Topic 4.5 Metallic bonding

Understandings and Applications:

• A metallic bond is an electrostatic attraction between the lattice of positive ions and the surrounding delocalized electrons.
• The strength of a metallic bond depends on the charge of the ions and the radius of the metal.
• Alloys usually contian more than one metal and have enhanced properties.
• Explanation of the electrical conductivity and malleability in metals.
• Explanation of trends in melting points.
• Explanation of the properties of alloys in terms of non-directional bonding.

Metallic bonds

• Regular giant lattice of positive metal ions surrounded by a sea of delocalized electrons.

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Delocalized electrons

• Not associated with a particular nucleus of a metal, but instead free to move throughout the entire crystalline lattice which forms the “sea” of mobile electrons.

Strength of metallic bonds

• Depends on:
• The number of valence electrons that can become delocalized.
• The charge of the metal ion.
• The ionic radius of the metallic positive ion.

Properties of metals

• Electrical conductivity: good conductors because of delocalized electrons.
• Malleability: positive ions can slide past one another and not repel because they are cushionned by the electrons (no disruption in the metallic bonding when they are pushed on)
• Melting points: High because metallic bonds are strong

Alloys

• Mixture of two or more metals or
• Metal(s) combined with alloying element composed of more non-metals (ex: cast iron Fe with C)
• Have enhanced properties such as strength, hardness, durability.

Alloys

Alloys are produced by adding one metal element to another metal (or carbon) in the molten state, so that the different atoms can mix.

As the mixture solidifies, ions of the different metals are scattered through the lattice and bound by delocalized electrons, so they contain metallic bonds.

The production of alloys is possible because of the non-directional nature of the delocalized electrons, and the fact that the lattice can accomodate ions of different size.

Alloys

Alloys have properties that are distinct from their component elements due to the different packing of the cations in the lattice.

The alloy is often more chemically stable, and also often stronger and more resistant to corrosion.

You don’t need to learn all the examples, but you need to be able to use the example to explain why the structure of the alloys gives them enhanced properties compared with their component metals.