12
1
Structures and
Properties of
Substances
12.1
Classification of Substances According to
Structures
12.2
Classification of Substances According to
the Nature of Bonding
12.1
Classification of
Substances According
to Structures
2
12.1 Classification of substances according to structures (SB p.302)
Some pre-knowledge
solid
forces of
attraction
between
particles weaken
Particles (may be
molecules, ions or
metallic atoms)

gas



3
liquid
forces of
attraction
between particles
broken
12.1 Classification of substances according to structures (SB p.302)
Molecular structures
•
Consist of discrete molecules
•
Atoms in the molecules are held by strong
covalent bonds
•
Molecules are held together by weak
intermolecular forces
•
2 types: simple molecular structures,
macromolecular structures
4
12.1 Classification of substances according to structures (SB p.302)
Substances with simple molecular
structures may be solids, liquids
or gases at room conditions
Substances with
macromolecular
structures are always
solids at room conditions
5
12.1 Classification of substances according to structures (SB p.302)
Giant structures
•
All the atoms or ions are linked by strong
bonding
•
Classified into 3 types: giant covalent, giant
ionic and giant metallic structures
6
12.1 Classification of substances according to structures (SB p.302)
Substances with giant covalent (graphite), giant ionic
(sodium chloride) and giant metallic (copper) structures
7
12.1 Classification of substances according to structures (SB p.303)
Check Point 12-1
8
Classification of substances according to their
structures
12.2
Classification of
Substances According
to the Nature of
Bonding
9
12.2 Classification of substances according to the nature of bonding
(SB p.304)
Simple molecular substances
Examples
forces of
attraction
between
particles weaken
liquid iodine
(by raising the temp.)
solid iodine
(at room temp.)
Gaseous
iodine
10
forces of attraction
between particles broken
(by further raising the temp.)
12.2 Classification of substances according to the nature of bonding
(SB p.304)
Properties of simple molecular
substances
1. Low melting points and boiling points
2. Soft and easily distorted
3. Non-conductors of electricity
4. Usually insoluble in polar solvents but
soluble in non-polar solvents
Like dissolves like
11
Like dissolves like
Non-polar solvents dissolve non-polar
substances best
E.g. hexane dissolves I2
Polar solvents dissolve polar substances
best.
E.g. water dissolves NH3 / SO2 / HCl / NaCl
12
12.2 Classification of substances according to the nature of bonding
(SB p.305)
Giant covalent substances
•
Non-metallic atoms are joined together by
strong covalent bonds
•
No discrete molecules are present
•
Examples: diamond, graphite, quartz
13
12.2 Classification of substances according to the nature of bonding
(SB p.305)
Giant covalent substances
The giant covalent
structure of diamond
14
The giant covalent
structure of quartz
12.2 Classification of substances according to the nature of bonding
(SB p.306)
Properties of giant covalent substances
1. High melting points and boiling points
2. Hard and rigid
3. Poor conductors of heat and electricity
4. Insoluble in any solvents
15
12.2 Classification of substances according to the nature of bonding
(SB p.306)
Unique properties of graphite
The giant covalent structure of graphite
(a layered structure)
16
12.2 Classification of substances according to the nature of bonding
(SB p.306)
Unique properties of graphite
• Soft and can act as a lubricating agent
 layers of carbon atoms are held
together by weak van der Waals’ forces
• Good conductor of electricity
 unhybridized  electrons of carbon atoms
are delocalized within the layers
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12.2 Classification of substances according to the nature of bonding
(SB p.306)
Macromolecules
• Long chains of molecules containing a very
large number of atoms
• Examples: plastics, proteins, some
carbohydrates
• Higher m.p./b.p. than simple molecules due
to much stronger van der Waals’ forces
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12.2 Classification of substances according to the nature of bonding
(SB p.307)
Ionic substances
• Cations and anions are held together by ionic
bonds which are
strong non-directional electrostatic attraction
• Oppositely charged ions are closely packed
together to give a 3-dimensional giant lattice
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12.2 Classification of substances according to the nature of bonding
(SB p.307)
Properties of ionic substances
1. High melting and boiling points
2. Hard but brittle
The giant ionic structure fractures when
a stress is applied
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12.2 Classification of substances according to the nature of bonding
(SB p.307)
Properties of ionic substances
3. Good conductors of electricity in molten or
aqueous state
4. Soluble in polar solvents but insoluble in
non-polar solvents
Like dissolves like
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12.2 Classification of substances according to the nature of bonding
(SB p.307)
The formation of aqueous sodium chloride solution, in which
the ions are attracted and surrounded by water molecules
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12.2 Classification of substances according to the nature of bonding
(SB p.308)
Metallic substances
• Consist of a regular lattice of metallic cations
and a “sea” of delocalized electrons
• The lattice is held by strong electrostatic
attraction between the delocalized electrons
and metallic cations known as metallic bonds
23
12.2 Classification of substances according to the nature of bonding
(SB p.308)
Properties of metallic substances
1. High melting and boiling points
2. Good conductors of heat and electricity
3. High density
24
12.2 Classification of substances according to the nature of bonding
(SB p.308)
Properties of metallic substances
4. Malleable and ductile
Illustration of the malleability and ductility of a metal
25
12.2 Classification of substances according to the nature of bonding
(SB p.309)
Example 12-2
26
Check Point 12-2
The END
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12.1 Classification of substances according to structures (SB p.303)
Back
Classify the following elements or compounds into different
types of molecular or giant structures:
•
Lithium fluoride
(a) Giant ionic structure
•
Argon
(b) Simple molecular structure
•
Uranium
(c) Giant metallic structure
•
Polyester
•
Glucose
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(d) Macromolecular structure
(e) Simple molecular structure
Answer
12.2 Classification of substances according to the nature of bonding
(SB p.305)
Explain the ability of buckminsterfullerence to partially
conduct electricity in relation to its structure.
It is because each carbon atom in
buckminsterfullerene forms three 
bonds only and delocalized  electrons
are present which conduct electricity.
Back
29
Answer
12.2 Classification of substances according to the nature of bonding
(SB p.306)
Answer
Unlike diamond and graphite, fullerenes can be
dissolved in various solvents. Why?
Diamond and graphite have giant covalent structure.
They do not dissolve in any solvents because of the
presence of strong covalent bonds between all carbon
atoms.
However, fullerenes have simple molecular structure.
Only weak van der Waals’ forces have to be overcome in
the process of dissolution.
Back
30
12.2 Classification of substances according to the nature of bonding
(SB p.309)
What are the types of attractive forces present in each of the
following substances? Show how the physical properties are
related to their structure and bonding.
(a) Dry ice
(b) Sodium chloride
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Answer
12.2 Classification of substances according to the nature of bonding
(SB p.309)
(a) In dry ice, the atoms are joined together within the molecules by
strong covalent bonds while only weak van der Waals’ forces exist
between molecules. The melting and boiling points of dry ice are low
since only weak van der Waals’ forces are needed to be overcome
during the processes of melting and boiling. Besides, dry ice consists
of simple molecules with no mobile electrons. Thus, it does not
conduct electricity. Carbon dioxide is only slightly soluble in water
because it is non-polar in nature.
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12.2 Classification of substances according to the nature of bonding
(SB p.309)
(b) Strong ionic bonding exists between oppositely charged ions
throughout the whole lattice of a sodium chloride crystal. Thus, the
melting and boiling points of sodium chloride are very high. Moreover,
sodium chloride is hard since the ions are closely packed and strong
ionic bonds hold the ions together. It is soluble in water due to the
solvation of ions (i.e. the attraction between the ions and water
molecules which are polar) and is insoluble in non-polar solvents.
Sodium chloride conducts electricity in the molten or aqueous state
due to the presence of mobile ions in these states.
Back
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12.2 Classification of substances according to the nature of bonding
(SB p.309)
(a) Explain the following characteristic properties of metals
in terms of their structure and bonding.
(i) Malleable
(ii) High density
(iii) High melting point
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Answer
12.2 Classification of substances according to the nature of bonding
(SB p.309)
(a) (i) When stress is applied on a piece of metal, one plane of
metal ions slides over another. The delocalized electrons
take up new positions and the metallic bonds can be
maintained. Therefore, metals are malleable.
(ii) Metals have high densities as metal ions are closely
packed together in the metallic lattice.
(iii) Metals have high densities as metal ions are held tightly in
the giant lattice. A large amount of energy is required to
overcome the strong metallic bonds between the
cationic lattice and the delocalized electrons.
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12.2 Classification of substances according to the nature of bonding
(SB p.309)
(b) Describe briefly how the structures of the following
substances are related to their physical properties.
(i) Quartz
(ii) Magnesium
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Answer
12.2 Classification of substances according to the nature of bonding
(SB p.309)
(b) (i) Quartz is a substance with a giant covalent structure. In the
quartz lattice, each silicon atom is bonded tetrahedrally to four
neighbouring oxygen atoms whereas each oxygen atom is
bonded to two neighbouring silicon atoms. This gives rise to a
tetrahedral diamond-like structure. As the atoms are held
together by strong covalent bonds, a large amount of energy is
required to overcome the strong covalent bonds in the
processes of melting and boiling. Thus, quartz has high melting
and boiling points. Besides, quartz is hard and rigid as the atoms
are fixed in their positions by strong covalent bonds. Since there
are no mobile electrons, quartz is a poor conductor of heat and
electricity. Quartz is insoluble in both polar and non-polar
solvents
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12.2 Classification of substances according to the nature of bonding
(SB p.309)
Back
(b) (ii) Magnesium is a substance with a giant metallic structure. It has
high melting and boiling points as the magnesium ions are held
tightly in the giant lattice. A large amount of energy is required to
overcome the strong metallic bonds between the cationic lattice
and the delocalized electrons. Due to the presence of mobile
electrons, magnesium is a good conductor of heat and electricity.
It also has a high density as the magnesium ions are closely
packed together in the metallic lattice. Moreover, magnesium is
malleable and ductile. It is because when stress is applied, one
plane of metal ions will slide over another, and the delocalized
electrons will take up new positions. As a result, the metallic
bonds can be maintained.
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