Science is often described as a ‘hard’ subject. By the time students in secondary school approach public examinations, success in science depends on the ability to handle abstract concepts and models as well as being, numerate, literate and able to recall a body of factual knowledge. Teachers help students to improve their understanding by providing structured experiences that help students to develop increasingly sophisticated mental models, moving from concrete to abstract.
Physical models use real objects to represent parts of a system. Mental models are also representations of systems and processes, but are usually more abstract than physical models. They are schematic rather than concrete, and often use diagrams and equations. The Class X textbook chapter, ‘Carbon and its compounds’, introduces students to different ways of representing molecular structures. If students are to become proficient in their understanding of the topic, they need to make effective use of mental models of molecular structures. All models have their strengths and limitations, and students need to be aware of the strengths and limitations of the models that they use.
In this unit the focus will be on helping your students to develop mental models in the context of carbon and its compounds. What you learn about developing mental models will also be applicable to other topics.
The study of carbon compounds is something that many students find challenging. There is a lot of new terminology to learn, and success depends on the ability to relate the properties of compounds to their molecular structure. Students have to retain and manipulate mental models to represent chemical reactions and they have to think about the molecular structure of the reactants.
As in the rest of science, when students learn about carbon and its compounds they need to start from concrete experiences and prior learning. They need to be supported in developing increasingly abstract approaches – the activities and case studies in this unit will you provide that support. Initially, you might show your students some three-dimensional molecular models, but they will still have to make the connection between real three-dimensional molecular structures and two-dimensional diagrams that represent the same structure. Students have to learn the conventions of these diagrams but also have to remember what is not shown by the diagrams but is still relevant to the chemistry of the situation.
When students learn to use electron dot structures to represent carbon compounds, these diagrams relate a molecular formula such as CH4 or C2H6 to the number of covalent bonds available as a consequence of the number of valence electrons for each kind of atom.
Once this concept is established, electron dot structures are not a convenient way to represent anything other than the simplest molecules, as large numbers of circles and dots can become a distraction: it will be a useful approach again when students need to learn about reaction mechanisms, but for now the model has served its purpose and students begin to use diagrams of molecular structure which represent each covalent bond by a single line.
Both of these models identify the numbers of bonds in a molecule, but they do not convey information about the true shape of the molecules, or that parts of molecules are able to rotate relative to each other. Students have previously learnt that particles in a gas are in rapid motion relative to each other, but have treated the molecules as simple spheres in rapid motion. Those who go on to study chemistry further will learn about infra-red spectroscopy and will relate spectral features to rotation and vibration around particular bonds within molecules. Class X students are still building their use of mental models, however, as a first step they need to recognise that the diagrams they have been using are two-dimensional representations of three-dimensional molecular structures.
One of the problems for students in learning about carbon compounds is that they sometimes treat a diagram of structure too literally. For example, when they are asked to work out structural isomers for a given formula, many students cannot see that some of the structures that they have drawn are equivalent. It is much easier to do this once students have seen a physical model of the molecule and have seen the effect of rotating it.
[If you have access to computer applications that allow students to generate molecular structures and see them rotating, this would also be very useful.]
Pause for thought
Mrs Gupta constructed some molecular models and compared these to diagrams of the structures.
I wanted to teach about functional groups, and from past experience, I knew that many of my students would find it difficult to see that certain substitution positions are actually equivalent, if I only relied on using diagrams of the structure. I decided to use molecular models to help my students review what they already knew about alkanes in order to prepare for learning about functional groups. At the same time, I wanted to remind my class that the diagrams in the textbook are only one way of representing a molecule’s structure. Using the physical models would help them to realise some of the limitations of using diagrams.
Before the lesson, I constructed a model of a methane molecule and of a hexane molecule. I kept the hexane molecule out of sight and began the lesson by showing the model of a methane molecule to my class. I told them it was a molecular model of methane and asked them what they noticed about it. They told me they could see four white balls attached to a black ball by plastic rods. They said that the four white balls must be hydrogen atoms, the black ball must be a carbon atom and the rods must be the bonds, like the lines in the diagrams.
Next, I told them all to look at Table 4.2 in the textbook and asked, ‘What is different about the model from what you can see in the table?’ Nobody answered. ‘Look at the angles’, I said. ‘Can you see any right angles? Is the molecule flat?’ Now they knew what to look for, of course they could see that the model was not flat, but formed a tetrahedron with all the hydrogen atoms as far away from each other as possible.
I got out the other model that I had made before the lesson. I told one of my students, ‘Count how many carbon atoms there are and how many hydrogen atoms, then tell the rest of the class.’ Then I asked what they thought it must be. They identified it as hexane, because there were six carbon atoms.
We looked again at the two models and compared them to the diagrams in the table. This time, my students were ready to tell me that these molecular models were certainly not flat, and in fact were beginning to look a bit wiggly. The ‘spine’ of the molecule was not straight; the carbon atoms at each could rotate around, so the hydrogen atoms and their bonds were like propellers on a boat or plane.
I passed the methane model to a student and asked him to turn it into a butane molecule. At the same time, I passed the hexane model to a student and told her to turn it into another model of a butane molecule.
So I then had two identical butane molecules. I explained that it was possible to make a new carbon compound by replacing one of the hydrogen atoms by a chlorine atom and that we would represent the chlorine atom in the models by using a green ball with one link point. I gave a ‘butane molecule’ and a ‘chlorine atom’ to two students and asked them to make a new molecule.
Both students had replaced a hydrogen atom on one end of the molecule. I held both models up for everyone to see ‘Are they the same or different?’ I asked. As one hydrogen seemed to be at the opposite end from the other hydrogen, everyone thought they must be different, but as I rotated the molecules around the ‘spine’ of the molecule, then end over end and then made the ends of the molecule rotate, they began to see that the molecules were actually the same. I drew three diagrams of the structure that looked different but actually represented the same molecular structure [Figure 2].
I asked my students how I could make one of the molecules different from the other. After a few moments thought, someone suggested that replacing a hydrogen connected to a carbon in the middle of the chain should make a different molecule. I drew a diagram of the structure [Figure 3].
Then one student suggested that we could move the chlorine atom to another carbon atom. We tried this out, and it was indeed different. Using the other model and moving the chlorine atom so that it was attached to one of the carbon atoms in the middle, we also established that it was impossible to tell middle carbon atoms apart once the model was rotated, even though the diagrams might look different [Figure 4].
Even though I only have one modelling kit, I think using it with my students helped them to understand the relationship between the physical models and the diagrams in the book. Now that I’ve used the models in my lessons, I might organise my students into groups and let each group have a turn at building with the modelling kit.
This activity will help you to develop your planning and in-class teaching.
You will need a molecular modelling kit for this activity (either a ‘ball and stick’ type or a ‘space-filling’ type). Alternatively, you can use toothpicks and modelling clay to represent bonds and atoms.
Look at Section 4.2.2 in the Class X textbook.
Figures 4.8a, 4.8b and 4.9 show the carbon skeleton of a molecule and then the complete molecule.
Now create models for each of the structures shown in Figures 4.8a, 4.8b and 4.9. Compare these models to the related diagrams in the textbook.
One of the wonderful things about carbon chemistry is the myriad of compounds that are possible. The huge number of different compounds would be very difficult to study if we had to learn about each compound individually. Fortunately, it is possible to organise compounds into groups sharing common structural features and common chemical properties. The simplest example of these is the alkanes, which can all be represented by a common formula CnH2n+2. Once we recognise common features we can start to make predictions about how compounds in a particular family will behave based on what we know about the behaviour of one or two members of the family and about the effect of differences such as the mass of the molecule.
In order to generalise and make predictions about carbon compounds, your students will learn to apply the concepts of a homologous series and functional groups.
The first mental model they will have to build is that every member of a homologous series shares the same general molecular formula, such as CnH2n+2 for alkanes or CnH2n for alkenes. For each of these series, adding a –CH2 unit to a molecule will produce the next molecule in the series.
The next mental model that your students need, is that you can replace one of the hydrogen atoms attached to a carbon atom by something else called a functional group such as a chlorine or bromine atom, or one of the other groups shown in Tables 4.3 or 4.4 in the textbook. The chemical properties of the new compound will depend on what functional group you use to replace a hydrogen atom.
Combining these two models allows your students to produce a homologous series of alcohols, carboxylic acids or other chemical families, such as those shown in Table 4.4. Every compound in a homologous series will have similar chemical properties to all the other compounds in the series.
These can be difficult concepts for students to understand, it is wise to try to introduce the content in small ‘chunks’, a small amount of new information in each lesson so that students have time to process the information and see patterns.
Mr Bulsara used diagrams and mental models to teach about homologous series.
I decided to introduce the idea of homologous series to my students as a ‘spot the connection’ game. I asked everyone to close their textbook and I began by writing the molecular formula for methane, ethane and propane on the blackboard:
CH4 C2H6 C3H8
I asked my students, ‘Predict the next two items in the sequence and tell me how you worked it out.’
They told me that the next two items would be C4H10 and C5H12, and that each time you had to add one C and two Hs.
Then I asked, ‘Work out what the answer should be if it started with C10, and to explain how they arrived at the answer.’ I asked them to discuss the answer in pairs, before I chose someone to reply.
Ravi told me that the answer should be C10H22, and one of my students said that whatever number you had for C, you should double it and add 2 to get the number for H. I wrote this down as CnH2n+2, then told them that this was a general formula for the family known as the alkanes. A family like this is also called a homologous series.
I also wanted my students to relate what they had just discussed to the structural diagrams for molecules in these two series, as they would need to recognise both ways of representing the same molecule.
I provided the structural diagrams for CH4 and C2H6 and asked everyone to draw diagrams for C4H10 and C5H12. As they already understood the rules of the homologous series, everyone managed this easily.
I suggested that next time they visited an internet café, they should find some images of carbon molecules so they could see some of the different ways of representing them.
This activity will help you to develop your planning and in-class teaching.
Look at the list of functional groups in Table 4.4 in the Class X textbook. It shows the effect of substituting the functional group for one or more hydrogen atoms on a propane molecule.
Read Resource 1, ‘Planning lessons’, for more information.
In learning about carbon and its compounds, your students use a range of ways of representing information about molecules. One recurring model is the electron dot structure model resulting from carbon having four valence electrons. Structural diagrams are another way of encapsulating the same information about ‘where the bonds are?’. Every time a new type of compound or reaction is introduced, students need to go back to one or more models of molecular structure, to refresh, recall recent concepts and to scaffold a new understanding.
For a great deal of the time, one or both of these two-dimensional approaches may be sufficient for your students, but occasionally it will be helpful to remind your students of the three-dimensional nature of the molecules by using physical models.
When learning about soaps and detergents, your students may be puzzled why the structures called micelles are shown as zig-zags with a Na+ on one end. Why a zig-zag? Well, it’s a convention. Yes, but why this convention? Using physical models of alkanes with increasing numbers of carbon atoms makes it very evident that the ‘spine’ of a carbon compound is more like a zig-zag than a straight line.
Using physical models can help to remind students that diagrams only represent some aspects of a molecule’s structure, but can be actively misleading. The diagram of the branched structure for C4H10 in Figure 4.8 in the textbook and the related carbon skeleton in Figure 4.7 suggests that two of the carbon atoms are closer together, for example, whereas a physical model would show that the skeleton is a tetrahedral structure with rotational symmetry about more than one axis (Figure 5).
Sometimes, a combination of approaches is useful to help students to understand what is happening in a reaction. For example, students learn that one of the reactions of ethanoic acid is with an alcohol to produce an ester and water. There are several ways that you could examine this reaction with your students:
CH3COOH + CH3CH2OH
(ethanoic acid) (ethanol)
CH3COOCH2CH3 + H2O
CH3CH2COOH + CH3CH2OH
(propanoic acid) (ethanol)
CH3CH2COOCH2CH3 + H2O
Using this approach can help to make it more clear what the molecular structures are.
This is a more abstract approach, but shows ‘where’ the reaction happens and emphasises that the rest of the reactant molecule is unchanged.
Each of these models provides a different way of viewing the same event.
Pause for thought
This activity will help you to develop your planning and in-class teaching about the chemical properties of carbon compounds. You may find it useful to carry out this activity as a discussion with a colleague.
In this unit you have learned about some of the things that make carbon compounds a difficult topic, and how you can use different approaches to help your students to develop appropriate mental models.
Whatever topic you are teaching, it is important to use a range of approaches to help your students develop appropriate mental models. This unit has focussed on a small range of strategies. A key element in all of these is the use of questions to help your students to become aware of the features of the model they are using, including any possible limitations. It is also important to use these models to structure and reinforce the learning process, don’t try to introduce too many ideas in any one lesson.
Good lessons have to be planned. Planning helps to make your lessons clear and well-timed, meaning that students can be active and interested. Effective planning also includes some in-built flexibility so that teachers can respond to what they find out about their students’ learning as they teach. Working on a plan for a series of lessons involves knowing the students and their prior learning, what it means to progress through the curriculum, and finding the best resources and activities to help students learn.
Planning is a continual process to help you prepare both individual lessons as well as series of lessons, each one building on the last. The stages of lesson planning are:
When you are following a curriculum, the first part of planning is working out how best to break up subjects and topics in the curriculum into sections or chunks. You need to consider the time available as well as ways for students to make progress and build up skills and knowledge gradually. Your experience or discussions with colleagues may tell you that one topic will take up four lessons, but another topic will only take two. You may be aware that you will want to return to that learning in different ways and at different times in future lessons, when other topics are covered or the subject is extended.
In all lesson plans you will need to be clear about:
You will want to make learning active and interesting so that students feel comfortable and curious. Consider what the students will be asked to do across the series of lessons so that you build in variety and interest, but also flexibility. Plan how you can check your students’ understanding as they progress through the series of lessons. Be prepared to be flexible if some areas take longer or are grasped quickly.
After you have planned the series of lessons, each individual lesson will have to be planned based on the progress that students have made up to that point. You know what the students should have learnt or should be able to do at the end of the series of lessons, but you may have needed to re-cap something unexpected or move on more quickly. Therefore each individual lesson must be planned so that all your students make progress and feel successful and included.
Within the lesson plan you should make sure that there is enough time for each of the activities and that any resources are ready, such as those for practical work or active groupwork. As part of planning materials for large classes you may need to plan different questions and activities for different groups.
When you are teaching new topics, you may need to make time to practise and talk through the ideas with other teachers so that you are confident.
Think of preparing your lessons in three parts. These parts are discussed below.
1 The introduction
At the start of a lesson, explain to the students what they will learn and do, so that everyone knows what is expected of them. Get the students interested in what they are about to learn by allowing them to share what they know already.
2 The main part of the lesson
Outline the content based on what students already know. You may decide to use local resources, new information or active methods including groupwork or problem solving. Identify the resources to use and the way that you will make use of your classroom space. Using a variety of activities, resources, and timings is an important part of lesson planning. If you use various methods and activities, you will reach more students, because they will learn in different ways.
3 The end of the lesson to check on learning
Always allow time (either during or at the end of the lesson) to find out how much progress has been made. Checking does not always mean a test. Usually it will be quick and on the spot – such as planned questions or observing students presenting what they have learnt – but you must plan to be flexible and to make changes according to what you find out from the students’ responses.
A good way to end the lesson can be to return to the goals at the start and allowing time for the students to tell each other and you about their progress with that learning. Listening to the students will make sure you know what to plan for the next lesson.
Look back over each lesson and keep a record of what you did, what your students learnt, what resources were used and how well it went so that you can make improvements or adjustments to your plans for subsequent lessons. For example, you may decide to:
Think about what you could have planned or done even better to help students learn.
Your lesson plans will inevitably change as you go through each lesson, because you cannot predict everything that will happen. Good planning will mean that you know what learning you want to happen and therefore you will be ready to respond flexibly to what you find out about your students’ actual learning.
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