Teaching Structure and Bonding (Part 4): Metallic Structures

Having introduced how I approach teaching the KS4 Structure and Bonding topic, I’ve described the teaching sequence I use for ionic structures in detail. These posts give a clear indication of the journey through each structure type which I take students on, from the macroscopic properties they are familiar with, to the sub-microscopic structure – the particles and forces involved in holding them together – which enables these properties to be explained. In this post I’m going to focus on a few key details and traps to avoid which I think are really important in ensuring a secure understanding of metallic structures. 

On the surface, it is easy for students to confuse ionic and metallic structures – they both consist of a lattice of ions, held together by electrostatic attractions, and when seen individually, the differences might not be immediately apparent to a novice learner. Therefore, once I have introduced students to the metallic lattice, I always show them diagrams of ionic and metallic structures side by side and highlight the differences – the charges on the ions, what particles the electrostatic attractions are between, the delocalisation of electrons in the metallic lattice and what this actually means in terms of their mobility.

In developing students’ understanding of metallic properties, I continue to compare back to the ionic properties throughout. This not only serves as a helpful comparison to highlight differences in structure which result in very different properties, but is an opportunity for students to recall and strengthen their understanding of ionic structures and their properties. For example, I would use diagrams similar to those shown below, to remind students that ionic structures are brittle when forces are applied, whereas metallic ones are malleable. Having the two diagrams side by side is really helpful in making the structural differences which cause these properties clear – the repulsion in one structure when the lattice is disturbed, compared to the continuing attraction in the other because of those all important delocalised electrons.

Similar comparisons can be made in explaining the different electrical conductivities. Students should recall that freely moving charged particles are required for conductivity. They are usually able to apply this to explain electrical conductivity in terms of the delocalised electrons being free to carry charge through the structure, whereas the metal ions are locked in place within the solid lattice, thus unable to conduct.

Having gone through the structure property-relationships in metals, I allow students time to complete this summary sheet which picks up on the key explanations and enables me to check that they are using terminology correctly.

I would then go on to consider the melting points of a selection of metals with different ion charges – a great opportunity to review atomic structure and ion formation. Why does magnesium have a higher melting point than sodium? Why does sodium have a higher melting point than potassium? Why is potassium easier to cut that sodium? All these can be explained in terms of electrostatic attractions between different charges or across different distances and relate back to explanations of reactivity trends.

Finally, I consider alloys and the alteration to the lattice structure caused by adding atoms of additional elements, a key point to highlight here being that malleability and reactivity can be significantly altered, but electrical conductivity is not – the delocalised electrons are still present and are still free to carry charge through the structure.

Metals are probably the most straightforward structures to understand, I think because students are most familiar with them in their bulk state, making their properties easy to remember. This, in turn, provides a secure platform on which to build the structural knowledge which explains these properties. However, there is a danger of ionic and metallic bonding diagrams being confused if the differences are not made explicit.