1 Structures and Properties of Substances 12.1Classification of Substances According to Structures 12.2Classification of Substances According to the Nature.

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Structures and Structures and Properties of Properties of SubstancesSubstances

12.112.1 Classification of Substances According to Classification of Substances According to StructuresStructures

12.212.2 Classification of Substances According to Classification of Substances According to the Nature of Bondingthe Nature of Bonding

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Classification of Classification of Substances Substances According to According to StructuresStructures

12.12.11

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Some pre-Some pre-knowledgeknowledge

solid

Particles (may be molecules, ions or metallic atoms)

forces of attraction between particles weaken

liquid

gas

forces of attraction between particles broken

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

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Molecular Molecular structuresstructures

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)

Substances with simple molecular structures may be

solids, liquids or gases at room conditions

Substances with macromolecular

structures are always solids at room

conditions

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

Giant Giant structuresstructures

• 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)

Substances with giant covalent (graphite), giant ionic (sodium chloride) and giant metallic (copper)

structures

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12.1 Classification of substances according to structures (SB p.303)

Classification of substances according to their structures

Check Point 12-1Check Point 12-1

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Classification of Classification of Substances Substances

According to the According to the Nature of Nature of BondingBonding

12.12.22

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Simple molecular Simple molecular substancessubstancesExamples

forces of attraction between particles weaken

(by raising the temp.)

forces of attraction between particles broken

(by further raising the temp.)

solid iodine(at room temp.)

solid iodine(at room temp.)

liquid iodineliquid iodine

Gaseous iodine

Gaseous iodine

12.2 Classification of substances according to the nature of bonding (SB p.304)

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Properties of simple Properties of simple molecular substancesmolecular 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

12.2 Classification of substances according to the nature of bonding (SB p.304)

Like dissolves like

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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

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Giant covalent Giant covalent substancessubstances

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 Giant covalent substancessubstances

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

The giant covalent structure of

diamond

The giant covalent structure of quartz

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Properties of giant covalent Properties of giant covalent substancessubstances

1. High melting points and boiling points

2. Hard and rigid

3. Poor conductors of heat and electricity

4. Insoluble in any solvents

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

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Unique properties of graphiteUnique properties of graphite

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

The giant covalent structure of graphite (a layered structure)

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Unique properties of graphiteUnique 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

unhybridized electrons of carbon atoms are delocalized within the layers

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MacromoleculeMacromoleculess

• 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

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

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

Ionic Ionic substancessubstances• 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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Properties of ionic substancesProperties of ionic substances

1. High melting and boiling points

2. Hard but brittle

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

The giant ionic structure fractures when a stress is applied

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Properties of ionic substancesProperties of ionic substances

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

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 Metallic substancessubstances

• 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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Properties of metallic Properties of metallic substancessubstances1. High melting and boiling points

2. Good conductors of heat and electricity

3. High density

12.2 Classification of substances according to the nature of bonding (SB p.308)

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Properties of metallic Properties of metallic substancessubstances

12.2 Classification of substances according to the nature of bonding (SB p.308)

4. Malleable and ductile

Illustration of the malleability and ductility of a metal

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Example 12-2Example 12-2 Check Point 12-2Check Point 12-2

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)

Classify the following elements or compounds into different types of molecular or giant structures:

• Lithium fluoride

• Argon

• Uranium

• Polyester

• Glucose

(a) Giant ionic structure

(b) Simple molecular structure

(c) Giant metallic structure

(d) Macromolecular structure

(e) Simple molecular structure

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Answer

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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.

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It is because each carbon atom in buckminsterfullerene forms three bonds only and delocalized electrons are present which conduct electricity.

Answer

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

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.

Answer

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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 Answer

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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.

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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 Answer

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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 Answer

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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)

(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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