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10. 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. Nature of Metallic Bonding. Li(g) + Li(g) Li + Li  (g) (1). Li(g) + Cl(g) Li + Cl  (g) (2).

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slide1

10

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

slide3

Li(g) + Li(g) Li+ Li(g) (1)

Li(g) + Cl(g) Li+ Cl(g) (2)

Not ionic : -

∵ atoms of the same electronegativity

No favourable

favourable

slide4

Li(g) + Li(g) Li+ Li(g) (1)

Li(g) + Cl(g) Li+ Cl(g) (2)

Q.57 Explain, with the aid of suitable enthalpy change cycles, why reaction (1) is energetically less favourable than reaction (2).

(1st EA : Li = 60 kJ mol1, Cl = 349 kJ mol1)

slide5

H2

H4

Li+(g) + Li(g)

Li+(g) + Cl(g)

H1

Li(g) + Li(g) Li+ Li(g)

1st E.A. of Li

1st I.E. of Li

H1 = 1st I.E. of Li + (60 kJ mol1) + H2

H3

Li(g) + Cl(g) Li+ Cl(g)

1st E.A. of Cl

1st I.E. of Li

H3 = 1st I.E. of Li + (349 kJ mol1) + H4

slide6

 H4 is more negative than H2

 Cl is smaller than Li

slide7

H1 = 1st I.E. of Li + (60 kJ mol1) + H2

H3 = 1st I.E. of Li + (349 kJ mol1) + H4

Li(g) + Li(g) Li+ Li(g) (1)

Li(g) + Cl(g) Li+ Cl(g) (2)

H1

No favourable

H3

favourable

H4 is more negative than H2

H3 is more negative than H1

slide8

Not covalent : -

∵ efficiency of orbital overlap  as the bonding atoms get larger

slide9

Explanation

1. Classical approach

Distance between shared pair and the bonding nuclei :

H2 < Li2 < Na2 < K2 < Rb2 < Cs2

Bond strength :

H–H > Li–Li > Na–Na > K–K > Rb–Rb > Cs–Cs

slide10

LiA Li2 LiB

1s*

2sA

2sB



1s

2. MO approach (Not required in AL)

slide11

Be(A) Be2 Be(B)

1s*

2s*

2sA

2sB

1s

2s

Q.58

There is no gain of stability when the AOs of two Beryllium atoms overlap.

2 e involved in bonding

2 e involved in antibonding

Overall : -

No e involved in bonding

Bond order = 0

slide12

Conclusion : -

1. Metals tend to form giant structures rather than discrete molecules.

2Li  Li – Li (low bond enthalpy)

nLi  Lin (high bond enthalpy)

Stronger bonds are formed due to extensive delocalization of valence electrons.

slide13

Conclusion : -

2. The electron-sea model

The valence electrons do not belong to any specific atoms (not localized) but delocalize throughout the whole crystal structure.

slide14

Conclusion : -

2. The electron-sea model

Mobile es  electron sea

Stationary +ve ions

slide15

Conclusion : -

2. The electron-sea model

The electrostatic attractive forces between the delocalized electron cloud and the positive ions are called the metallic bonds

slide16

Since metallic bonds are non-directional, they exist in significant extent even in molten state.

The boiling points of metals are much higher than the corresponding melting points.

E.g. Na m.p.  97.8oC ; b.p.  903.8oC

NaCl m.p. = 801C ; b.p. = 1413C

slide17

Conclusion : -

3. MO approach : Band theory

Spacing  as the no. of molecular orbitals 

Li

Li2

Li3

Li4

Lin

slide20

10.2 Metallic radius (SB p.262)

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

slide21

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
slide23

The strength of metallic bond can be estimated by

melting point,

boiling point,

enthalpy change of fusion or

enthalpy change of vapourization.

Higher m.p./b.p./Hfusion/Hvap

 stronger metallic bond

slide24

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

The metallic bond strength increases with:

1. decreasing size of the metal atom (i.e. the metallic radius);

2. increasing number of valence electrons of the metal atom.

slide25

Let's Think 1

10.3 Factors affecting the strength of metallic bond (SB p.263)

Effect of number of valence electrons on metallic bond strength

slide26

10.3 Factors affecting the strength of metallic bond (SB p.263)

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

slide27

Metal

Ni

Cu

Ag

Pb

Hg

Au

Density (g cm3)

8.91

8.94

10.49

10.66

13.53

19.30

Typical properties of metals

1. High density

due to close packing of atoms in metallic crystal (h.c.p./f.c.c. co-ordination number  12)

slide28

Typical properties of metals

1. High density

Exception : Alkali metals have low densities

(< 1 for Li, Na and K )

(a) they have more open structures

(b.c.c. /co-ordination number  8)

(b) their atomic radii are the highest in their own Periods.

E.g. Size : Na > Mg > Al

slide29

Typical properties of metals

2. High melting point and boiling point

Extensive delocalization of valence electrons  stronger bonds

Bond strength : -

ionic bond  covalent bond  metallic bond

slide30

Typical properties of metals

3. High flexibility

Malleability :

The ability to be deformed under compression

Ductility :

The ability to be deformed under tension

slide31

Typical properties of metals

3. High flexibility

Reasons : -

(a) The presence of layers in the crystal lattice

i.e. the layers can slide over one another under strain

(b) Metallic bonds are non-directional.

i.e. electrons can take up new positions and reform metallic bond after the deformation

slide33

Lin

Half-filled 2s band

Since the gap between energy levels are extremely small, radiation of any frequency in visible region can be absorbed and emitted.

Typical properties of metals

 Silvery and shiny

4. Surface lustre

slide34

Half-filled s band

E = 220 kJ mol1

full-filled d band

Typical properties of metals

Cu : 3d10, 4s1

Reddish brown

slide35

Half-filled s band

E = 300 kJ mol1

full-filled d band

Typical properties of metals

Au : 5d10, 6s1

Golden yellow

slide36

UV light absorbed

Half-filled s band

E = 380 kJ mol1

full-filled d band

Typical properties of metals

Ag : 4d10, 5s1

Silvery

slide37

Typical properties of metals

5. High Thermal and Electrical Conductivity

Due to the free movement of delocalized electrons

closed packed structure

10.4 Metallic crystals (SB p.263)

Closed-packed structure
  • Closed-packed structure is made possible with identical particles
  • Two types : -
  • 1. Hexagonal closed-packed, h.c.p.
  • Cubic closed-packed, c.c.p. Or
  • Face-centred cubic, f.c.c.
slide41

10.4 Metallic crystals (SB p.263)

Hexagonal close-packed structure

abab…

slide42

10.4 Metallic crystals (SB p.265)

Hexagonal close-packed structure

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

Packing efficiency = 74 %

Co-ordination no. = ?

slide44

rotate by 45

10.4 Metallic crystals (SB p.265)

Cubic close-packed / Face-centred cubic structure

c.c.p. or f.c.c.

abcabc…

Packing efficiency = 74 %

Co-ordination no. = 12

open structure

10.4 Metallic crystals (SB p.266)

Open structure
  • Structures with more empty space between the atoms
  • Most common: body-centred cubic structure
slide46

10.4 Metallic crystals (SB p.267)

Body-centred cubic structure

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

Packing efficiency = 68 %

Co-ordination no. = 8

slide47

Given : Density of Cu = 8.94 g cm3

Relative atomic mass of Cu = 63.546

Atomic radius of Cu = 0.128 nm

Cu adopts f.c.c. structure

Calculate the Avogadro’s constant

a2 + a2 = (4r)2

slide51

10.5 Alloys (SB p.268)

Alloys

  • Made by mixing a metal with one or more other elements (metals or non-metals)
slide52

10.5 Alloys (SB p.268)

Structure and Properites 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
slide53

10.5 Alloys (SB p.268)

Structure of alloy

  • In an alloy, atoms of different sizes are present
slide54

10.5 Alloys (SB p.268)

Structure of alloy

  • Changes the regular arrangement of the layers of atoms in the metal
  • Slipping of layersof atoms becomes more difficult
  • Harder and stronger
slide55

10.5 Alloys (SB p.269)

Types of alloys

  • 2 common types of alloys:
  • Substitutional alloy
  • Interstitial alloy
slide56

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
slide57

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
slide58

10.5 Alloys (SB p.269)

Some common alloys - Steel

  • An alloy of iron
  • The presence of directional carbon-iron bonds makes the resulting alloy harder, stronger and less ductile than pure iron.
  • Amount of carbon present affects the properties of steel
  • Mild steel: contains <0.2 % carbon, ductile, malleable (used for nails, cables and chains)
slide59

10.5 Alloys (SB p.269)

Some common alloys - Steel

  • Medium steel: contains 0.2 – 0.6 % carbon, harder
  • used in rails and structural steel beams
  • High-carbon steel: contains 0.6 – 1.5 %, tough and hard
  • used for springs tools and cutlery
slide60

10.5 Alloys (SB p.269)

Some common alloys - Steel

Articles made from stainless steel

slide61

10.5 Alloys (SB p.269)

Some common alloys – Alloy Steel

  • A mixed form of interstitial (carbon) and substitutional (other metals) alloys
  • Example : Stainless steel (steel + Cr + Ni)
  • The presence of Cr and Ni greatly increases the resistance to corrosion of the alloy.
slide62

10.5 Alloys (SB p.269)

Some common alloys – Alloy Steel

Example : Tool steel (steel + W + Co)

It is very hard and has a very high m.p.

It is used for making high-speed cutting tools

slide63

10.5 Alloys (SB p.270)

Some common alloys – Copper alloys

  • Brass - an alloy of copper and zinc
  • Attractive golden appearance
  • Harder and more corrosion resistant than copper and zinc.
  • Used to make ornaments, buttons, musical instruments, plugs and sockets, and water taps.
slide64

Article made from brass

10.5 Alloys (SB p.270)

Some common alloys – Copper alloys

  • Brass - an alloy of copper and zinc
slide65

10.5 Alloys (SB p.270)

Some common alloys – Copper alloys

  • Coinage metals
slide66

Silver coins = cupronickel (Cu + Ni)

10.5 Alloys (SB p.270)

Some common alloys – Copper alloys

slide67

Copper coins (copper + tin + zinc)

10.5 Alloys (SB p.270)

Some common alloys – Copper alloys

slide68

Brass (Cu + Zinc)

10.5 Alloys (SB p.270)

Some common alloys – Copper alloys

slide69

10.5 Alloys (SB p.269)

Some common alloys – Duralumin

  • An alloy of aluminium with Cu, Mg and Mn
  • It is light and is stronger and more corrosion resistant than aluminium.
  • It is used for making spacecrafts and jet fighters.
slide70

10.5 Alloys (SB p.270)

Some common alloys – Solder

  • An alloy of lead and tin
  • It has a lower m.p.(about 180C) than that of lead and tin.
  • It is used in joining metals together.
  • It melts easily to fill the gaps between metals without melting them.
  • On cooling, it solidifies and completes the circuit.
slide71

Check Point 10-5

10.5 Alloys (SB p.270)

Some common alloys – Solder

  • An alloy of lead and tin
slide72

10.5 Alloys (SB p.270)

Some common alloys – Carat Gold

  • An alloy of gold with silver and copper.
  • Pure gold is too soft to make jewellery.
  • Carat gold is harder than pure gold
  • Pure gold is called 24 carat (24K) gold
  • 18 carat (18K) gold contains
  • 18/24 or 75% gold.
slide74

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.

slide75

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.

slide76

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

slide77

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

10.4 Metallic crystals (SB p.267)

Example 10-4

slide78

(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

10.4 Metallic crystals (SB p.267)

Example 10-4

Back

slide79

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

slide80

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

10.4 Metallic crystals (SB p.268)

Check Point 10-4

slide81

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

slide82

(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.

10.4 Metallic crystals (SB p.268)

Back

Check Point 10-4

slide83

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.
slide84

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.

slide85

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 be 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.