Some possible models and analogies for teaching about elements and compounds. Additional guidance

Transcription

1 Some possible models and analogies for teaching about elements and compounds Remind participants that using ideas of particles to clarify the differences between elements, compounds and mixtures is an essential part of the Year 8 yearly teaching objectives. Slide 3.3 Show slide 3.3 and ask which image in this slide represents an element and which a mixture? Elements or mixtures? Slide 3.3 A B C T Slide 3.4 Additional guidance Images A and B both represent elements. Image C represents a mixture. Now show slide 3.4 and ask participants why this represents a compound rather than a mixture. Take comments and allow brief discussion for a few minutes about the strengths and limitations of this model. Compound or mixture? Slide 3.4

2 67 Strengthening teaching and learning of particles in Key Stage 3 science Session 3 T Additional guidance Both compounds and mixtures contain more than one element (in this case represented by mint and toffee), but there are two significant differences between compounds and mixtures. In a compound the elements are combined and cannot be separated by physical methods. The constituent parts of a mixture can be separated more easily. (In this case the toffees and mints can be picked out.) In a compound the elements are combined in a fixed ratio. In a mixture the constituents can be mixed in any proportions. If you can manage to obtain examples of mints, toffees and mint toffees, use these to augment your presentation and engage interest. andout 3.5 If you require a further model to explain the meaning of the term element, use handout 3.5 to show the paper clip analogy. owever, do not spend too much time on it. andout 3.5 A paper clip model of an element Paper clip models Iron metal Element iron nail a pile of 20 paper clips 10 paper clips 10 paper clips iron filings individual paper clips iron powder heap single paper clip single iron atom Break into fragments paper clip fragments are not a paper clip atomic fragments are not iron Source: Adapted from Teaching chemistry to KS4, by Elaine Wilson, odder and Stoughton, 1999, ISBN Slide 3.6 Task F The task is summarised on slide minutes Task F Examining some physical particle models for Slide 3.6 elements and compounds Try out one type of model. Decide on the strengths and limitations. Write these on sticky notes. Stick your notes on the poster. andout 3.6 rganise participants into groups of about six. Ask them to retrieve handout 3.6. Arrange three posters for participants sticky note responses. There is one poster for each model and each poster requires two columns: one for strengths and one for limitations. 57 Strengthening teaching and learning of particles in Key Stage 3 science Session 3

3 68 Strengthening teaching and learning of particles in Key Stage 3 science Session 3 andout 3.6 Examining some particle models 1 of 2 for elements and compounds You will have the materials needed in order to try three different models. molecular models plastic bricks cards Use each of these systems to create models to represent as many of these compounds as you have time for: aluminium chloride (AlCl 3) aluminium oxide (Al 2 3) aluminium nitride (AlN) chloride (Cl 2) oxide () nitride ( 3N 2) chloride (Cl) water ( 2) ammonia (N 3) (hydrochloric acid) methane (C 4) carbon dioxide (C 2) tetrachloromethane (CCl 4) As you try out any of the models identify any strengths or limitations you see in their use for pupils learning. Write these on a sticky note and add it to the appropriate space on the communal displays. Molecular models You are provided with the following model atoms: carbon (black, 4 holes) (silver, 2 holes) aluminium (silver, 3 holes) (green, 1 hole) nitrogen (blue, 3 holes) (white, 1 hole) (red, 2 holes) You link the models together using the grey plastic links, which represent bonds. Plastic bricks Use these bricks to represent the following atoms: black for carbon yellow for aluminium, blue for nitrogen grey for, red for tan for, brown for Link the bricks together like this to represent molecules or formulae: Cards Each card represents the atom of an element. The name of the element is printed on each card. Fit the cards together to represent the compounds like this: This handout also includes a photocopy original if you wish to make similar cards for use at school. A card game, Atom@tak, has previously been provided free to secondary schools by the Royal Society of Chemistry and could be used to extend this model. Give out to each group one set of plastic bricks; molecular models; task F cards. Say that: The purpose of this task is to become more familiar with three types of physical model. These will be used again, in greater depth, in session 4. Participants are to use the models to represent the compounds shown on handout 3.6. Participants should quickly evaluate some of the strengths and limitations of each model and write these on sticky notes. The sticky notes are to be added to the communal display (posters) for each of the three models. There are about 10 to 15 minutes available for this activity. andout 3.15 When sufficient time has been spent on the activity, distribute handout 3.15 (not in participants packs) which contains an analysis of the strengths and limitations of the four models examined so far this session. Review the three communal displays for additional strengths and limitations not included in this handout so that participants can add them to their own copy if they wish. Analysis of four models for teaching about elements and compounds andout Strengthening teaching and learning of particles in Key Stage 3 science Session 3 LIMITATINS STRENGTS Toffees and mints Molecular models Plastic bricks Element cards It helps to suggest that the Shows how atoms can be Relatively familiar materials There is some indication of two elements are combined in arranged in specific ways which atoms might react elps show that elements the compound together Reinforces the consensus combine in simple ratios Provides a good visual display model of atoms as spherical elps show that elements The brick is obviously a model of a mixture combine in simple ratios elps show that elements for an atom Novelty and interest value combine in simple ratios The card is obviously a model Simple to build up a giant for an atom Relatively familiar materials structure as well as molecules Cheap Doesn t convey the size of Doesn t convey the size of Doesn t convey the size of Doesn t convey the size of atoms atoms atoms atoms Doesn t show atoms are Doesn t show which Doesn t show how atoms are Doesn t show how atoms are joined elements will react joined joined Doesn t show which elements Pupils can confuse the Doesn t show which elements will react models with real atoms will react Pupils are tempted to eat the particles 58 Strengthening teaching and learning of particles in Key Stage 3 science Session 3

4 Allow a few moments for any participants comments about these models. Say that it is important for pupils to realise that these are only physical models and that our mental pictures of particles are also models. They need to realise that these models have limits and weaknesses. Ask for participants suggestions about how we can help pupils to understand the limitations of these models. T Additional guidance andout 3.15 is not included in participants packs. This will prevent the activity in this task from being pre-empted. You will need to have sufficient photocopies for each participant. Whilst these three models can be used to represent atoms of elements and molecules of many compounds, not all compounds are composed of molecules. Nearly all compounds of a non-metal with a non-metal are composed of discrete molecules, e.g. carbon dioxide, but Si 2 in sand is an exception. Many compounds of metals with non-metals do not form discrete molecules but form a giant structure of millions of particles. Examples are oxide, aluminium oxide and chloride. Therefore do not talk about molecules of these compounds. owever, it is still possible to use physical models to represent the ratios in which the elements combine in these compounds. (Aluminium chloride is an exception and exists as molecules, often paired into an Al 2 Cl 6 structure.) At this level there is no need to introduce pupils to ions, valency or giant structures. Close this task by saying that some of these ideas and models will be helpful in the next task and that these models will be revisited in session 4. Using appropriate teaching and models to address misconceptions Task G 25 minutes 15 minutes Slide 3.7 Task G is summarised on slide 3.7. Task G Using appropriate teaching and models to Slide 3.7 address misconceptions Find handout 3.7. Work on one misconception. Decide how you would overcome it. Briefly record your teaching sequence. Say that: There are a number of misconceptions that pupils may have about particles. Research suggests that several of these misconceptions are quite common. 59 Strengthening teaching and learning of particles in Key Stage 3 science Session 3

5 70 Strengthening teaching and learning of particles in Key Stage 3 science Session 3 andout 3.7 Ask participants to retrieve handout 3.7, which sets out a number of these possible misconceptions. Quickly run through the instructions on the first page of the handout. Ask participants to work in pairs. There are 15 minutes available for this task. andout 3.7 Task G Using appropriate 1 of 2 teaching and models to address misconceptions ere is a list of some frequent misconceptions about particles held by pupils at about this age. 1 When a solid dissolves in water it is no longer there; its particles have disappeared. 2 Particles exhibit the macro properties of the material. For example, the particles expand when a material is heated; a solid melts when heated because the particles melt; copper metal is ductile because copper atoms are ductile. 3 The particles are destroyed when a substance is burned, so it loses mass. 4 Compounds are mixtures. The elements can be mixed in any proportions. The name isn t systematically related to the constituent elements. 5 Reactions where gases are formed result in a loss of mass. 6 When two elements react together the atoms from the reactants are transmuted into new atoms (of the products). The reaction is a magical change. Select one of these misconceptions to work on. Decide how you would plan to overcome it in your teaching. Explain the teaching sequence, the practical work and/or the model(s) you might use. Complete your notes on page 2 of this handout. Treatment of one misconception is illustrated below. When a solid dissolves in water it is no longer there; its particles have disappeared. Weigh samples of salt (sodium chloride) and water prior to mixing. Then mix the two together to dissolve the salt. Ask the question what evidence do we need to show whether or not the salt has disappeared? to provoke thought about reweighing the solution. Weigh the solution to show that the mass of the salt has not been lost. Under hygienic conditions show that the taste of the salt is still there in the solution. Allow a group of pupils to evaporate a sample of the solution to demonstrate that there is still salt there. Demonstrate the bright yellow colour produced by adding a few grains of salt to a Bunsen burner flame. Then show the flame colour produced by the salt solution, indicating the presence of the salt. Use small beads (or rice grains) to represent the particles in salt. Add these to a large measuring cylinder of larger beads which represent water particles. The way the salt particles are able to fit between the water particles is one possible explanation for, although not evidence for, the way salt appears to vanish when dissolved in water. Circulate around the groups as they work. Use this time both to offer support and to look for any particular suggestions to pick out in a brief report back from groups. T Additional guidance If most of the participants are hesitant in their understanding of particles, focus most on misconceptions 3, 4 and 5 on the handout, for which it is easier to suggest teaching sequences. You may wish to nominate a particular misconception for each group, to ensure that all are tackled. You may wish to demonstrate the suggested model (with beads or spheres of two different sizes) that illustrates one possible mechanism for dissolving.

6 A paper clip model of an element andout 3.5 Paper clip models Element Iron metal iron nail a pile of 20 paper clips 10 paper clips 10 paper clips iron filings individual paper clips iron powder heap single paper clip Fe single iron atom Break into fragments paper clip fragments are not a paper clip atomic fragments are not iron Source: Adapted from Teaching chemistry to KS4, by Elaine Wilson, odder and Stoughton, 1999, ISBN Strengthening teaching and learning of particles in Key Stage 3 science Session 3

7 andout 3.6 Examining some particle models 1 of 2 for elements and compounds You will have the materials needed in order to try three different models. molecular models plastic bricks cards Use each of these systems to create models to represent as many of these compounds as you have time for: aluminium chloride (AlCl 3 ) aluminium oxide (Al 2 3 ) aluminium nitride (AlN) chloride (Cl 2 ) oxide () nitride ( 3 N 2 ) chloride (Cl) water ( 2 ) ammonia (N 3 ) (hydrochloric acid) methane (C 4 ) carbon dioxide (C 2 ) tetrachloromethane (CCl 4 ) As you try out any of the models identify any strengths or limitations you see in their use for pupils learning. Write these on a sticky note and add it to the appropriate space on the communal displays. Molecular models You are provided with the following model atoms: carbon (black, 4 holes) (silver, 2 holes) aluminium (silver, 3 holes) (green, 1 hole) nitrogen (blue, 3 holes) (white, 1 hole) (red, 2 holes) You link the models together using the grey plastic links, which represent bonds. Plastic bricks Use these bricks to represent the following atoms: black for carbon yellow for aluminium, blue for nitrogen grey for, red for tan for, brown for Link the bricks together like this to represent molecules or formulae: Cards Each card represents the atom of an element. The name of the element is printed on each card. Fit the cards together to represent the compounds like this: This handout also includes a photocopy original if you wish to make similar cards for use at school. A card game, Atom@tak, has previously been provided free to secondary schools by the Royal Society of Chemistry and could be used to extend this model. 68 Strengthening teaching and learning of particles in Key Stage 3 science Session 3

8 andout 3.6 Template for preparing more 2 of 2 element cards carbon C carbon C carbon C carbon C nitrogen N nitrogen N aluminium Al aluminium Al nitrogen N nitrogen N aluminium Al aluminium Al 69 Strengthening teaching and learning of particles in Key Stage 3 science Session 3