Endpoint: Keith Taber has the last word

Periodic table with question marks in place of elements

Source: iStock

A Y9 student was telling me about the new topic she had started in science: 'we were looking at the Periodic Table, and it had stuff like iron, and you had to know what the symbol for it was, and if it had certain ions you balanced them'.

'Stuff' featured often in this student's reports of her science lessons, and indeed she was being taught about the nature of 'stuff'. This is chemistry - learning about different types of 'stuff', and how it reacts with other 'stuff'. However, chemists characterise 'stuff' as elements, compounds, mixtures and so forth which do not obviously relate to perceived properties of materials, and we explain much of the subject in terms of various particle models. These ideas provide the conceptual basis of our science,1 but students often have difficulty making sense of such abstract notions. 

In this particular lesson the student was being introduced to the key chemical concept of an element, so I asked her about this.  

  • What's an element? Student: like sodium and stuff. 
  • Why is sodium an element? Student: I think it's made up of atoms, or molecules. 
  • So is this wooden table made up of atoms or molecules? Student: It's made up of particles, isn't it? 
  • So is the table made up of particles? Student: yes. 
  • And an element is made up of atoms or molecules? Student: yes. 
  • What are atoms or molecules then? Student: I don't know. 

The student had come to this new topic with an existing concept of particles, which she had learnt in Y7. For her the table was 'a solid', so 'the particles are all closely packed', rather than 'a gas, [where] the particles are arranged really loosely, and there's lots of air between them', or 'a liquid [where] they're packed a bit more closely, but not as closely as a solid'. 

A common misconception

Clearly some aspects of the basic particle model that this student had been taught two years earlier had been understood. However, her description of the arrangement of particles in a gas showed a significant (if common) misinterpretation of the model.2 From everyday experience, it is obvious that air will fill up any spaces between objects. For this student, there was no room for air to get between the particles in a solid (which were closely packed), but plenty of space in a gas where there were significant distances between the particles.  

I then shifted her attention from the table, a complex composite material, to 'stuff' that she might recognise as an element: the tubular support for the stools we were sitting on. She acknowledged this was 'made of metal', and I said that I thought this metal was 'basically iron'. I was interested to know if she thought this was made up of particles (as a solid) or atoms (if considered as an element) or, correctly 'particles that we might label atoms'. She opted for particles since 'everything in the world is made of particles'. I pointed out that earlier she had implied that iron was made of atoms or molecules.  

She suggested how the iron could contain both particles and atoms: 'the particles were circular, and circles don't all fit together. This leaves little gaps in between and room for the atoms which are much smaller than particles'. So there was air between the particles in a gas, whereas in a solid there was only room for atoms! 

In schools we teach a generic particle model, which we later differentiate by introducing more specific particle types. Obviously (to us) we are talking about the same basic entities when we talk about particles, and then later atoms, ions and molecules. We might assume this is obvious to students, but the Y9 student clearly did not make this connection. 

Time to talk about 'stuff'

Sadly classroom teachers seldom have the time to talk in-depth to their students about their understanding of topics. Such conversations can provide insight into learning difficulties that impede effective classroom learning.3 In this student's case I would tentatively diagnose three contributing factors: the counter-intuitive nature of particle theory, the failure to make a connection between prior and new learning, which was abetted by our specialist language - the technical terms 'atom', 'molecule' and 'ion' do not obviously link to the everyday term 'particle'. Teachers need to find, or be given, the time for such diagnostic work if they are to help their students really understand 'stuff'. 

Dr Keith S. Taber is a senior lecturer in science education in the faculty of education at the University of Cambridge, Hills Road, Cambridge CB2 8PQ.

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