Metallic Bonding

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Metallic Bonding
10.1 Metallic Bonding 10.2 Metallic
Radius 10.3 Factors Affecting the Strength of
Metallic Bond 10.4 Metallic Crystals 10.5 Alloys
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Metallic Bonding
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Electron sea model of bonding in metals
10.1 Metallic bonding (SB p.261)

• 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

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Metallic Radius
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10.2 Metallic radius (SB p.262)
Atoms in a metallic crystal
Metallic radius (r) is defined as half of the
internuclear distance between atoms in a metal
crystal.
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10.2 Metallic radius (SB p.262)
Trend of metallic radius in the Periodic Table

• Moving down a group, metallic radii increase
• Going across a period, metallic radii decrease

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Factors Affecting the Strength of Metallic Bond
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10.3 Factors affecting the strength of metallic
bond (SB p.262)
Factors affecting the strength of metallic bond
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.
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10.3 Factors affecting the strength of metallic
bond (SB p.263)
Effect of number of valence electrons on metallic
bond strength
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10.3 Factors affecting the strength of metallic
bond (SB p.263)
Effect of metallic radius on metallic bond
strength of Group IA metals
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Metallic Crystals
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Closed-packed structure
10.4 Metallic crystals (SB p.263)
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10.4 Metallic crystals (SB p.265)
Hexagonal close-packed structure
(a) normal side view (b) exploded view
(c) a unit cell
Co-ordination no. ?
Empty space 26
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10.4 Metallic crystals (SB p.265)
Cubic close-packed / Face-centred cubic structure
(a) normal side view (b) exploded view
(c) a unit cell
Co-ordination no. ?
Empty space 26
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10.4 Metallic crystals (SB p.266)
Holes in close-packed structures

• Tetrahedral hole formed when a sphere sits on
  the depression formed by three spheres in an
  adjacent layer

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10.4 Metallic crystals (SB p.266)
Holes in close-packed structures

• Octahedral hole formed between three spheres in
  one layer and three in an adjacent layer

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Open structure
10.4 Metallic crystals (SB p.266)

• Structures with more empty space between the
  atoms
• Most common body-centred cubic structure

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10.4 Metallic crystals (SB p.267)
Body-centred cubic structure
(a) normal side view (b) exploded view
(c) a unit cell
Empty space 32
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10.4 Metallic crystals (SB p.267)
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Alloys
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10.5 Alloys (SB p.268)
Alloys

• Alloy a material with metallic properties
• Made by mixing a metal with one or more other
  elements

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10.5 Alloys (SB p.268)
Structure 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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10.5 Alloys (SB p.268)
Structure of alloy

• In an alloy, atoms of different sizes are present

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10.5 Alloys (SB p.268)
Structure 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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10.5 Alloys (SB p.269)
Types of alloys

• 2 common types of alloys
• Substitutional alloy
• Interstitial alloy

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10.5 Alloys (SB p.269)
Substitutional alloy

• Some of the host metallic atoms are replaced by
  other metallic atoms of similar sizes
• e.g. in brass

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10.5 Alloys (SB p.269)
Interstitial 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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10.5 Alloys (SB p.269)
Some common alloys - Steel

• An alloy of iron
• Amount of carbon present affects the properties
  of steel
• Mild steel contains lt0.2 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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10.5 Alloys (SB p.269)
Some common alloys - Steel
Articles made from stainless steel
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10.5 Alloys (SB p.270)
Some common alloys Copper alloys

• Brass - an alloy of copper and zinc

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10.5 Alloys (SB p.270)
Some common alloys Copper alloys

• Coinage metals

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10.5 Alloys (SB p.270)
Some common alloys Solder

• An alloy of lead and tin

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The END
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10.3 Factors affecting the strength of metallic
bond (SB p.263)
Back
Let's Think 1
It is said that bonding in most metals is strong
but non-directional. Can you think of some facts
to support the above statement?
Answer
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.
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10.4 Metallic crystals (SB p.266)
Back
Let's Think 2
How many tetrahedral holes and octahedral holes
are there adjacent to each sphere in cubic
close-packed structure?
Answer
In cubic close-packed structure, there are 6
octahedral holes and 8 octahedral holes adjacent
to each sphere.
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10.4 Metallic crystals (SB p.267)
Example 10-4

• 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
• aluminium and
• potassium

Answer
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10.4 Metallic crystals (SB p.267)
Example 10-4
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10.4 Metallic crystals (SB p.267)
Example 10-4
Back
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10.4 Metallic crystals (SB p.268)
Check Point 10-4

• 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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10.4 Metallic crystals (SB p.268)
Check Point 10-4
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10.4 Metallic crystals (SB p.268)
Check Point 10-4
(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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10.4 Metallic crystals (SB p.268)
Back
Check Point 10-4
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10.5 Alloys (SB p.271)
Check Point 10-5

• (i) Give two advantages of steel compared to the
  pure iron.
• (ii) Why is tungsten added to certain types of
  alloy steels?

Answer

• (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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10.5 Alloys (SB p.271)
Check Point 10-5
(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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10.5 Alloys (SB p.271)
Back
Check Point 10-5

• (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 no 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, it
solidifies and joins the two metals together.