The KS4 Structure and Bonding topic builds links between so many aspects of chemistry and it’s one of the first parts of the course which really starts to give meaning to a subject which can so often seem very abstract to students. This is why it has always been one of my favourite topics to teach. My own interest in chemistry really began when I started to understand that everything around me was composed of atoms, and that the nature of, and interactions between these atoms accounted for the huge variety of natural and synthetic materials and their disparate properties. I was fascinated by the microscopic world and how it enabled me to make sense of what I could observe with my own senses. This topic has the potential to bring this microscopic world to life and lay a sure foundation for understanding so many other chemical concepts. Yet, it is a topic which students find notoriously difficult. This year, having watched various talks about teaching these concepts during the lockdown period (Niki Kaiser and Kate Nichols researchED Norwich presentations, and the CogSciSci Curriculum chat on bonding), I have changed my approach to teaching the topic. This series of blogs will outline how I used to teach this (very briefly), some of the challenges presented and how I taught the topic this year much more successfully, I think!
How I used to teach structure and bonding
Prior to this year, I have begun this topic by teaching the types of bonding – covalent, ionic and metallic – before introducing the different structure types and their properties, relating these back to the bonding. Usually students would appear to be quite happy with the different types of bonding and be able to draw accurate dot and cross diagrams to represent them. However, deeper questioning revealed that students were following a set of rules, with little understanding of what any of it actually meant, and most were unable to relate the diagrams they were drawing to any concrete examples in the real world. They were drawing circles, dots and crosses on a sheet of paper without really grasping what these were representing.
This lack of deep understanding always became apparent when I introduced structure types and started trying to get students to think about the type of bonding involved in different substances and how this explained their various properties and behaviours. Students would often appear to follow an explanation, for any given example, but when required to transfer and apply this knowledge to previously unseen examples, they struggled (I’ve written about how I have changed my assessment practice in this topic to identify where students’ understanding was breaking down). I think this was largely down to two things – a lack of clarity in some explanations, and the sequence in which I was introducing the concepts.
Challenges and misconceptions
The biggest challenge in this topic is the need to switch between different ways of seeing substances – we view macroscopic properties, but they are explained by the interactions between particles on the microscopic level which students find hard to visualise and link to what they are observing. In addition to this, we expect students to represent these structure types using bonding diagrams and symbols which often only represent one unit of the giant structure in a very abstract way. I have found Johnstone’s triangle (introduced here by Niki Kaiser and also in her researchED Norwich talk) to be a really helpful way of making this more explicit, and supporting students to make links between these different levels and forms of representation.
As well as this overarching challenge, there are a lot of difficult concepts in this topic, and much scope for students to develop misconceptions. Some of the most prevalent challenges and misconceptions which seemed to crop up year after year in my teaching have been:
Ionic Bonding – The AQA specification states that, when a metal atom reacts with a non-metal atom electrons in the outer shell of the metal atom are transferred. It then goes on to say, The electron transfer during the formation of an ionic compound can be represented by a dot and cross diagram, e.g. for sodium chloride:
Teaching this (which I’m not really happy with as a description of ionic bonding at all as it encourages misconceptions about how NaCl is actually formed in most instances) before introducing the concept of a giant ionic lattice frequently leads to students having an understanding that when, for example, NaCl is formed, electrons are directly transferred from a sodium atom to a chlorine atom and these form a discrete NaCl ‘molecule’ (we’ll come back to this) where the ions are paired up. The true concept of electrostatic attractions acting in all directions between the ions in the lattice is completely absent. This was brought home to me even more forcibly recently when teaching bonding to Y12, a very able student asked the following question, “Does that mean that a grain of salt doesn’t just contain one sodium and one chloride ion?”. Early on in the same Y12 topic I asked students to answer this question:
Sodium oxide has an ionic structure with the formula Na2O. In your own words, explain what the formula Na2O tells you about sodium oxide.
A selection of typical responses serve to illustrate the prevalence of this misconception:
The formula sodium oxide tells me that there are two sodium atoms joined to one oxygen atom.
Each oxygen bonds with two sodiums.
Every one molecule contains two sodium atoms bonded to one oxygen.
Similar problems arise in interpreting the formulae of giant covalent structures, such as SiO2.
Covalent bonding – What actually is a covalent bond? Ionic and metallic bonding are generally explained in terms of electrostatic attractions (between oppositely charged ions, or between ions and delocalised outer shell electrons, respectively). Students can understand this, and grasp that these electrostatic attractions result in strong bonding which explains the high melting points in ionic and metallic structures. In contrast, covalent bonding is often described only as “sharing pairs of electrons” without reference to why this should hold the atoms together (this is the case in most exam specifications and textbooks I have seen). This, along with the additional presence of weak intermolecular forces within simple molecular structures, tends to leave students viewing covalent bonding as weaker, and they then struggle to link the different properties of giant and simple covalent structures to the appropriate aspects of their bonding.
I have found that explicitly teaching the electrostatic attraction between the shared pair of electrons and the atomic nuclei as the reason that sharing electrons results in strong bonds, helps students to grasp the structure-property links, and to differentiate between covalent bonds and intermolecular forces in molecular structures.
What is a molecule? – This is a question which crops up so frequently. Students tend to use the words ‘atom’ and ‘molecule’ to describe any particles which make up a structure, regardless of the true nature of the particles. I think this stems from a lack of understanding of what a molecule actually is and how it is different from (or sometimes the same as) a compound or an element. Defining molecules is a challenge, but I’ll explain how I try to make it clear when I get to writing about how I teach covalent structures.
Intermolecular forces – Students tend to pepper their answers about structure-property relationships with references to intermolecular forces, even when writing about giant structures. I think this stems from the lack of clarity in the two previous points – the confusion around what a molecule is, and the nature of covalent bonding.
The Octet rule – This is a tricky one, which I don’t feel I have a good answer to at KS4 level. Why do atoms form bonds at all? Why do they give or take, or share electrons such that outer shells become full (or empty)? I have settled on teaching this as a rule which can always be applied to bonding at GCSE as I think this is more faithful to truth than personifying atoms as beings which ‘want’ a full outer shell. If anyone has a good explanation accessible to KS4 students which explains why this is, I’d love to hear it.
So, having Identified what I think are the key challenges in teaching this topic well, the remainder of this blog series will describe how I now teach Structure and Bonding to try to overcome some of these challenges and prevent these misconceptions from arising.
In Part 2 I describe the lesson I teach to lay the foundations for the topic and give a very brief topic sequence. Part 3 onwards will then cover some specific aspects of the topic in more detail (I haven’t written these yet so they might be a little while in coming…).
I’m always interested to hear how others approach teaching this topic, so do get in touch if you have ideas you’d be willing to share or if there are other aspects of this topic which you feel present challenges.
elements of learning 
On the octet rule. My simple mantra is “full shells are stable” then try to get ionic and covalent linked to that.
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