used as a rough *indicator of acidity
or alkalinity, both in solution and as
litmus paper (absorbent paper soaked
in litmus solution).
litre Symbol l. A unit of volume in
the metric system regarded as a special name for the cubic decimetre. It
was formerly deÜned as the volume
of 1 kilogram of pure water at 4°C at
standard pressure, which is equivalent to 1.000 028 dm
3 .
lixiviation The separation of mixtures by dissolving soluble constituents in water.
LNG See liquefied petroleum gas.
localization The conÜnement of
electrons to a particular atom in a
molecule or to a particular chemical
bond.
localized bond A *chemical bond
in which the electrons forming the
bond remain between (or close to)
the linked atoms. Compare delocalization.
lock-and-key mechanism A
mechanism proposed in 1890 by
Emil Fischer (1852–1919) to explain
binding between the active site of an
enzyme and a substrate molecule.
The active site was thought to have a
Üxed structure (the lock), which exactly matched the structure of a
speciÜc substrate (the key). Thus the
enzyme and substrate interact to
form an *enzyme–substrate complex.
The substrate is converted to products that no longer Üt the active site
and are therefore released, liberating
the enzyme. Recent observations
made by X-ray diffraction studies
have shown that the active site of an
enzyme is more Ûexible than the
lock-and-key theory would suggest.
lodestone See magnetite.
logarithmic scale 1. A scale of
measurement in which an increase
or decrease of one unit represents a
tenfold increase or decrease in the
quantity measured. Decibels and pH
measurements are common examples of logarithmic scales of measurement. 2. A scale on the axis of a
graph in which an increase of one
unit represents a tenfold increase in
the variable quantity. If a curve y = x
n
is plotted on graph paper with logarithmic scales on both axes, the result is a straight line of slope n, i.e.
logy = nlogx, which enables n to be
determined.
London formula A formula giving
an expression for the induceddipole–induced-dipole interaction
between molecules (called the dispersion interaction or London interaction). The London formula for the
interaction energy V is given by V=
–C/r
6 , where C = ⅔α′ 1 α′ 2 I 1 I 2 /(I 1 + I 2 ).
Here α′ 1 and α′ 2 are the polarizability
volumes of molecule 1 and 2 respectively, I 1 and I 2 are the ionization
energies of molecules 1 and 2 respectively, and r is the distance between
the molecules. The London formula
is named after Fritz London (1900–
54), who derived it. The interaction
described by the London formula is
usually the dominant term in intermolecular forces (unless hydrogen
bonds are present).
lone pair A pair of electrons having
opposite spin in an orbital of an
atom. For instance, in ammonia the
nitrogen atom has Üve electrons,
three of which are used in forming
single bonds with hydrogen atoms.
The other two occupy a Ülled atomic
orbital and constitute a lone pair (see
illustration). The orbital containing
these electrons is equivalent to a single bond (sigma orbital) in spatial orientation, accounting for the
pyramidal shape of the molecule. In
the water molecule, there are two
lone pairs on the oxygen atom. In
331
lone pair
l
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or alkalinity, both in solution and as
litmus paper (absorbent paper soaked
in litmus solution).
litre Symbol l. A unit of volume in
the metric system regarded as a special name for the cubic decimetre. It
was formerly deÜned as the volume
of 1 kilogram of pure water at 4°C at
standard pressure, which is equivalent to 1.000 028 dm
3 .
lixiviation The separation of mixtures by dissolving soluble constituents in water.
LNG See liquefied petroleum gas.
localization The conÜnement of
electrons to a particular atom in a
molecule or to a particular chemical
bond.
localized bond A *chemical bond
in which the electrons forming the
bond remain between (or close to)
the linked atoms. Compare delocalization.
lock-and-key mechanism A
mechanism proposed in 1890 by
Emil Fischer (1852–1919) to explain
binding between the active site of an
enzyme and a substrate molecule.
The active site was thought to have a
Üxed structure (the lock), which exactly matched the structure of a
speciÜc substrate (the key). Thus the
enzyme and substrate interact to
form an *enzyme–substrate complex.
The substrate is converted to products that no longer Üt the active site
and are therefore released, liberating
the enzyme. Recent observations
made by X-ray diffraction studies
have shown that the active site of an
enzyme is more Ûexible than the
lock-and-key theory would suggest.
lodestone See magnetite.
logarithmic scale 1. A scale of
measurement in which an increase
or decrease of one unit represents a
tenfold increase or decrease in the
quantity measured. Decibels and pH
measurements are common examples of logarithmic scales of measurement. 2. A scale on the axis of a
graph in which an increase of one
unit represents a tenfold increase in
the variable quantity. If a curve y = x
n
is plotted on graph paper with logarithmic scales on both axes, the result is a straight line of slope n, i.e.
logy = nlogx, which enables n to be
determined.
London formula A formula giving
an expression for the induceddipole–induced-dipole interaction
between molecules (called the dispersion interaction or London interaction). The London formula for the
interaction energy V is given by V=
–C/r
6 , where C = ⅔α′ 1 α′ 2 I 1 I 2 /(I 1 + I 2 ).
Here α′ 1 and α′ 2 are the polarizability
volumes of molecule 1 and 2 respectively, I 1 and I 2 are the ionization
energies of molecules 1 and 2 respectively, and r is the distance between
the molecules. The London formula
is named after Fritz London (1900–
54), who derived it. The interaction
described by the London formula is
usually the dominant term in intermolecular forces (unless hydrogen
bonds are present).
lone pair A pair of electrons having
opposite spin in an orbital of an
atom. For instance, in ammonia the
nitrogen atom has Üve electrons,
three of which are used in forming
single bonds with hydrogen atoms.
The other two occupy a Ülled atomic
orbital and constitute a lone pair (see
illustration). The orbital containing
these electrons is equivalent to a single bond (sigma orbital) in spatial orientation, accounting for the
pyramidal shape of the molecule. In
the water molecule, there are two
lone pairs on the oxygen atom. In
331
lone pair
l
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