Section 4D Bonding and Structure IV (Metallic Bonding)

Modern College F.6 Chemistry (2009 – 10)

Section 4D

Name: ______________________________ Class: _______________ Class No.: ____________ Prepared by Mr. Chau Chi Keung, Richard

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Modern College F.6 Chemistry (2009 – 10)

Section 4D

4.16 Metallic Bonding Revisited 4.16.1. Formation of metallic bonding 

The structure of metal consists of a giant structure of cationic lattice (regularly arranged and closely packed cations) immersed in a sea of mobile valence electrons. (在金屬晶格中， 金屬陽離子會以緊密裝填並有規則地排列在一起。它們均被由價電子所形成的電子海 包圍。)

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The non-directional electrostatic attraction between the delocalized valence electrons and the metal ions is the metallic bonding. (金屬陽離子和離域電子之間的無方向性的靜 電引力，稱為金屬鍵。)

4.16.2. General properties of metals 

Thermal conductivity – Presence of mobile electrons.

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Electrical conductivity – Presence of mobile electrons.

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High density – Metals tend to adopt close-packed structures which minimize the amount of empty space between the atoms. High melting and boiling point – Metallic bonds are usually quite strong. A lot of energy is required to break them. Malleability and ductility – Layers of metal ions can slip over one another through the sea of electrons to new positions. After that, non-directional metallic bonds can still hold the metal ions together. Shiny surface – The mobile electron can be excited and re-emits the energy in form of light.

4.16.3. Metallic radius 

Metallic radius (r): Half of the internuclear distance between atoms in a metallic crystal.

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Modern College F.6 Chemistry (2009 – 10)

Section 4D

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Moving down a group, metallic radii increase (∵screening effect ↑)

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Going across a period, metallic radii decrease (∵nuclear charge ↑)

4.16.4. Strength of metallic bonds 

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The strength of metallic bonds increases as, 

The number of valence electrons of the metal atom increases

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The metallic radii decrease

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The packing efficiency of the metallic crystal increases (to be discussed later)

The strength of metallic bonds can be reflected from melting and boiling points of metals. 

Example 1: Alkali metals – Effect of metallic radius Metallic radius m.p.(°C) b.p.(°C)

Li 0.152 180.5 1330

Na 0.186 97.7 892

K 0.231 63.4 759

Rb 0.244 39.3 688

Cs 0.262 28.4 671

Note: Metallic radius ↑ ⇒ no. of inner electron shells ↑ ∴ more screening effect on the valence e– ∴attraction between outermost e– and nucleus ↓ ( ⇒ bond strength ↑) 

Example 2: Na, Mg and Al – Effect of number of valence electrons

No. of valence e m.p.(°C) b.p.(°C)

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Na 1 97.7 892

Mg 2 650 1091

Al 3 660.3 2519

Note: No. of valence e– ↑ ⇒ more electron-nuclei attractions ⇒ bond strength ↑

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The strength of metallic bonds, ionic bonds and covalent bonds can roughly compared as shown in the following table: Type of Bonding

Estimated by

Ionic bond (non-directional)

Lattice enthalpy

Covalent bond (directional)

Bond dissociation enthalpy

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Approximate range (in kJmol–1) –780 (NaCl) to –3791 (MgO) 158 (E (F–F)) to 944 (E (N≡N))

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Modern College F.6 Chemistry (2009 – 10) Metallic bond (non-directional)

Section 4D

Atomization enthalpy

107.3 (Na) to 514.2 (V)

4.17 Metallic Crystals 4.17.1. Structures of metallic crystals – An overview 

Metallic crystals have 2 general types of structure: 

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Close packing structure (緊 密 裝 填 結 構 ): The metal atoms are packed together as close as possible so that the packing efficiency is high (≈74%). Open structure (開 放 結 構 ): The metal atoms are not closely packed together so that there will be more empty space between the metal atoms (i.e. lower packing efficiency, ≈68%).

Some important terms: 

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Unit cells: The smallest identical block of metal atoms which can be stacked together to fill space completely and to reproduce the whole regular arrangement. Coordination number: The number of atoms closest to a particular atom.

4.17.2. Close packing structure I – Hexagonal-closed packing (h.c.p.) (六方緊密裝填結構 ) 

Examples: Magnesium, titanium, cobalt, zinc and cadmium

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The following figure shows an example of hexagonal-closed packed atoms

a

b Normal side view a

Exploded view

Figure 1 A unit cell

Figure 2

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Modern College F.6 Chemistry (2009 – 10)

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Section 4D

In figure 1, each metal atom in the first layer (a) is in contact with 6 atoms in the same layer (hexagonal). In the second layer (b), each atom is also in contact with 6 atoms in the same layer, but also in contact with 3 atoms in the first layer (put on the depression between 3 atoms). Orientation of the third layer is the same as the first one. For this reason, this packing pattern is called an “abab” pattern. The whole structure is made up of alternating layers ababa…… (You may refer figure 2 as well). In hexagonal-closed packing, each metal atom is surrounded by 12 atoms (∴coordination number = 12).

4.17.3. Close packing structure II – Cubic -closed packing (c.c.p.) (立方緊密裝填結構 ) 

Also known as face-centred cubic structure (f.c.c.).

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Examples: Aluminium, calcium, copper, nickel and silver

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Same as hexagonal-closed packing, the coordination number for c.c.p. is also 12.

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The following figure shows an example of cubic-closed packed atoms

a

b

c Normal side view

A unit cell Exploded view

Figure 3

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Figure 4

The first layer of metal atoms has a different orientation when compared with the third layer (see Figure 4). For this reason, this packing pattern is called an “abcabc” pattern. By looking at a four layers unit cell, there is 1 atom at the first layer, 6 at the second layer, 6 at the third layer and 1 at the fourth layer, a face centred cubic unit cell can be constructed

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Modern College F.6 Chemistry (2009 – 10)

Section 4D

(see Figure 3). Summary (Important): Coordination number Packing efficiency (% of space filled) Packing pattern Unit Cell

Cubic-closed packing

Hexagonal-closed packing

abcabc……

ababab……

Number of metal atoms per unit cell

4.17.4. Tetrahedral holes and octahedral holes 

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Although the crystal is closely packed, there is still some empty space between the atoms. They are called holes. There are two types of holes – tetrahedral hole (四面體洞) and octahedral hole (八面體洞). Tetrahedral hole is surrounded by 4 atoms. It is formed when a sphere sits on the depression formed by three spheres in an adjacent layer.

Octahedral hole is surrounded by 6 atoms. It is the space between two layers of triangularly arranged atoms. From another angle, it can be seen that the six atoms are arranged in a form of octahedron.

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Modern College F.6 Chemistry (2009 – 10)

Section 4D

4.17.5. Open structure 

Also known as body-centred cubic structure (b.c.c.).

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Examples: All alkali metals, iron and chromium

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The metal atoms are not closely packed together (∴lower packing efficiency, ≈68%).

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Each metal atom is surrounded by 8 atoms (∴ coordination number = 8).

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The following figure shows a body-centred cubic structure:

Normal side view

A unit cell Exploded view

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Some metals may exist as two or more structures at different conditions. 

For example, iron has a body-centred cubic structure below 906°C. When iron is heated between 906 and 1401°C, the b.c.c. structure becomes an f.c.c. structure.

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Example: HKALE 2004 Paper II Q.4(d)

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Modern College F.6 Chemistry (2009 – 10)

Section 4D

4.18 Alloys (合金) 4.18.1. Types and structures of alloys  

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Alloys are made by mixing a metal with one or more other elements (metal or non-metal). Have more desirable properties as compared with pure metals (e.g. hardness ↑, corrosion resistance ↑). Metals will readily form alloys since the metallic bond is non-specific. The presence of small quantities of a second element in the metal frequently increases its strength.  

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Atoms of the second metal are different in size to those of the original metal. These differently sized atoms interrupt the ordered arrangement of atoms in the lattice and prevent them sliding over each other.

Pure metal Alloy There are two major types of alloys:  Substitutional alloy ( 取代 合金 ): Some of the host metallic atoms are replaced by other metallic atoms of similar sizes (e.g. brass)

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Interstitial alloy ( 間 隙 合 金 ): Formed when some of the holes among the closely packed host metallic atoms are occupied by atoms of smaller atomic sizes (e.g. steel)

4.18.2. Some common alloys 

Steel: Iron + Carbon (0.2 – 2.2%). Percentage of C added affects its hardness.

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Stainless steel: Steel + chromium (+ manganese / nickel ) (substitution).

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Modern College F.6 Chemistry (2009 – 10)

Section 4D

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Duralumin: Aluminium + copper (about 4%) + magnesium (0.5% – 1%) + manganese (