New Way Chemistry for Hong Kong A-Level Book 1 1 Metallic Bonding 10.1Metallic Bonding 10.2Metallic Radius 10.3Factors Affecting the Strength of Metallic

New Way Chemistry for Hong Kong A-Level Book 1

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Metallic BondingMetallic Bonding

10.110.1 Metallic BondingMetallic Bonding

10.210.2 Metallic RadiusMetallic Radius

10.310.3 Factors Affecting the Strength of Metallic Factors Affecting the Strength of Metallic BondBond

10.410.4 Metallic CrystalsMetallic Crystals

10.510.5 AlloysAlloys

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10.10.11 Metallic Metallic

BondingBonding


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Electron sea model of bonding in Electron sea model of bonding in metalsmetals

• The structure of metal consists of a giant structure of cationic lattice immersed in a sea of mobile valence electrons

• The electrostatic attraction between the delocalized electron cloud and the metallic ions is the metallic bonding

10.1 Metallic bonding (SB p.261)


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RadiusRadius


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Metallic radius (r) is defined as half of the internuclear distance between atoms in a metal crystal.

Metallic radius (r) is defined as half of the internuclear distance between atoms in a metal crystal.

Atoms in a metallic Atoms in a metallic crystalcrystal

10.2 Metallic radius (SB p.262)


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Trend of metallic radius in Trend of metallic radius in the Periodic Tablethe Periodic Table

10.2 Metallic radius (SB p.262)

• Moving down a group, metallic radii increase

• Going across a period, metallic radii decrease


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10.10.33 Factors Factors

Affecting the Affecting the Strength of Strength of

Metallic BondMetallic Bond


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The metallic bond increases with:

1. decreasing size of the metal atom (i.e. the atomic/metallic radius); 2. increasing number of valence electrons of the metal atom.

Factors affecting the strength of Factors affecting the strength of metallic bondmetallic bond

10.3 Factors affecting the strength of metallic bond (SB p.262)


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Effect of number of valence Effect of number of valence electrons on metallic bond electrons on metallic bond strengthstrength

Metal Number of valence electrons(s)

Melting point (oC)

Sodium

Magnesium

Aluminium

1

2

3

98

650

660



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Effect of metallic radius on metallic Effect of metallic radius on metallic bond strength of Group IA metalsbond strength of Group IA metals

Metal Metallic radius (mm)

Melting point (oC)

Lithium

Sodium

Potassium

Rubidium

Caesium

0.152

0.186

0.231

0.244

0.262

180

98

64

39

29



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CrystalsCrystals


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Closed-packed structureClosed-packed structure

10.4 Metallic crystals (SB p.263)


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(a) normal side view (b) exploded view (c) a unit cell

Co-ordination no. = ?Co-ordination no. = ? Empty space = 26 %Empty space = 26 %

Hexagonal close-packed structureHexagonal close-packed structure



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Co-ordination no. = ?Co-ordination no. = ? Empty space = 26 %Empty space = 26 %

Cubic close-packed / Face-centred cubic strucCubic close-packed / Face-centred cubic structureture



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Holes in close-packed Holes in close-packed structuresstructures• Tetrahedral hole: formed when a sphere sits on the depression formed by three spheres in an adjacent layer

A tetrahedral hole formed by four spheres


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Holes in close-packed Holes in close-packed structuresstructures• Octahedral hole: formed between three spheres in one layer and three in an adjacent layer

A octahedral hole formed by six spheres


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Open structureOpen structure10.4 Metallic crystals (SB p.266)

• Structures with more empty space between the atoms

• Most common: body-centred cubic structure


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Empty space = 32 %Empty space = 32 %

Body-centred cubic structureBody-centred cubic structure10.4 Metallic crystals (SB p.267)


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Example 10-4Example 10-4 Check Point 10-4Check Point 10-4


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10.10.55 AlloysAlloys


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10.5 Alloys (SB p.268)

AlloyAlloyss

• Alloy – a material with metallic properties

• Made by mixing a metal with one or more other elements


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Structure of alloyStructure of alloy• Have structures and properties different

from that of a pure metal

• In a pure metal, all the atoms are of the same size


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Structure of alloyStructure of alloy• In an alloy, atoms of different sizes are

present


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Structure of alloyStructure of alloy

Changes the regular arrangement of the layers of atoms in the metal

Slipping of layers of atoms becomes more difficult

Harder and stronger


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Types of Types of alloysalloys• 2 common types of alloys:

Substitutional alloy

Interstitial alloy


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Substitutional alloySubstitutional alloy• Some of the host metallic atoms are

replaced by other metallic atoms of similar sizes

• e.g. in brass


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Interstitial alloyInterstitial alloy• Formed when some of the interstices among

the closely packed host metallic atoms are occupied by atoms of smaller atomic sizes

• e.g. in steel



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Some common alloys - Some common alloys - SteelSteel• An alloy of iron

• Amount of carbon present affects the properties of steel

• Mild steel: contains <0.2 % carbon, ductile, malleable

• Medium steel: contains 0.2 – 0.6 % carbon, harder

• High-carbon steel: contains 0.6 – 1.5 %, tough and hard


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Some common alloys - Some common alloys - SteelSteel

Articles made from stainless steel


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Some common alloys – Copper Some common alloys – Copper alloysalloys• Brass - an alloy of copper and zinc

Article made from brass


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Some common alloys – Copper Some common alloys – Copper alloysalloys• Coinage metals


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Some common alloys – SolderSome common alloys – Solder

• An alloy of lead and tin

Check Point 10-5Check Point 10-5


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The END


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It is said that bonding in most metals is strong but non-directional. Can you think of some facts to

support the above statement?

Metals are durable and have high melting (and boiling) points. These indicate that metallic bonds are strong. On the other hand, metals can be pulled into wires or hammered into sheets (I.e. it is relatively easy to change the shape of most metals). This shows that metal atoms can slide over each other which is a consequence of the non-directional nature of the metallic bond.

Answer


Back


35

How many tetrahedral holes and octahedral holes are there adjacent to each sphere in cubic close-

packed structure?

In cubic close-packed structure, there are 6 octahedral holes and 8 octahedral holes adjacent to each sphere.

Answer

Back



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X-ray crystallography shows that aluminium and potassium have f.c.c. and b.c.c. structures respectively. Calculate the number of atoms in a unit cell of

(a) aluminium; and

(b) potassiumAnswer



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(a)

For the face-centred cubic structure of aluminium, an atom on each face of the unit cell is shared by two cell and so of

the atom belongs to the unit cell; an atom at each corner is shared by eight cells and so of the atom belongs to the unit cell.

Number of aluminium atoms in a unit cell =

= 4

21

81

81

821

6


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(b)

For the body-centred cubic structure of potassium, an atom at the centre of the unit cell is not shared with other cells and totally belongs to the unit cell; an atom at each corner is shared by eight cells and so of the atom belongs to the unit cell.

Number of potassium atoms in a unit cell =

= 2

81

81

81

Back


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(a)X-ray crystallography shows that copper has the cubic close-packed structure. Calculate the number of atoms in a unit cell of copper.

Answer



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(a) For the cubic close-packed structure of copper, an atom on each face of the unit cell is shared by two cells and of the atom belongs to the unit cell; an atom at each corner is shared by 8 cells and so of the atom belongs to the unit cell.

Number of Cu atoms in a unit cell =

= 4

21

81

81

821

6


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(b) It is a known that sodium metal has a body-centred cubic structure.

(i) Draw a unit cell of sodium.

(ii) Is this structure a close-packed structure? Explain this in terms of the coordination number of sodium.

Answer



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Back

(b) (i) A unit cell of sodium is drawn as follows:

(ii) Refer to the unit cell drawn in (b)(i), one atom is at each of the eight corners of a cube, and one atom is at the centre touching these eight atoms, so the coordination number of the central atom is 8. Thus, the structure is not a close-packed structure.


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(a)(i) Give two advantages of steel compared to the pure iron.

(ii) Why is tungsten added to certain types of alloy steels?

Answer


(a) (i) Steel is harder and stronger than iron. It is also less ductile.

(ii) The addition of metal tungsten to certain types of alloy steels make them become hard and strong with a very

high melting point. These materials are ideal for making high- speed cutting tools.


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(b) Cupronickel replaced earlier silver coins which contained silver.

Give two reasons for the replacement.Answer


(b) The main reason for the replacement was due to the relatively high cost of silver, as the cost of making a pure silver coin was higher than the value of the coin. Besides, cupronickel is much harder and more durable than pure silver.


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(c) (i) Why the low melting point of solder makes it useful in joining metals together?

(ii) Explain how soldering joins up metals.Answer


(c) (i) Due to the low melting point of solder, it needs not to ne heated up to a high temperature. As a result, there is n

o risk for the metals to be joined to melt during soldering.

(ii) Solder is melted by an electrically heated rod. When it melts, it flows over the two metal parts. When it cools, i

t solidifies and joins the two metals together.

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New Way Chemistry for Hong Kong A-Level Book 1 1 Metallic Bonding 10.1Metallic Bonding 10.2Metallic Radius 10.3Factors Affecting the Strength of Metallic