A method
of testing for phenols. A small sample of the test substance and a crystal
of sodium nitrite are dissolved in
warm sulphuric acid. The solution is
then poured into excess aqueous alkali, when the formation of a bluegreen colour indicates the presence
of a phenol.
Lennard-Jones potential
324
l
www.AzShimi.ir www.AzShimi.com