Senior Form Chemistry – The roles of models in understanding science The roles of models in understanding science Teacher’s Version (with video exemplars) 1. NOS/STSE Features A model can be regarded as a representation of an idea, an event, a process or a system. 2. Every model has its strengths and weaknesses. 3. The strengths and weaknesses of a model depend on the context or purpose. 4. A model is not a direct copy of reality. You can click an icon like this to access the activities in this package. You can click an icon like this to access the PowerPoint files in this package. You can click an icon like this to access the exemplar videos in this package. Icons of different colours indicate exemplars from different teachers. If the links do not work, try again by pressing the “Ctrl” key and clicking the icon at the same time. Table of Contents Introduction (i) Activity 1: The most helpful map P.1 Activity 2: The three states of water P.2 Activity 3: Which model is the best? P.4 Further Exercise/ Homework Exercise P.5 Appendix: Lesson Plan P.7 Appendix: Lesson exemplar (Transcript) P. 10 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Introduction Rationale (a) What is a ‘model’? A model can be regarded as a representation of an idea, an event, a process or a system. Models in science are created to test ideas. They will change dramatically as ideas develop. Models also help us to represent our ideas , yet they are subject to some limitations. Therefore, a model is not a direct copy of reality. In order to better serve the purpose, when people develop models, it is most common that they will highlight the more relevant areas selectively while neglecting less important aspects. (b) What we have learnt about students’ concepts of models: 1. Some students regard models as copies of reality and they think that the goodness of a model should be judged by its similarity to the real situation. 2. They prefer simpler models, even when they know of more advanced models (e.g. they might still prefer a shell model of a molecule when the teacher tries to explain orbital hybridization.) 3. They may fail to appreciate the scope and limitations of different models or ways of representation. As a result, they may not be able to choose the appropriate model to help them to solve a problem. (e.g. When students fail to see redox in terms of gain of oxygen, they might feel difficulty in understanding that carboxylic acid is the oxidation product of an alkanal.) 4. They may confuse models (e.g. Arrhenius vs Bronsted-Lowry models of acid/base.). This may be due to the quality of textbooks which also do not differentiate these models clearly. Example: ‘When HCl dissolves in water, it releases hydrogen ions as the only positive ions. Because the chloride ion is a very weak conjugate base, it is unlikely to accept the proton. Therefore HCl is a strong acid.’ 5. They may place greater reliance on the material or concrete model than on other models (e.g. some students insist that the ball-and-stick model is inferior to the space-filling model). 6. They may receive wrong messages from models. For example, students may wrongly conceive from a 3D model of NaCl(s) that each sodium ion is only bonded to six chloride ions. This is due to their literal interpretation of the 3D model of NaCl where they see the “sticks” representing ‘ionic bonds’. (The rods are simply supports, not the representations of bonds. This is probably a confusion between the ionic model and the ball-and-stick model.) (i) © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science (c) Aims of this package It is hoped that students can understand (i) the role of models in making sense of scientific knowledge; (ii) the scope and limitation of different models in representing the physical world. Level of study Number of lessons Form 4 – Form 5 (Activities 1 to 2) 2 lessons (each lesson 35-40 minutes) Form 6 – Form 7 (Activities 1 to 3) [Related Topic: Covalent Bonding (This could be a round-up lesson for the topic); Organic chemistry (This could be an introductory session to the topic)] Contents: 1. Introducing the ideas relating to Models to students, exemplified by the different types of maps we use in daily life. 2. Introducing the uses of models in science through an examination of water as ‘particles’ and water as ‘molecules’. 3. A re-examination of different chemical models. Students will work out the benefits and drawbacks of different models. Teaching package includes 1. Worksheet (Teacher’s Version and Student’s version) 2. A Teaching PowerPoint (ii) © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Activity 1 The most helpful map Key ideas: (i) Definition of models as representations of an idea, an event, process or system. (ii) When people develop a model, they will selectively highlight the more relevant areas while neglecting something less important in that particular context. (iii) Every model has its strengths and weaknesses. Q1 Which types of map do you think is the best? Explain? (Notes: Students should be encouraged to express their views.) (Slide 3: Map showing Tsim Sha Tsui rail station exits Slide 4: MTR system map Slide 5: Geographic map of the MTR) Notes: After some brainstorming from students, teacher (or better students) may bring out that: 1. ‘Best’ can be defined in terms of either how similar the map is to reality or how useful the map is. (Skip the realist and instrumentalist view of science knowledge.) For example, the geographic map (slide 5) may not be helpful at all if you get lost in a street or if you want to know how to travel by MTR from Prince Edward station to North Point station. However, the MTR system map (slide 4) is useful, but it doesn’t necessary tell the distance between stations. It would be wrong to believe that the map is drawn to scale. 2. A map can be a model of the geography of HK. The MTR map is a model of the routes travelled by MTR trains. A model can be regarded as a representation of an idea, an event, a process or a system. Q2 When people construct maps, they do not draw everything they see? Why not? When people develop a model, it is common that they will selectively highlight the more relevant areas while neglecting something less important. (Key question: when people draw these maps, do they copy everything they see? If not, what criteria guide their drawing/selection?) [Notes: In devising the MTR system map (slide 4), the designer only focuses on the names of different stations and highlights how to change from one line to another (satellite maps, no matter how accurate they are, cannot do this). Other information such as the name of the streets are neglected. However, such delibrate selectivity actually makes the MTR map better for its purpose. Therefore, in order to better serve the purpose, when people develop models, it is most common that they will highlight the more relevant areas selectively while neglecting something less important.] P.1 Every model has its strengths and weaknesses. © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Activity 2 The three states of water Key ideas: Strengths and weaknesses of a model depend on the context or purpose. Therefore teachers will need to ascertain the value of models that students have learnt in their junior forms – they are not wrong, but serve different purposes. Notes: Teacher should bring out the notion that in science, we often use different models or different types of representation to convey our ideas. In order to make our ideas clearer and easier to understand, we will focus on something relevant while neglecting something insignificant. This is illustrated by a comparison of two models of the circulatory system of the blood. Q3 In the spaces below, draw pictures to illustrate the arrangement of water molecules in solid, liquid and gas states. ice water steam (Notes: Teachers will need to ascertain the value of the models that students have learnt in their junior forms.) Q4 In what ways do the drawings you have made act as models for the three states of water? Each sphere represents a water molecule. The arrangement of the spheres shows the differences in the arrangement of water molecules in the three states. The arrangement explains the differences in the properties of three states. P.2 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Q5 Explain why it is not necessary, for instance, to use the space-filling model to describe the three states of water. To explain the three states of water, the most important information is the arrangement of the water molecules. In this case, the information shown by a space-filling model (e.g. relative size of different atoms, composition of water molecule) becomes far less important and may distract people’s understanding. Q6 Suggest a concept which would require a ball-and-stick or a space-filling model of water to more accurately represent it. Ball-and-stick model: bond angles Space-filling model: relative size of atoms Notes: In order to better understand the properties of water resulting from hydrogen bonds between water molecules e.g. surface tension, high boiling point, high specific heat capacity, the bonding and constituents of water molecules become important. So we could use a ball-and-stick model of the molecules rather than representing water molecules as spheres. Q7 What have you learnt from this activity about the uses of different models? In order to understand the three states of matter (e.g. ice, water, steam), we use spheres to represent water particles. In this model, the composition of each sphere is not our concern. It is not important and may be distracting to our understanding of the arrangement of particles in different states. Notes: As a round up, teacher might present a sphere and a space-filling model and emphasize that on different occasions, each of these have different strengths and weaknesses. The strengths and weaknesses of a model depend on the context or purpose. P.3 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Activity 3 Which model is the best? Key ideas: (i) Models are not copies of reality. (ii) Different models give us different pieces of information. We will have a more complete picture when putting the ideas gained from different models together. Here are different models of a methane molecule. 1. 3. 5. 2. 4. 6. CH4 Q8 Notes: Teacher may ask students, e.g. in the form of a class vote, among the different models, which one is more similar to what they perceive to be a methane molecule and why. In a group of 3-4, compare the information that each model tells and does not tell you. Information conveyed The relative size of different atoms (it shows covalent radii). A space-filling model is particularly useful when considering steric hindrance in SN reactions. Drawbacks Atoms or molecules are not hard spheres. 2. Ball-and -stick Good three-dimensional representation. Bond angles can be shown more clearly. 3. Dot-andcross diagram Number of outermost shell electrons involved in chemical bonding. Bond order. Sometimes, the balls are of the same size; the bond length and bond order may not be represented accurately. open 4. 3D diagram The spatial arrangement of atoms can be shown in a piece of paper (i.e. in 2D) without the trouble of building space-filling and ball-and-stick model. Bond order (single, double or triple bond); the structure of a molecule (i.e. which atom is bonded to which atoms). The atoms and their number in a molecule. It is a very convenient shorthand to represent a molecule, particularly when the molecule is made of many atoms (e.g. C6H12O6 is more convenient and simpler than it structural formula). 1. Space filling 5. Structural formula 6. Chemical formula P.4 open open open © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Q9 Can we have a true picture of a methane molecule when we put the information from all these models together? Even seeing all of these models all together cannot constitute a complete representation of the molecules or a real methane molecule. For example, the nuclear structure (e.g. the number of neutrons), the size of the nucleus and the movement of electrons have not been considered. Models are not copies of reality. Further Exercise/ Homework Exercise Task 1 Construct a table to compare the strengths and weaknesses of each model of sodium chloride. (a) 2D picture of NaCl (showing the relative size of Na+ and Cl-) Cl - Cl - Cl - Cl - Cl - Na + Cl - Na + Cl - Na+ Cl - Na+ Na + Cl - Na+ Cl - Na + Cl - Na + Cl - Cl - Na+ Cl - Na + Cl - Na + Cl - Na+ Na + Cl - Na+ Cl - Na+ Cl - Na+ Cl - Cl - Na+ Cl - Na + Cl - Na + Cl - Na+ Na + Cl - Na + Cl - (b) A 3D model of a NaCl lattice Cl - Cl - (c) The formula ‘NaCl (s)’ Notes: Some students hold the view that each sodium ion is bonded to six chloride ions. This may be due to their literal interpretation of the 3D model of NaCl where they see the sticks representing the ‘ionic bonds’. P.5 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Task 2 In your view, which model is better to illustrate the digestive system in our bodies? Why? Task 3 Suggest some examples from your integrated science lessons where the teachers or the textbooks use different models to present their ideas. Task 4 (for Form 6) Compare the strengths and weaknesses of the shell model and the orbital model of an atom. P.6 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Appendix: Lesson plan Activity 1: The most helpful map (Introduction to the ideas of Model and Multiple Representations) Time Intended Learning Outcomes Students should be able to define models state the multiple representations of models state that models usually highlight the more relevant areas while neglecting those less important 15 min Teaching Flow P.7 Materials Students compare three kinds of maps all showing MTR stations (PowerPoint slides) Teacher probes students about what they perceive to be the “best” map. Key questions: Which type of map do you think is the best? Teacher may bring out the idea that ‘best’ can be defined in terms of either how similar the model is to reality or how useful the model is. Teacher brings out that model can be regarded as a representation of an idea, an event, a process or a system. Students discuss the necessity of drawing everything they seen on a map. Teacher brings out that when developing a model, it is most common that people will selectively highlight the more relevant areas while neglecting something less important. Remarks PowerPoint (slides 2 – 15) Worksheet © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Activity 2: The three states of water (Linking the concepts discussed in Part 1 to Science) P.8 Time Intended Learning Outcomes 15 min Students should be able to appreciate the use of different models in science point out that the strengths and weaknesses of a model depend on the context or purpose value different types of models in aiding understanding in junior and senior forms Teaching Flow Teacher shows two models of the circulatory system (in the PowerPoint). Students draw pictures to illustrate the arrangement of water particles in solid, liquid and gas state. Students discuss the reasons for using spheres to represent water particles. Teacher points out that the strengths and weaknesses of a model depend on the context or purpose. Teacher would ascertain the value of model that students learn in their junior forms- they are not wrong, but serve different purposes. Materials Remarks PowerPoint (slides 16 – 21) Worksheet We should not judge a model that students learn in their earlier stage of study (e.g. particles as spheres) to be ‘wrong’. While these models may suffer from their breath of explanation, they have an apparent advantage of simplicity and serve good purpose. © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Activity 3: Which model is the best? (A reexamination of different chemical models) Time Intended Learning Outcomes Teaching Flow Students should be able to work out the benefits and drawbacks of different models point out that Models are not copies of reality 35 min Further Exercise (To reinforce or assess students’ understanding to models.) P.9 Materials Teacher shows methane molecules represented in PowerPoint different ways. (slides 22 – 39) Teacher may ask students, e.g. in the form of a Worksheet class vote, among different models, which one is more similar to what they perceive to be a methane molecule and why. Students compare the benefits and drawbacks of each model and the information that each model conveys and does not convey. (Group discussion) Teacher asks students whether they have a true picture of the methane molecule when they put all the information from these models together. Teacher summarizes that even seeing all models of methane collectively, we cannot construct a complete representation of a real methane molecule. Remarks It is very important to be aware of the scope and limitation of each model. A deeper understanding can result from the comprehension of multiple representations © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Appendix: Lesson exemplar (Transcript) T: teacher (Ms Koo Wing Fun, King’s College) S: student (Form six science students) T: Models can be regarded as a representation of an idea, an event, a process or a system. Okay? Now, let see some examples. (Showing a diagram of the circulatory system in human) What is this system? You are living with this system. S: Circulatory system. T: The circulatory system? Right! Another example would be…a process. S: Haber process. T: This is Haber process. You have learnt it before. Nitrogen combines with hydrogen at about 400 ℃. Okay? Then another model would be… Do you notice that there are two models? This one is the one you learnt in Form three. It shows the electronic arrangement. How about that one? How can you represent hydrogen atom? What is the electronic arrangement you have learnt in Form six? S: … (S’s voice was too soft.) T: Yes, 1s1. Maybe it is too simple. How about sodium? How do you represent sodium in Form three? S: 2, 8, 1. T: How about in Form six? S: 1s2 2s2 2p6 3s1. T: Okay. There are two different models, right? What is it? It should be very obvious. Model can also represent an event. S: History. P.10 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science T: Right, it’s about the history of China. Okay, when scientists develop models, do they copy everything they see? What do you think? If not, what criteria guide their drawing? Let see an example again. (T showed the diagram of circulatory system.) Do they copy everything they see? S: No. T: Then what do they copy? S: Something important. T: Okay. Something important. Then they neglect something, right? How colours? Is the blood really in red and blue colours? Mr. Chan? about the S: It represents oxygen. T: Oxygen. There will be more oxygen in this kind of … S: Oxygenated blood. T: What? S: Oxygenated blood. (S repeated loudly.) T: Yes? S: The red one. T: Yes. The red one represents more oxygen inside the blood. Then this one (T pointed to the blue colour.) represents less oxygen. But is it really red or blue in colour? S: No. T: How about this process (Haber process)? Are there any drawbacks of this diagram? Does the diagram contain everything you know? P.11 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science S: Compression room… T: Okay, the things happened in compression room. What else? S: Mechanism… T: Yes, it cannot show what really happened in the reaction chamber and also the mechanism. It hasn’t showed you the reaction, right? For the electronic structure, there are two models representing the same atom. When people develop models, they just selectively highlight the relevant areas and neglect some less important things. Right? Now, come back to something that you have learnt in junior forms. It’s about water particles. How can you draw pictures to represent the three states of water? You are Form six students now, can you briefly describe or draw the pictures on the board? How do Form one students draw water molecules? (A student drew a diagram showing the solid state of water.) T: Is it a solid or liquid? (Student wrote “solid”.) T: Okay. How can you draw water in liquid or gas states? (Student continued with his drawing. Spheres are used to represent water molecules.) T: Thank you. Now, of course you know how to draw. But the question is…what is used to represent water molecules? You used spheres to represent H2O. Are you satisfied about that? Some information is neglected. We use spheres to represent water molecules. The composition of the sphere (water molecule) is not our concern here. That’s why we can use spheres. If you just want to show the arrangement of water molecules in the three states, spheres are used. In junior forms, you just want to highlight the more relevant areas and neglect something less important. Then, what do you think now? What are the other things used to represent water molecules? Maybe sphere is not used to represent a water molecule anymore. We may use this model (T pointed to a diagram showing a space-filling model). What is this model? S: … P.12 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science T: Okay. Let me show you. This is a space-filling model. Then why do we use a space-filling model instead of a sphere to represent water molecules? What ideas or information do you get from a space-filling model instead of a sphere? S: … T: When you use a sphere, you neglect the composition of a water molecule. But it is important now. There is an oxygen atom and two hydrogen atoms. What else does this model tell you? S: The atomic size. T: The atomic size and the arrangement of oxygen and hydrogen atoms. What else? S: Bond angle. T: Yes. This model tells us some other things. It tells us the arrangement of the atoms, the bond angle and the relative size. That’s why we use the space-filling model instead of sphere. Okay, this is a methane molecule. As you can see, you may have six models to represent a methane molecule. You have something to do. You have seven groups, right? Groups 1 to 6 will do these things. You have to state the name of the model. You also need to describe the information that each model conveys and the drawbacks of each model. Drawback is the weakness of the model. (T distributed the questions to each group.) T: Then, what will be group 7’s work? Group 7 will do another thing. To answer “Can we have a true picture of methane molecule when we put all the information of these models together?” will be the job of group 7. Okay? You need to write down your answer in the colour paper. You should assign one of your group mates to present your ideas. Okay? (Students started their discussions.) P.13 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science Group discussion Group 3’s discussion S1: It seems that the circles are identical. S2: The actual structure of carbon and hydrogen cannot be shown. S3: How about the bond angle? S2: We can’t see that. Group 4’s discussion S1: What is the question about? S2: This one is the 3D structure. S3: It’s about optical isomers. S2: Is it the molecular formula? S3: How about atomic size and radius? T: What is the name of this model? S3: 3D model. T: That’s right. Does it convey any information? And are there any drawbacks? S2: Bond angle. S3: How about bond length? S1: How can we observe bond length from it? S3: Are we talking about model no. 2? S1 & 4: No. We are group 4. P.14 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science S3: Oh! Then it can show the optical… S4: That’s 3D arrangement. S2: It should be the spatial arrangement of atoms. S1: Each carbon atom is bonded to four hydrogen atoms. S3: We should state also the drawbacks. S4: We cannot see the atomic radius S1: And the colour of different atoms! S2, S3, S4: Ah! S3: These may confuse the reader. The reader may think these are different kinds of bond. S2: How about the drawbacks? S3: It can’t show the bond length. S4: Orbital activity. S2: It’s good. S1: It doesn’t show hybridization. S2: No. We can see it from the diagram. S1: How? S2: After we have learnt it. S1: That’s not what the model shows. S2: You don’t know what hybridization is if you haven’t learnt it. P.15 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science S3: It can’t show the bond length. S1: How about lone pair? S4: Lone pairs will be drawn if present. S1: How about polarity, the polarity of bond? S4: How to spell “polarity”? Student presentation Group 1 S: There are many advantages of that. It shows the bond angle, the relative atomic size of atoms, atomic structure, as well as the type of bonding. In this example, it is covalent bond. However, there are lots of drawbacks. Firstly, atoms are not spherical in shape. It is only an assumption. Also atoms are actually mainly occupied by space. This kind of representation doesn’t show that atoms are mainly occupied by space. Also, it doesn’t show the bond length. And last, no electron cloud distortion and hybridization are shown. Thank you. T: Okay, just keep it here. No electron cloud distortion. Okay. He gave more information than I did, right? Let me just write down something. It doesn’t show the hybridization. It doesn’t show the polarization also…atoms are not spherical in shape. He did tell us some information, right? (T’s voice was too soft.) Do you remember steric hindrance? There are various atomic sizes. I think it can give you some information about steric hindrance. Okay. Thank you. How about group 2? Group 2? Group 2 S: After discussion, our classmates think that this model is called a ball-and-stick model. It represents the position of the atoms. You can see in the PowerPoint its bond angle, and the composition of the atomic structure. It is a tetrahedral structure. But we don’t know the electronic arrangement, the relative size, and which ball represents which atom, bond strength, electronegativity, bond enthalpy…and the bonds may occupy too much space relative to the other models. Thank you. T: Great! He gave more information than I, right? There are so many things. Okay. So P.16 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science many items, right? It shows you the bond angles. Okay, very good, and then the bonding is not presented accurately. Okay. Then how about group 3? Group 3 S: The model, we’d say it’s a dot-and-cross diagram. The dot-and-cross diagram represents the bonding electrons. You can see how carbon and hydrogen form the methane molecule. Four covalent bonds are formed by sharing of electrons. Even though we can see the electronic arrangement, we cannot see the 3D structure of the methane molecule. We cannot see the relative size of carbon and hydrogen. It seems that the 5 circles are identical. Also, we cannot see the bond angles. The actual structure of it is a tetrahedral structure. Also the bond length and other things cannot be seen. T: Okay. Now you see, it’s really…It shows the electron shell, the sharing of electrons and the bond order. But it cannot tell us about the 3D structure. I hope you can see it. Now, group 4, come here. Group 4 S: Let’s look at model 4. We can see the bond angles of the molecule and the 3D arrangement. This diagram can tell us that each carbon is bonded to four hydrogen. Also we can see whether it has orbital activity. About the drawbacks, we can’t determine the atomic radius. And hybridization cannot be observed. And the variety of bonds cannot be observed. T: Okay. You said so many things. He told us more then what are written in here (T pointed to the PowerPoint.). There is just one drawback here. There are so many drawbacks mentioned by you. It (model 4) shows the 3D structure on a piece of paper. Do you remember how to present the 3D structure? These two and this one are pointing towards us. And this one is pointing away from us. Okay? This is one triangle and this is another triangle. Okay. Now it comes to group 5. Group 5 S: The 5th model is called the structural formula. It shows that the molecule consists of carbon and hydrogen atoms. The model also tells us which kinds of bond it is consisted of. For example, single bond, double bond, etc. Moreover, this model also tells us the number of bonds and the number of atoms consisted in the bond. However, there are some drawbacks. For example, this model cannot tell us the positions of the atoms. Also, the bond angle of the bonding is not shown in this model. And the electronic P.17 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science arrangement cannot be observed. The last one is, the relative atomic size of these atoms are not shown. T: Okay. He told us so many things they observed and a lot of information they got, right? They mentioned about the bond angles, the electronic arrangement. Then how about group 6? Okay, please come out. Group 6 S1: We’re going to talk about the 6th model, that is CH4. This is a condensed structural formula. It can only show us the constituents of this (molecule). We can only see this (constituents of the molecule) from the formula, so it has fewer advantages than other models. S2: There are some drawbacks of the model. The atomic size and bond angles cannot be seen. Also, we can’t see the spatial arrangement and the electronic configuration (of the atoms). T: Okay. So it’s a condensed structural formula. And then this is the molecular formula. But it would have drawbacks. How many atoms are there? It cannot tell us the electronic arrangement, hybridization, and atomic size. Right? Okay. Group 7 will tell us something else. Suppose you combine all these information together, could you have a true picture of a methane molecule? Group 7 S: We think that these models cannot give an actual description of the electron density because the electron density in atom is always dynamic. All these models assume the electron density to be static in a specific atom. Therefore, the bond angle and bond length are fixed. We think that electrons are continuously moving, relative to the neutrons and protons at the centre. The other drawback is that no neutron or protons is shown in the atom. T: Now, thanks. He gives us much information, right? But we know that electrons are concentrated in orbital, right? The positions of the electrons are changing. It does not look like the dot-and-cross diagram which shows electrons in fixed positions. And then, he has talked about nucleus, right? There is no information about the nucleus, the structure of nucleus, the size of the nucleus and the movement of the electrons. This is the model answer. Now we cannot have a complete representation. Even though you P.18 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science combine all these models together you cannot have a true picture. Okay? Right? Another example is ammonia. T: Okay now, what is the conclusion? Now you have multiple models to convey ideas. The scientists have multiple models. Yet they cannot tell us the true picture. Each model tells us some information within a certain scope. Each has some drawbacks and limitations. Okay? They cannot tell us all information. Okay then, in junior form you use spheres to represent water molecules. Do you think it is wrong? Or does it have any function? You can still have this right picture, right? When you are in Form 1, spheres are used to represent water molecules because only the arrangement of water molecules is concerned. What is inside the water molecule is not important. How about your present study? Would we use other models to represent similar ideas? You all have a very good understanding of the shell model and the orbital model of atoms. We have discussed it already. Just like sodium atom, right? It’s represented as “2,8,1” in junior form, and now it’s “1s2 2s2 2p6 3s1”. Okay? Anything else? Okay, redox again. Fai (a student), you have to tell us the definition of redox reactions. How many definitions do you know? S: Decrease in… T: Oxidation first, just oxidation. We have to… S: Increase in oxygen. T: Yes, increase in the number of oxygen atoms. S: Decrease in hydrogen. T: Yes, right. Decrease in the number of hydrogen atoms. It is an oxygen-hydrogen transfer. Just tell us about oxidation. S: Electron… T: Yes, that’s right. And how about oxidation number? S: Increase in oxidation number. P.19 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science T: Increase in oxidation number. Do you think you will use this model to represent redox reaction? Then, how about this concept? This is the Arrhenius model and Bronsted-Lewis model, there should be one more. What would be the other one? Maybe it’s not included in the exercise. Any other definition for acid and base besides these models? S: Lewis acid T: Yes. Then what would be the definition for acid according to Arrhenius model? Either one, just tell us the different models, e.g. Arrhenius model of acid? This is the definition you learnt in junior form. S: Hydrogen ions and hydroxide ions. T: Yes, right! Hydrogen ion represents acid. For the whole definition, acid is what? S: Covalent compounds that dissolve in water and release hydrogen ion. T: Yes, acid can be defined as what? Once it is dissolved in water, it gives off hydrogen ion as the only positive ion. And then bases are those that produce hydroxide ion. Then how about Bronsted-Lewis model? S: Proton donor is an acid. Proton acceptor is a base. T: Yes. We have already mentioned the definition for Lewis model. The electron pair donor is an acid and the electron pair acceptor is a base. Then, how about the equilibrium position for Le Chatelier’s principle and reaction quotient? Maybe you cannot remember the quotient. I think you know the Le Chatelier’s principle. Can any of you tell us the Le Chatelier’s principle and the reaction quotient? Can anybody tell us the Le Chatelier’s principle? S: Whenever there’s a stress, the reaction tends to restore the things to cancel the stress. T: Yes, the equilibrium position will shift so as to reduce the stress, right? If you provide a stress on a system which is at equilibrium, the equilibrium will shift so as to release the stress. (T was writing on the board) And reaction quotient is a…you are comparing K (equilibrium constant) with Q (reaction quotient)… You may compare the values… This P.20 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved. Senior Form Chemistry – The roles of models in understanding science would be the equilibrium. You may just compare the values. If Q is greater than K, what happens? “Greater than” means “too many”. Then, what happens? S: The equilibrium… (S’s voice was too soft) T: Yes, right! Then the equilibrium position would shift to this side. You may use a different model to represent an equilibrium shift. Then, what is the roles of models in science? The first thing you know is a model can be regarded as a representation of an idea, an event, a process or a system. Then it can be changed or modified to serve different purposes. A model is not a direct copy of the reality. Okay, to serve the purpose, people would just selectively highlight the more relevant ide P.21 © 2007 The University of Hong Kong – Faculty of Education. All rights reserved.