analysed systematically using *group
theory. It is possible to make deÜnite
statements about certain properties
of molecules, such as whether they
can have an electric dipole moment
or exhibit optical activity on the
basis of symmetry.
molecular volume See molar volume.
molecular weight See relative
molecular mass.
molecule One of the fundamental
units forming a chemical compound;
the smallest part of a chemical compound that can take part in a chemical reaction. In most covalent
compounds, molecules consist of
groups of atoms held together by covalent or coordinate bonds. Covalent
substances that form *macromolecular crystals have no discrete molecules (in a sense, the whole crystal
is a molecule). Similarly, ionic compounds do not have single molecules,
being collections of oppositely
charged ions.
mole fraction Symbol X. A measure of the amount of a component in
a mixture. The mole fraction of component A is given by X A = n A /N, where
n A is the amount of substance of A
(for a given entity) and N is the total
amount of substance of the mixture
(for the same entity).
molÜle format A format using a
connection table and atom coordinates to hold information about
chemical structures. It was produced
by Elsevier MDL and is widely used in
chemical drawing programs. Files
have the extension .mol.
Molisch’s test See alpha-naphthol
test.
molybdenum Symbol Mo. A silvery hard metallic *transition element; a.n. 42; r.a.m. 95.94; r.d.
10.22; m.p. 2617°C; b.p. 4612°C. It is
found in molybdenite (MoS 2 ), the
metal being extracted by roasting to
give the oxide, followed by reduction
with hydrogen. The element is used
in alloy steels. Molybdenum(IV) sulphide (MoS 2 ) is used as a lubricant.
Chemically, it is unreactive, being
unaffected by most acids. It oxidizes
at high temperatures and can be dissolved in molten alkali to give a
range of molybdates and polymolybdates. Molybdenum was discovered
in 1778 by Karl *Scheele.
A
• Information from the WebElements site
moment of inertia Symbol I. A
quantity associated with a body that
is rotating about an axis. If a body
consists of i particles of mass m a perpendicular distance r from an axis of
rotation, the moment of inertia I of
the body about that axis of rotation
is given by I = ∑ m i r i
2 . The moment of
inertia of a body is an important
quantity because it is the analogue
for rotational motion of mass for linear motion.
In a molecule all rotational motion
can be analysed using three perpendicular axes of rotation. Each of these
has a moment of inertia associated
with it. To a Ürst approximation, a
molecule can be regarded as a rigid
rotor, i.e. a body that is not distorted
by its rotation. There are four types
of rigid rotor:
In a spherical top all three moments
of inertia are equal (e.g. SF 6 ).
In a symmetric top two of the moments of inertia are equal (e.g. NH 3 ).
In an asymmetric top all three moments of inertia are different (e.g.
H 2 O).
In a linear rotor the moment of inertia about the axis of the molecule is
zero (e.g. HCl, CO 2 ).
The type of rotor depends on the
symmetry of the molecule. If a molecule has cubic or icosahedral sym359
moment of inertia
m
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theory. It is possible to make deÜnite
statements about certain properties
of molecules, such as whether they
can have an electric dipole moment
or exhibit optical activity on the
basis of symmetry.
molecular volume See molar volume.
molecular weight See relative
molecular mass.
molecule One of the fundamental
units forming a chemical compound;
the smallest part of a chemical compound that can take part in a chemical reaction. In most covalent
compounds, molecules consist of
groups of atoms held together by covalent or coordinate bonds. Covalent
substances that form *macromolecular crystals have no discrete molecules (in a sense, the whole crystal
is a molecule). Similarly, ionic compounds do not have single molecules,
being collections of oppositely
charged ions.
mole fraction Symbol X. A measure of the amount of a component in
a mixture. The mole fraction of component A is given by X A = n A /N, where
n A is the amount of substance of A
(for a given entity) and N is the total
amount of substance of the mixture
(for the same entity).
molÜle format A format using a
connection table and atom coordinates to hold information about
chemical structures. It was produced
by Elsevier MDL and is widely used in
chemical drawing programs. Files
have the extension .mol.
Molisch’s test See alpha-naphthol
test.
molybdenum Symbol Mo. A silvery hard metallic *transition element; a.n. 42; r.a.m. 95.94; r.d.
10.22; m.p. 2617°C; b.p. 4612°C. It is
found in molybdenite (MoS 2 ), the
metal being extracted by roasting to
give the oxide, followed by reduction
with hydrogen. The element is used
in alloy steels. Molybdenum(IV) sulphide (MoS 2 ) is used as a lubricant.
Chemically, it is unreactive, being
unaffected by most acids. It oxidizes
at high temperatures and can be dissolved in molten alkali to give a
range of molybdates and polymolybdates. Molybdenum was discovered
in 1778 by Karl *Scheele.
A
• Information from the WebElements site
moment of inertia Symbol I. A
quantity associated with a body that
is rotating about an axis. If a body
consists of i particles of mass m a perpendicular distance r from an axis of
rotation, the moment of inertia I of
the body about that axis of rotation
is given by I = ∑ m i r i
2 . The moment of
inertia of a body is an important
quantity because it is the analogue
for rotational motion of mass for linear motion.
In a molecule all rotational motion
can be analysed using three perpendicular axes of rotation. Each of these
has a moment of inertia associated
with it. To a Ürst approximation, a
molecule can be regarded as a rigid
rotor, i.e. a body that is not distorted
by its rotation. There are four types
of rigid rotor:
In a spherical top all three moments
of inertia are equal (e.g. SF 6 ).
In a symmetric top two of the moments of inertia are equal (e.g. NH 3 ).
In an asymmetric top all three moments of inertia are different (e.g.
H 2 O).
In a linear rotor the moment of inertia about the axis of the molecule is
zero (e.g. HCl, CO 2 ).
The type of rotor depends on the
symmetry of the molecule. If a molecule has cubic or icosahedral sym359
moment of inertia
m
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