Chapter 27
Structures and properties of molecular
crystals
27.1 Molecular crystals
27.2 Structure and properties of ice
27.3 Structure and properties of C60
Key terms
Progress check
Summary
Concept map
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27.1 Molecular crystals
Crystals having an ordered arrangement of
molecules are called molecular crystals.
Examples of molecular crystals:
Iodine
Ice
Table sugar (sucrose)
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(a)
(b)
Figure 27.2 Examples of molecular crystals. (a) Ice (b) Table sugar.
27.1 Molecular crystals
Class practice 27.1
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27.2 Structure and properties of ice
Structure and bonding of ice
empty space
hydrogen bond
a water molecule
Figure 27.3 The structure
of ice.
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Tetrahedral arrangement of water
molecules in ice
a water molecule
hydrogen bond
The oxygen atom of each
water molecule forms
hydrogen bonds with
two hydrogen atoms of
nearby water molecules.
Keys:
hydrogen
oxygen
27.2 Structure and properties of ice
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hydrogen bond
The two hydrogen atoms of
each water molecule also
form hydrogen bonds with
oxygen atoms of nearby
water molecules.
Keys:
hydrogen
oxygen
27.2 Structure and properties of ice
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Each water molecule can form a total of four
hydrogen bonds with four neighbouring water
molecules tetrahedrally.
1
2
4
3
27.2 Structure and properties of ice
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Learning tip
•
•
The O atom of each H2O molecule forms hydrogen
bonds with two H atoms of nearby H2O molecules.
The two H atoms of each water molecule also form
hydrogen bonds with O atoms of nearby H2O
molecules.
27.2 Structure and properties of ice
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In ice, the tetrahedral arrangement of water
molecules repeats over and over again, resulting in
a regular network structure.
Because of the empty space among the water
molecules, the structure is described as an open
structure.
Learning tip
In liquid water, hydrogen bonds still exist between
molecules. However, they form and are broken
continuously. There is no regular packing of water
molecules.
27.2 Structure and properties of ice
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Hexagonal arrangement of oxygen atoms in ice
Figure 27.5 The oxygen atoms in the structure of ice are arranged in
a hexagonal shape.
27.2 Structure and properties of ice
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Figure 27.6 The hexagonal shape of a snowflake reflects
the structure of ice.
27.2 Structure and properties of ice
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Properties of ice
Density
Ice has a lower density than liquid water.
ice
water
Figure 27.7 Ice is less dense than
liquid water and so floats on the
water.
27.2 Structure and properties of ice
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In ice, water molecules are arranged in an orderly
manner in an open structure because of the
extensive hydrogen bonding between them.
In this structure, water molecules are further apart
than they are in liquid water. Thus, they take up a
larger volume.
When ice melts, the open structure collapses.
The water molecules pack more closely together.
Example 27.1
27.2 Structure and properties of ice
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Melting point
The melting point of ice is 0°C.
Water has a high melting temperature
compared with substances of similar relative
molecular masses.
Relative molecular
Melting point (°°C)
Substance
mass
Methane
16.0
–183
Ammonia
17.0
–78
Water
18.0
0
Table 27.1 Melting points of some substances with similar relative
molecular masses.
27.2 Structure and properties of ice
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When ice melts, a considerable amount of energy
is required to overcome some of the hydrogen
bonds between water molecules.
As some hydrogen bonds are broken, the open
structure of ice collapses.
Key point
The presence of extensive hydrogen bonding
between water molecules results in an open
structure of ice. Because of this special structure
lower density than liquid
and bonding, ice has a ______
high melting point.
water and a relatively ______
Class practice 27.2
27.2 Structure and properties of ice
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27.3 Structure and properties of C60
Fullerenes are molecules composed entirely of
carbon atoms, in the form of hollow spheres
or hollow tubes.
C60 and carbon nanotube are commonly known
fullerenes.
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Structure and bonding of C60
The first fullerene discovered was C60 (or
buckminsterfullerene, buckyball).
C60 is composed of 60 carbon atoms in a
hollow sphere.
(a)
(b)
Figure 27.8 (a) The structure of C60. (b) A soccer ball.2
27.3 Structure and properties of C60
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The 60 carbon atoms in a molecule of buckyball
are bonded together to form a hollow sphere.
Each carbon atom is bonded to three other carbon
atoms.
The atoms are arranged in a pattern of 20
hexagons and 12 pentagons on the surface of
the sphere.
27.3 Structure and properties of C60
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Properties of C60
Melting point
C60 molecules are held together by weak van
der Waals’ forces.
The melting point of C60 (1070°C) is not that
high compared with those of graphite (3730°C)
and diamond (3550°C).
27.3 Structure and properties of C60
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Solubility
C60 is a molecular solid which is non-polar.
It cannot dissolve in polar solvents like water.
It can dissolve in non-polar solvents like
benzene.
27.3 Structure and properties of C60
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Strength and hardness
The molecules of C60 in the solid state are
packed closely together.
C60 molecules are relatively strong and hard
compared with most other molecular solids.
C60 molecule
Figure 27.9 The C60 molecules are
packed closely together in the solid
state.
27.3 Structure and properties of C60
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Electrical conductivity
Pure C60 does not conduct electricity well.
However, it can react with potassium to make
a superconducting material with the formula of
K3C60.
The electrical resistance of this material falls to
zero below –243°C.
C60 molecule
potassium atom
Example 27.2
Class practice 27.3
Figure 27.10 The C60
with potassium atoms
filling the spaces
between its molecules is
a superconductor. Its
formula is K3C60.
27.3 Structure and properties of C60
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Key point
C60 is an example of fullerenes. It is a molecule
composed of 60 carbon atoms, in the form of a
sphere
hollow _______.
Experiment 27.1
27.3 Structure and properties of C60
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Activity 27.1
Ice
•
Bonding
•
Hydrogen bonding •
and van der Waals’
forces between H2O
•
molecules
Covalent bonding
between H and O
atoms
C60
Van der Waals’
forces between
C60 molecules
Covalent bonding
between C atoms
Table 27.2 A summary of the bonding, structures and properties of ice and C60.
27.3 Structure and properties of C60
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Ice
C60
a carbon atom
empty
space
Structure
hydrogen
bond
a water
molecule
covalent bond
Table 27.2 A summary of the bonding, structures and properties of ice and C60.
27.3 Structure and properties of C60
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Ice
C60
• Less dense than
• Lower melting point when
liquid water
compared with graphite
and diamond
• Higher melting
point compared
• Soluble in non-polar
with substances of
solvents like benzene
similar relative
• Relatively strong and hard
molecular masses
Properties
when compared with most
other molecular solids
• Does not conduct
electricity when pure;
becomes superconducting
when metal atoms (e.g. K)
are present in the structure
Table 27.2 A summary of the bonding, structures and properties of ice and C60.
STSE connections 27.1
STSE connections 27.2
27.3 Structure and properties of C60
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Key terms
1.
2.
3.
4.
5.
C60/buckminsterfullerene C60/布克碳
ice 冰
molecular crystal 分子晶體
open structure 敞開結構
regular network structure 規律的網絡結構
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Progress check
1. What is meant by a ‘molecular crystal’?
2. What are the attractive forces that hold water
molecules together in ice?
3. How are the water molecules arranged in ice?
4. Why is ice less dense than liquid water?
5. Why does ice have a higher melting point than
substances of similar relative molecular masses?
6. How the carbon atoms are bonded in a C60
molecule?
7. How are the properties of C60 related to its
bonding and structure?
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Summary
27.1 Molecular crystals
1.
Molecular solids in which molecules have an
ordered arrangement are known as molecular
crystals.
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27.2 Structure and properties of ice
2.
For each water molecule in ice, the oxygen atom
forms hydrogen bonds with two hydrogen atoms
of the neighbouring molecules. The two
hydrogen atoms also form hydrogen bonds with
oxygen atoms of the neighbouring water
molecules. Hence, each water molecule in ice
can form four hydrogen bonds with four water
molecules tetrahedrally.
Summary
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3.
Ice has a regular network structure in which
each water molecule is surrounded by four
other water molecules. Because of the empty
space among water molecules, the structure is
described as an open structure.
4.
In ice, the water molecules are arranged in an
orderly manner because of the extensive
hydrogen bonding between them. They are
further apart than they are in liquid water. This
explains why liquid water is denser than ice.
Summary
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5.
During the melting of ice, a considerable amount
of energy is required to overcome some of the
hydrogen bonds between the water molecules.
This explains why ice has a relatively high
melting point.
27.3 Structure and properties of C60
6.
Each molecule of C60 has 60 carbon atoms
which are bonded together to form a hollow
sphere.
Summary
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7.
Because of the special structure of C60, it is
strong, hard and soluble in non-polar solvents.
They have relatively lower melting points than
diamond and graphite. In pure form, C60 does
not conduct electricity well. However, when it
reacts with potassium, it becomes a
superconducting material.
Summary
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Concept map
Molecular substances
ordered
with ________
arrangement of
molecules
MOLECULAR CRYSTALS
examples
Ice
C60
_____
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MOLECULAR CRYSTALS
examples
Ice
physical
properties
C60
______
physical
properties
Low
High
Poor
Strong Lower melting
density
_______ _______
melting
electrical
and
point than
point
_______ conductor _____
hard graphite and
diamond
Concept map
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