Two teachers sit in a staffroom; one holds a mug of coffee protectively in her hands. She turns to the other and says, ‘I do love 8C, they are fantastic; but I just wish they didn’t know so much.’ The other teacher nods absent-mindedly as a unicorn passes by the window and his Kit Kat floats towards the ceiling because clearly this is absurd and not the world we live in.

If I ask you to think of a good teacher, what comes to mind? Someone with enthusiasm, passion and energy for their subject? Perhaps chemistry taught by a mad professor who just can’t get more excited about the different kinds of vibration of bonds in infrared spectroscopy.

You’ve probably stepped over the question of knowledge. It’s pretty much a given, isn’t it? You need extensive knowledge of a topic to teach it. But is this an assumption we should question? Well, not directly – but learning chemistry (or indeed anything) is not just about knowing facts. The ability to efficiently pass knowledge to students does not in itself make a good teacher.

In fact, giving students too much knowledge probably creates a bad teacher. This is because of the way our brains are structured – our cognitive architecture.

Hard working memory

The memory system in healthy brains can be coarsely divided into your working memory (which you may know as your short term memory) and your long term memory. We’ll ignore for now that memories can be procedural, emotional, semantic, and some other kinds. Let’s just call a memory a memory.

We can measure how long information stays in our working memory in seconds or, at most, a few minutes. Information will sit there until it either disappears or is transferred to long term memory.

It may surprise you how little information can sit in your working memory at one time. George Miller was the first psychologist to attempt to quantitatively measure the working memory’s capacity. Miller coined the term ‘Magical Seven’ – the idea that working memory could hold seven plus or minus two items. Later work by Nelson Cowan suggested a lower maximum capacity of four.

But what accounts for the discrepancy in these numbers? How could one person measure up to nine, and another no more than four?

The answer lies in chunking. Chunking is how we organise and collect information – how we relate ideas. By chunking related pieces of information together we can fit more into our working memory. For example, if I gave you a series of 20 random digits and asked you to recall them in order, you’d probably remember a few, but I doubt you’d make it to 20. But if I asked you to recall your house number, your landline number, your mobile number, you would easily recite 20 to 30 digits in the right order.

Our ability to chunk together different kinds of memory allows us to carry out a practical task without being completely blinded by the complexity of it.

It’s clear then how easily we can overfill our working memory. When we do, we induce cognitive overload. Students who struggle to chunk new information effectively become overloaded and cannot fit more information in their working memory, not without discarding something else.

Read Michael Seery’s discussion of cognitive overload in his article on jump-starting lectures



Cognitive overload can create misconceptions and muddy up previously clear concepts. Too many cooks stop other cooks from getting into the kitchen.

Constructive learning

Learning is not just acquiring knowledge or facts, it is linking them together and freely connecting old and new knowledge. This is the idea of constructivism – that we are able to place Lego bricks of knowledge into our long term memory and then build on them where we need to.

A classic example of this is learning to drive. In driving, there’s a lot to do and a lot to watch for. Making decisions about which gear to use; operating the clutch and throttle together; where you should be in the road; roundabouts … Remember when turning a corner and changing a gear at the same time was in equal parts paralysing and terrifying? But with experience we no longer need to access the individual pieces of driving knowledge and stitch them together in real time – it’s all together in a cognitive box labelled ‘driving’.

It’s the same with learning chemistry. As experienced chemists, it’s easy to forget how difficult chemistry is to learn. We might underestimate how much effort a learner must expend to overcome a learning curve. This is what Steven Pinker in his book A Sense of Style refers to as the curse of knowledge, and it’s a demonstrable characteristic of human psychology.

If a student states confidently that the double line in C=O means there are two electrons bonding the atoms – they’re wrong. But what makes them wrong? Is it that they don’t understand that '–' is different from '=', or that a covalent bond is one electron from each atom? Or did they mishear the question?

To teach effectively we need to be able to break down bigger concepts into the tiny ideas that make them up, and see them from a student’s point of view.

Next week, in part two, I’ll look at what this means for us as chemistry teachers – how we can plan our teaching to reduce cognitive load.

Updated: part two 'How to avoid cognitive overload in the classroom' is now online.

Tom Wilson is a chemical education PhD student at the University of Southampton, UK