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 volumeSee molar vol-
ume.
molecular weightSee relative
molecular mass.
molecule One of the fundamental
units forming a chemical compound;
the smallest part of a chemical com-
pound that can take part in a chemi-
cal reaction. In most covalent
compounds, molecules consist of
groups of atoms held together by co-
valent or coordinate bonds. Covalent
substances that form *macromolecu-
lar crystals have no discrete mol-
ecules (in a sense, the whole crystal
is a molecule). Similarly, ionic com-
pounds do not have single molecules,
being collections of oppositely
charged ions.
mole fractionSymbol X. A meas-
ure of the amount of a component in
a mixture. The mole fraction of com-
ponent A is given by XA= nA/N, where
nAis 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 formatA format using a
connection table and atom coordi-
nates 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.
molybdenumSymbol Mo. A sil-
very hard metallic *transition el-
ement; 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) sul-
phide (MoS 2 ) is used as a lubricant.
Chemically, it is unreactive, being
unaffected by most acids. It oxidizes
at high temperatures and can be dis-
solved in molten alkali to give a
range of molybdates and polymolyb-
dates. Molybdenum was discovered
in 1778 by Karl *Scheele.
A- Information from the WebElements site
moment of inertiaSymbol I. A
quantity associated with a body that
is rotating about an axis. If a body
consists of i particles of mass ma per-
pendicular distance rfrom an axis of
rotation, the moment of inertia I of
the body about that axis of rotation
is given by I = ∑miri^2. The moment of
inertia of a body is an important
quantity because it is the analogue
for rotational motion of mass for lin-
ear motion.
In a molecule all rotational motion
can be analysed using three perpen-
dicular axes of rotation. Each of these
has a moment of inertia associated
with it. To aÜrst 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 topall three moments
of inertia are equal (e.g. SF 6 ).
In a symmetric toptwo of the mo-
ments of inertia are equal (e.g. NH 3 ).
In an asymmetric topall three mo-
ments of inertia are different (e.g.
H 2 O).
In a linear rotorthe moment of iner-
tia 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 mol-
ecule has cubic or icosahedral sym-
359 moment of inertia
m