Teaching Structure and Bonding (Part 3): Ionic Structures

Part 1: Teaching Structure and Bonding – The Challenges

Part 2: Laying Foundations – The Particle Model

I’ve previously outlined some of the key challenges in teaching Structure and Bonding at KS4, and how I have found it useful to review the particle model (especially changes of state) before moving on to study the various types of bonding and the properties of the structures they produce.

I used to begin with the invisible – the bonding and the structure before building up to the macroscopic properties. This year, I changed tack and started from the macroscopic perspective, considering the tangible and demonstrable properties of each structure, before zooming in to the sub-microscopic level of the structure, an understanding of which is required to explain the observed properties. Students really did seem to grasp these concepts better and were able to make links between structure and properties more comfortably which this reversed sequencing.

This post outlines the teaching sequence I used when introducing ionic structures, their properties and bonding. My aim is to outline the approach I take in introducing and explaining ionic bonding, with a focus on trying to avoid some common pitfalls.

The macroscopic view

I begin by showing students some physical samples of ionic structures – sodium chloride and copper sulphate crystals – and discussing and demonstrating some of their macroscopic properties:

• Heat NaCl in a boiling tube to show high melting point.
• Put pressure on a crystal of copper sulphate to show it’s brittle nature (large CuSO₄ crystals under the visualiser work particularly well for this).
• Demonstrate that solid NaCl does not conduct electricity, but when dissolved in water, it does.

Although I’m not explaining these properties at this stage, I have found it helpful for students to have concrete examples of ionic structures and how they behave at the macroscopic level, before trying to understand them from a structural perspective.

In exploring the properties in more detail, I begin with the high melting point as this links so closely to the universal particle model which students will already understand (see Part 2). I ask students to draw a particle diagram for a solid (review of prior learning) before asking questions to start linking their understanding about changes of state to the ionic properties. 

• Ionic structures have very high melting points, what does this tell you about the interactions between the particles? They’re very strong.
• Why does having very strong interactions between particles mean that the melting point is very high? A lot of energy is needed to overcome the strong interactions between particles.

So, if we zoom into the structure of an ionic solid, we find that there are lots of particles arranged in a regular pattern (this is known as a giant lattice structure – giant because it contains many particles). We know that the interactions between the particles are very strong. We need to understand what the particles are and why they have strong interactions between them.

Ions

At this point I pause and recap what students know about ion formation from previous learning. I use the rule that atoms form ions by losing or gaining electrons so that their outer shell becomes full or empty. I revisit the explanation of why losing or gaining electrons results in ions having an overall charge (comparing proton and electron numbers) and give opportunity for students to practise drawing ion electron configurations and calculating and explaining their charges.

Having practiced drawing ions and calculating charges, we can summarise some rules:

• Metal atoms lose their outer shell electrons to become positive ions.
• Non-metal atoms gain electrons to complete their outer shell and become negative ions.
• An ionic substance will contain positive metal ions and negative non-metal ions. 

Depending on the group I may also do some work on working out ionic formulae (looking at the ratio of ions required to give an overall neutral formula) at this stage.

The sub-microscopic view

We then return to the particle diagram from earlier and I ask questions to draw out the key knowledge so far – giant ionic lattices have very high melting points because they contain particles which are held together by very strong interactions. Now it’s time to link this general particle understanding to the particular details of an ionic structure. I ask students to think about sodium chloride and draw the sodium and chloride ion electron diagrams and calculate their charges. Showing them the general particle diagram under the visualiser I ask questions to check understanding while explaining that the particles in a giant ionic lattice are the positive and negative ions.  This is a good time to recap interactions between charges:

• Why do positive and negative ions have such strong interactions between them? Opposite charges attract.
• What type of attraction is this? Electrostatic attraction.

I would then summarise that giant ionic lattices have very high melting points because a lot of energy is required to overcome the strong electrostatic attractions between the positive and negative ions in the structure. I would also add charges and further annotation to the particle diagram to build up students’ understanding of the sub-microscopic representations. 

Structure-property links

Melting point

I find that it’s helpful to build up students’ understanding of the sub-microscopic particle arrangement alongside the explanation of the high melting point as this links so well with their prior knowledge of the particle model and changes of state. The key points are for them to understand what the particles represent in each structure type and the nature and strength of the interactions between them. Before moving on to the other properties, I ask students to complete the following sentence in their books:

“Giant ionic lattices have very high melting points because…”

Brittle

Firstly, it’s important to explain what brittle means, students can often give examples of brittle substances but struggle to define the term – a brittle object will break or shatter when a force is applied to it. To help students understand why this is the case for ionic lattices I return to the particle diagram. I ask students to imagine they have shrunk down to the size of the ions and are pushing the top layers of ions in the structure along (i.e. applying a force to them to change the shape of the macroscopic structure). I then ask what will happen when they’ve moved it along by one ion? Think about the charges on the ions which are now next to each other. What force will be acting between these ions? Electrostatic repulsion. Similarly charged ions are now adjacent, so they will repel one another and the lattice will break into pieces.

Having explained these two properties I would give students time to consolidate by completing the following:

Electrical Conductivity

When explaining electrical conductivity it’s important for students to know what the necessary conditions for conductivity are – mobile charged particles. I usually describe this as, ‘charged particles which are free to move through the structure’. It’s important to emphasise to students that they always need to identify what these charged particles are for a particular example – in this case ions (but electrons, in the case of metals).

If you like a demo, this is the time to return to the demonstration of the conductivity of solid sodium chloride and ask students to think about why it does not conduct electricity. What is needed for a substance to conduct electricity? What are the particles in an ionic lattice? Are those particles charged? Why? (A good place to recap this.) Can the charged particles (ions) move through the structure? Why not?  In asking these questions, I’m modelling the thought process for considering whether any substance will conduct electricity. Going through this process over and over for different scenarios will help students to apply their knowledge of the conditions for electrical conductivity to unfamiliar examples.

Similar questions can then be asked about the dissolved sodium chloride, following a recap of what happens when a substance is dissolved (solute particles broken apart and spread out throughout the solvent). What is needed for a substance to conduct electricity? What are the particles spread out through the water when an ionic substance is dissolved? Are these particles charged? Are they able to move through the substance? Will the sodium chloride solution conduct? Why?

Before discussing the conductivity of molten ionic substances I ask students to complete the following sentences:

“A solid giant ionic lattice does not conduct electricity because…”

“A solid giant ionic lattice does not conduct electricity but…”

I will then give students opportunity to think about whether they expect a molten (melted) ionic structure to conduct electricity or not? After a bit of time for thinking, I ask the same questions as previously, What is needed for a substance to conduct electricity? What particles are present in a molten ionic substance? Are these particles charged? Are these particles free to move through the substance? Will molten sodium chloride conduct electricity? Why?

My aim has not been to give a detailed script for teaching this topic, but to outline a teaching sequence which I have found to be effective, and the thinking behind it. I have found this sequence of identifying the macroscopic properties, zooming in to explain the sub-microscopic structure, and then using this to explain the observed properties to be really helpful in developing students’ ability to make links between the structure and properties. Having gone through this explanatory sequence in detail, asking lots of questions to check for understanding, I would then give lots of practice written questions about ionic structures and properties, being sure to bring in a wide range of different substances (too often questions focus on sodium chloride resulting in students not recognising other examples as ionic) to consolidate learning.