Structures and Properties of Substances

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Structures and Properties of Substances
12.1 Classification of Substances According to
Structures 12.2 Classification of Substances
According to the Nature of Bonding
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Classification of Substances According to
Structures
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Some pre-knowledge
12.1 Classification of substances according to
structures (SB p.302)
solid
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Molecular structures
12.1 Classification of substances according to
structures (SB p.302)

• 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

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12.1 Classification of substances according to
structures (SB p.302)
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Giant structures
12.1 Classification of substances according to
structures (SB p.302)

• All the atoms or ions are linked by strong
  bonding
• Classified into 3 types giant covalent, giant
  ionic and giant metallic structures

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12.1 Classification of substances according to
structures (SB p.302)
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12.1 Classification of substances according to
structures (SB p.303)
Classification of substances according to their
structures
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Classification of Substances According to the
Nature of Bonding
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Simple molecular substances
12.2 Classification of substances according to
the nature of bonding (SB p.304)
Examples
liquid iodine
(by raising the temp.)
solid iodine(at room temp.)
Gaseous iodine
(by further raising the temp.)
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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

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Giant covalent substances
12.2 Classification of substances according to
the nature of bonding (SB p.305)

• Non-metallic atoms are joined together by strong
  covalent bonds
• No discrete molecules are present
• Examples diamond, graphite, quartz

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Giant covalent substances
12.2 Classification of substances according to
the nature of bonding (SB p.305)
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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
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Unique properties of graphite
12.2 Classification of substances according to
the nature of bonding (SB p.306)
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Unique properties of graphite
12.2 Classification of substances according to
the nature of bonding (SB p.306)

• 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
• ? unpaired electrons of carbon atoms are
  delocalized within the layers

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Macromolecules
12.2 Classification of substances according to
the nature of bonding (SB p.306)

• Long chains of molecules containing a very large
  number of atoms
• Examples plastics, proteins, some carbohydrates

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Ionic substances
12.2 Classification of substances according to
the nature of bonding (SB p.307)

• 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

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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
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12.2 Classification of substances according to
the nature of bonding (SB p.307)
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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

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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

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12.2 Classification of substances according to
the nature of bonding (SB p.308)
Properties of metallic substances
4. Malleable and ductile
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12.2 Classification of substances according to
the nature of bonding (SB p.309)
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The END
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12.1 Classification of substances according to
structures (SB p.303)
Back
Check Point 12-1

• Classify the following elements or compounds into
  different types of molecular or giant structures
• Lithium fluoride
• Argon
• Uranium
• Polyester
• Glucose

1. Giant ionic structure
2. Simple molecular structure
3. Giant metallic structure
4. Macromolecular structure
5. Simple molecular structure

Answer
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12.2 Classification of substances according to
the nature of bonding (SB p.305)
Let's Think 1
Explain the ability of buckminsterfullerence to
partially conduct electricity in relation to its
structure.
Answer
It is because each carbon atom in
buckminsterfullerene forms three bonds only and
delocalized electrons are present which conduct
electricity.
Back
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12.2 Classification of substances according to
the nature of bonding (SB p.306)
Let's Think 2
Unlike diamond and graphite, fullerenes can be
dissolved in various solvents. Why?
Answer
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
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12.2 Classification of substances according to
the nature of bonding (SB p.309)
Example 12-2
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
Answer
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12.2 Classification of substances according to
the nature of bonding (SB p.309)
Example 12-2
(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)
Example 12-2
(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)
Check Point 12-2

• Explain the following characteristic properties
  of metals in terms of their structure and
  bonding.
• (i) Malleable
• (ii) High density
• (iii) High melting point

Answer
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12.2 Classification of substances according to
the nature of bonding (SB p.309)
Check Point 12-2

• (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)
Check Point 12-2

• Describe briefly how the structures of the
  following substances are related to their
  physical properties.
• (i) Quartz
• (ii) Magnesium

Answer
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12.2 Classification of substances according to
the nature of bonding (SB p.309)
Check Point 12-2
(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
Check Point 12-2
(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.