Lok Sin Tong Leung Chik Wai Memorial School
F. 6 Chemistry
Chapter 12: Bondings, Structures and Properties
Chapter 12: p.1
THE RELATIONSHIP BETWEEN STRUCTURES AND PROPERTIES OF MATERIALS
I.
Introduction
There are four main types of solid structures : 1. giant ionic, 2. giant covalent,3.giant metallic
and 4. simple molecular. Their physical properties can be explained in terms of their structures.
II.
Giant Ionic Structure
Giant ionic structure is a three-dimensional lattice structure formed by cations and anions
which are held by strong electrostatic forces of attraction.
In ionic lattice structure, there are
• electrostatic (or coulombic) forces of attraction between oppositely charged ions and
• electrostatic (or coulombic) forces of repulsion between ions of the same charge.
Overall, the attraction win over the repulsions. This maintain the ionic lattice structure.
Example :
The forces interacting in the ionic lattice of sodium chloride are shown below
(A) Physical properties of giant ionic structure
<1> Brittle
•
Ionic crystals are generally brittle because movement of the planes of ions may result in
similarly charged ions being opposite one another.
•
The planes involved repel each other and tend to separate.
Lok Sin Tong Leung Chik Wai Memorial School
F. 6 Chemistry
Chapter 12: Bondings, Structures and Properties
Chapter 12: p.2
<2> High density
The ions are brought together by strong electrostatic forces and they pack closely in the unit cell.
This results in a high density.
<3> Hard and high melting points and boiling points
• It requires a lot of thermal energy to break down the giant ionic lattice through
overcoming strong electrostatic attraction between unlikely charged ions.
The m.p. of three ionic solid are in the order:
Na2O
< MgO
< A12O3
m.p. 920°C
2900°C
2980°C
Reason : ________________________________________________________________
<4> Electrical conductivity
• Ionic solids cannot conduct electric current because there are no delocalized of mobile
electrons ion the ionic lattice.
• However, if the solid is melted or dissolved in aqueous solution. the ions become mobile (i.e.
delocalized) and are able to carry current. Ionic compounds are invariable electrolytes.
<5> Solubility
• Ionic compounds are usually soluble in polar solvent such as water, in which ions are free to
move and interact with the solvent molecules.
• They are usually insoluble in non-polar solvents which do not allow free ,movement of ions
and interactions with solvent molecules.
III. Giant Covalent Structure
Giant covalent structure is formed when atoms are held to each other through strong covalent
bonds. thereby generating in a macromolecule.
(A)
Physical properties of giant covalent structure
<1> Very high melting points and boiling points
• The destruction of the lattice involves breaking a large number of strong covalent bonds. A
large amount of thermal energy is required.
<2> Solubility
• Macromolecular crystals are very stable and are insoluble in polar or non-polar solvent.
(B) Physical properties of diamond and graphite
Both allotropes of carbon, graphite and diamond, have giant covalent structure. They have same
properties in m.p. , b.p. and solubility. However, their physical properties differ in several aspects.
Lok Sin Tong Leung Chik Wai Memorial School
F. 6 Chemistry
Chapter 12: Bondings, Structures and Properties
Chapter 12: p.3
<1> Diamond
• It is extremely hard because each carbon atom is held firmly in position by covalent bonds
to four other carbon atoms. It can be used to make glass cutters and drills.
• All valency electrons of carbon are utilized in forming covalent bonds. Diamond cannot
conduct electricity as a result.
<2> Graphite
• Graphite is soft and slippery because the hexagonal layers are held together by weak Van
der Waals’ forces. These layers slide -over one another easily. Graphite can be used as high
temperature lubricant in machine as a result.
• Graphite can be used to make crucibles for molten metals and electrodes since it has a very
high melting point.
• Graphite can conduct electricity along its layers it has delocalized cloud electrons between
the layers.
Note : Graphite cannot conduct electricity in a direction perpendicular to the layers because
electrons cannot pass through them.
Exercise 1 :
Graphite is used as lubricant whereas diamond is used as glass cutters. Explain these two
properties with the aid of diagrams.
Lok Sin Tong Leung Chik Wai Memorial School
F. 6 Chemistry
Chapter 12: Bondings, Structures and Properties
Chapter 12: p.4
Exercise 2 Graphite is a macromolecular crystal
(a) Describe the physical properties of graphite with respect to its structure.
(b) Silicon(IV) oxide is another macromolecular crystal. Describe its structure and discuss how its
electrical conductivity differs from that of graphite.
Lok Sin Tong Leung Chik Wai Memorial School
F. 6 Chemistry
Chapter 12: Bondings, Structures and Properties
Chapter 12: p.5
IV. Giant Metallic Structure
(A)
Physical properties of giant metallic structure
<1> Hard and high melting points and boiling points
The metallic bonds between cationic lattice and mobile valency electrons are very strong.
Metals are therefore hard. Large amount of thermal energy is required to break the strong
metallic bonds, resulting in high melting and boiling points.
<2> High density
• The metal atoms are closely packed in the metallic structure, accounting for a high density of
metal.
<3> Shinning lustre
• The delocalized electrons are in energy levels that are close together. When light falls onto a
metal, the electrons are readily excited to a higher energy level.
• A large number of transitions between energy levels is possible, such that a whole range of
frequencies of light can be absorbed. When the electrons return to lower energy states, the
absorbed radiation is emitted immediately as light.
<4> Malleable and ductile
• When one plane of metal ions slides over another, the delocalized can move easily between
the planes to maintain metallic bonding.
<5> Electrical conductivity
• Metals can conduct an electric current because their valence electrons are mobile and
delocalized in the “sea of electrons” throughout the cationic lattice.
• If a potential is applied across the metal, electrons will flow in one direction from the
negative terminal to positive terminal.
<6> Thermal conductivity
• When metal is heated, the mobile electrons at the hot vibrate and gain kinetic energy. They
hit the electrons at the cool end, which starts to vibrate as a result.
• The transfer of kinetic energy through the system of delocalized electrons accounts for
thermal conductivity.
Lok Sin Tong Leung Chik Wai Memorial School
F. 6 Chemistry
Chapter 12: Bondings, Structures and Properties
Chapter 12: p.6
Exercise 3
Explain why the electrical conductivities of caesium chloride and copper differ in solid state.
V.
Simple Molecular Structure
The molecules in molecular crystals are held in position in a molecular lattice by weak
intermolecualr forces, i.e. van der Waals’ forces (and possibly dipole-dipole interactions and/or
hydrogen bonds).
(A)
<1>
Physical properties
Low melting point and boiling point
• Because of the weak intermolecular attractive forces between molecules, little energy is
required to separate the molecules.
<2>
Poor electrical conductivity
• There are no charged particles (ions or electrons) delocalized throughout the molecular
crystal lattice to conduct electricity. They cannot conduct electricity in either the solid or
molten state.
<3> Soft and low density
• Van der Waals forces are weak and non-directional. The lattice is readily destroyed and the
crystal are soft and have low density.
<4> Solubility
• Molecular crystals tend to dissolve in non-polar solvents such as alcohol.
• They are usually insoluble in polar solvents such as water. However, some may dissolve in
water as a result of forming hydrogen bonds with it.
Exercise 4
Account for the fact that SiO2 is a solid with high melting point, whereas CO2 ,is a gas at room
temperature.
Lok Sin Tong Leung Chik Wai Memorial School
F. 6 Chemistry
Chapter 12: Bondings, Structures and Properties
VI.
Chapter 12: p.7
Summary : Bonding and the nature of solids
The table below summarizes the structure, bonding and physical properties of the four main
types of solid.
Examples
Giant ionic
Giant covalent
Giant metallic
sodium chloride,
caesium chloride
graphite, diamond,
quartz (SiO2)
iron, copper, silver
Simple
molecular
iodine, dry ice (solid
C02), ice, methane
A. STRUCTURE
Constituents
Type of
substance
ions
metal/non-metal
compound with a
large difference in
electronegativity
between elements
atoms
atoms
molecules
non-metal element in metal element with
non-metal element or
Group IV or its
low electronegativity non-metal/non-metal
compounds
compound (element
with high
electronegativity)
B. BONDING
Nature of
bonding-
Electrostatic
Attraction between
positive
ions and negative
ions forms strong
ionic bond.
Strong covalent
bonds link one
atoms to another
through the whole
structure.
Packing of
constituents
regular packing of
ions
regular packing of
atoms
regular packing of
atoms
Resulting
structure
.
non-directional bond directional bonds,
giving high
low coordination
coordination and
high density
Attraction of strong
outer mobile
electrons (sea of
electron)
for positive nuclei
binds atoms together
through metallic
bonds.
Strong Covalent
bonds hold atoms
separate together in
molecules.
Separate
molecules are
held together by
weak
intermolecular
forces (van der
Waals’ forces,
hydrogen bonding,
dipole-dipole
interactions or
induced dipole induced dipole
interactions).
•
•
solid: discrete
molecules in
regular packing
liquid/gas:
discrete
molecules
randomly
located
non-directional bonds, short range
high coordination and intermolecular forces
high density
giving low
coordination and low
density
Lok Sin Tong Leung Chik Wai Memorial School
F. 6 Chemistry
Chapter 12: Bondings, Structures and Properties
Chapter 12: p.8
C. PHYSICAL PROPERTIES
Giant ionic
Giant covalent
Giant metallic
Simple molecular
melting point
high
very high
high
low
Boiling point
high
very high
high
low
Hardness/
malleability
hard and brittle
very hard
and brittle
hard but malleable
and ductile
soft
Electrical
conductivity
non-conductors when
solid, good conductor
when molten or in
aqueous solution
(electrolytes).
conduction by
mobile ions
non-conductor
(except graphite)
good conductors
non-conductors
when solid, liquid
and in aqueous
solution (but some
may react with water
to form electrolytes,
Solubility in
polar solvent,
e.g. water
soluble
insoluble
insoluble
Insoluble (except with
reaction)
Solubility in
non-polar
solvent.
e.g. CCl4
insoluble
insoluble
insoluble (but
soluble in liquid
metals - alloy)
soluble