This is one for the scientists..
I haven’t been teaching long. But any chemistry teacher worth their salt has taught GCSE bonding a shed load of times. The first time I was observed by my University tutor during my PGCE was a lesson on bonding (it was a disaster if you’re interested). I must have taught the same lessons in the same order about 15 times by now.

This year, with my Year 11’s (old spec AQA) I did the exact same thing. Start with atomic structure. Move on to ion formation. Ionic bonding, structure and properties. Then covalent bonding, structure and properties. Within covalent structures there are two main types; “simple molecules” then “giant covalent,” taught in that order. Every scheme of work and textbook I have seen teaches in that order, simple molecules then giant covalent.

This year’s lesson with the year 11’s went exactly the same as it has in the past. But it didn’t feel right. And that isn’t because I did some whizz-bangy mini-plenary with whiteboards, coloured cards or desk-mounted traffic light systems. It just didn’t feel right. Student answers to my questions weren’t confident. Students were looking at me blankly. Where I usually get a whole load of eager hands up and students getting straight on with the task; that day I got nothing. Specifically, they just weren’t following when I was talking about the properties of simple molecular substances; namely their low melting and boiling points.

I tried to figure out what wasn’t right. I’ve been trying this year to really focus on my instruction; to use small, cautious steps, to allow practice in-between steps, and not to introduce too much material at once. But on reflection, I hadn’t fulfilled that last point. I’m going to try and flesh out my thinking without getting too bogged down in the subject specifics.

To fully understand simple molecular substances, the below pieces of information are required:

  1. Knowledge of atomic structure
  2. Knowledge of covalent bonding
  3. Knowledge of how gaseous substances are ones where the constituent particles have been separated

Point 3 here is the interesting one. In simple molecular substances there are strong covalent bonds between the atoms within the molecules. But there are weak intermolecular forces between these molecules. This means it is easy to separate the molecules and therefore the substances have low melting points.

Compare this to giant covalent structures. In these substances billions of atoms are all held together by strong covalent bonds. In order to break them, a large amount of energy is needed and therefore they have high melting points.

I think that simple molecular substances are therefore more complex. They don’t just rely on an understanding of covalent bonds, but they introduce a new idea: that of intermolecular forces. Add to that the complexity of the contrast between these two ideas: intermolecular forces are weak, covalent are strong; Intermolecular are between molecules, covalent are between atoms. To me, this is just too much. You cannot compare and contrast two ideas until you actually understand each one individually. They need to be fully internalised by themselves before they can be tied together without overloading the students.

But if you start with giant covalent, then you have the ability to really concretise the concept of a strong covalent bond. You can really focus on this concept of a strong covalent bond and how that relates to melting points before you throw in the next step.

Seems obvious in hindsight, but every textbook and scheme of work I have seen did it in a different order.

The next lesson, I levelled with the year 11’s. I said to them I don’t think I explained it well enough. I went back to the drawing board and started with giant covalent. A couple of lessons later, when our understanding of that had been concretised to some extent, we move on and added some complexity. Second time around, eager hands and solid, thoughtful answers.

No doubt there are people out there who have thought about this and either have this, or another solution. But for me that process of real and deep reflection and then going back and reteaching was not something I had done before. And it felt great.

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