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 Bond 10.4Metallic Crystals 10.5Alloys 10

New Way Chemistry for Hong Kong A-Level Book Metallic Bonding

New Way Chemistry for Hong Kong A-Level Book 1 3 Electron sea model of bonding in metals 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)

New Way Chemistry for Hong Kong A-Level Book Metallic Radius

New Way Chemistry for Hong Kong A-Level Book 1 5 Metallic radius (r) is defined as half of the internuclear distance between atoms in a metal crystal. Atoms in a metallic crystal 10.2 Metallic radius (SB p.262)

New Way Chemistry for Hong Kong A-Level Book 1 6 Trend of metallic radius in the Periodic Table 10.2 Metallic radius (SB p.262) Moving down a group, metallic radii increase Going across a period, metallic radii decrease

New Way Chemistry for Hong Kong A-Level Book Factors Affecting the Strength of Metallic Bond

New Way Chemistry for Hong Kong A-Level Book 1 8 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 metallic bond 10.3 Factors affecting the strength of metallic bond (SB p.262)

New Way Chemistry for Hong Kong A-Level Book 1 9 Effect of number of valence electrons on metallic bond strength MetalNumber of valence electrons(s) Melting point ( o C) Sodium Magnesium Aluminium Factors affecting the strength of metallic bond (SB p.263)

New Way Chemistry for Hong Kong A-Level Book 1 10 Effect of metallic radius on metallic bond strength of Group IA metals MetalMetallic radius (mm) Melting point ( o C) Lithium Sodium Potassium Rubidium Caesium Factors affecting the strength of metallic bond (SB p.263)

New Way Chemistry for Hong Kong A-Level Book Metallic Crystals

New Way Chemistry for Hong Kong A-Level Book 1 12 Closed-packed structure 10.4 Metallic crystals (SB p.263)

New Way Chemistry for Hong Kong A-Level Book 1 13 (a) normal side view (b) exploded view (c) a unit cell Co-ordination no. = ? Empty space = 26 % Hexagonal close-packed structure 10.4 Metallic crystals (SB p.265)

New Way Chemistry for Hong Kong A-Level Book 1 14 (a) normal side view (b) exploded view (c) a unit cell Co-ordination no. = ? Empty space = 26 % Cubic close-packed / Face-centred cubic structure 10.4 Metallic crystals (SB p.265)

New Way Chemistry for Hong Kong A-Level Book 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 A tetrahedral hole formed by four spheres

New Way Chemistry for Hong Kong A-Level Book 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 A octahedral hole formed by six spheres

New Way Chemistry for Hong Kong A-Level Book 1 17 Open structure 10.4 Metallic crystals (SB p.266) Structures with more empty space between the atoms Most common: body-centred cubic structure

New Way Chemistry for Hong Kong A-Level Book 1 18 (a) normal side view (b) exploded view (c) a unit cell Empty space = 32 % Body-centred cubic structure 10.4 Metallic crystals (SB p.267)

New Way Chemistry for Hong Kong A-Level Book Metallic crystals (SB p.267) Example 10-4 Example 10-4 Check Point 10-4 Check Point 10-4

New Way Chemistry for Hong Kong A-Level Book Alloys

New Way Chemistry for Hong Kong A-Level Book Alloys (SB p.268) Alloys Alloy – a material with metallic properties Made by mixing a metal with one or more other elements

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book Alloys (SB p.268) Structure of alloy In an alloy, atoms of different sizes are present

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book Alloys (SB p.269) Types of alloys 2 common types of alloys: Substitutional alloy Interstitial alloy

New Way Chemistry for Hong Kong A-Level Book 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

New Way Chemistry for Hong Kong A-Level Book 1 27 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 10.5 Alloys (SB p.269)

New Way Chemistry for Hong Kong A-Level Book Alloys (SB p.269) Some common alloys - Steel 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

New Way Chemistry for Hong Kong A-Level Book Alloys (SB p.269) Some common alloys - Steel Articles made from stainless steel

New Way Chemistry for Hong Kong A-Level Book Alloys (SB p.270) Some common alloys – Copper alloys Brass - an alloy of copper and zinc Article made from brass

New Way Chemistry for Hong Kong A-Level Book Alloys (SB p.270) Some common alloys – Copper alloys Coinage metals

New Way Chemistry for Hong Kong A-Level Book Alloys (SB p.270) Some common alloys – Solder An alloy of lead and tin Check Point 10-5 Check Point 10-5

New Way Chemistry for Hong Kong A-Level Book 1 33 The END

New Way Chemistry for Hong Kong A-Level Book 1 34 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 10.3 Factors affecting the strength of metallic bond (SB p.263) Back

New Way Chemistry for Hong Kong A-Level Book 1 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 10.4 Metallic crystals (SB p.266)

New Way Chemistry for Hong Kong A-Level Book 1 36 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) potassium Answer 10.4 Metallic crystals (SB p.267)

New Way Chemistry for Hong Kong A-Level Book Metallic crystals (SB p.267) (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

New Way Chemistry for Hong Kong A-Level Book Metallic crystals (SB p.267) (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 Back

New Way Chemistry for Hong Kong A-Level Book 1 39 (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 10.4 Metallic crystals (SB p.268)

New Way Chemistry for Hong Kong A-Level Book Metallic crystals (SB p.268) (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

New Way Chemistry for Hong Kong A-Level Book 1 41 (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 10.4 Metallic crystals (SB p.268)

New Way Chemistry for Hong Kong A-Level Book Metallic crystals (SB p.268) 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.

New Way Chemistry for Hong Kong A-Level Book 1 43 (a)(i) Give two advantages of steel compared to the pure iron. (ii)Why is tungsten added to certain types of alloy steels? Answer 10.5 Alloys (SB p.271) (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.

New Way Chemistry for Hong Kong A-Level Book 1 44 (b) Cupronickel replaced earlier silver coins which contained silver. Give two reasons for the replacement. Answer 10.5 Alloys (SB p.271) (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.

New Way Chemistry for Hong Kong A-Level Book 1 45 (c) (i) Why the low melting point of solder makes it useful in joining metals together? (ii)Explain how soldering joins up metals. Answer 10.5 Alloys (SB p.271) (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. Back