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The roles of models in understanding science

Click the links below to access the activities:

- Activity 1

- Activity 2

- Activity 3

- Summary

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Compare the maps…

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Map showing TSTMTR system map

Geographic map of the MTR

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Map showing TSTMTR system map

Geographic map of the MTR

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Map showing TSTMTR system map

Geographic map of the MTR

6Map showing TSTMTR system map

Geographic map of the MTR

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This is a model of the land and sea of Tsim Sha Tsui, highlighting train station exits

MTR map is a model of the routes of the MTR trains.

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What is a model?

Gilbert and Boulter, 1998

A model can be regarded as a representation of an idea, an object, an event, a process or a system.

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A model as a representation of a real object

A model as a representation of a real object

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A model as a representation of a system

A model as a representation of a system

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A model as a representation of a process

A model as a representation of a process

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A model as a representation of events

A model as a representation of events

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A model as a representation of an idea

A model as a representation of an idea

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When people draw maps, do they copy everything they see?When people draw maps, do

they copy everything they see?

If not, what criteria guide their drawing?

15Figure 1 Figure 2

liver

lungheart

artery

vein

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? ? ?

Draw pictures to illustrate the arrangement of water particles in solid, liquid and gas state.

Activity 2:

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solid liquid gas

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What is used to represent each water particle in the model?

Spheres are used to represent water particles

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Information conveyed and drawbacks

In order to understand the three states of matter (e.g. ice, water, steam), we used to use spheres to represent water particles.

In this model, the composition of the sphere is not our concern.

The composition of the sphere is not important and may confuse our understanding

of the arrangement of particles in different states.

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Does anyone not use spheres to represent water molecule/particles?

Why?

Why do we use spheres even though we know that water molecules are not spherical?

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What do you value about the model that you learned in junior forms?

highlight the more relevant areas selectively

neglect some less important aspects

In order to better serve its purpose, when people develop a model, it is most common that they will:

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Methane molecules

Activity 3:

CH4

2.

1. 3.

4.

5.

6.

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CH4

In a group of 3-4,

(i) Name the type of model we used to represent methane

(ii) Describe the information that each model conveys

(iii) Describe the drawbacks of each model

2.

1. 3.

4.

5.

6.

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Name of the model Information conveyed Drawbacks

Space filling

• The relative size of different atoms (it shows covalent radii).

• Particularly useful when considering steric hindrance in SN reacti

on.

•Atoms or molecules are not hard spheres.

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Name of the model Information conveyed Drawbacks

Ball-and-stick

• Good three-dimensional representation.

• Bond angles can be shown more clearly.

• Sometimes, the balls are of the same size.

• The bond length and bond order (single, double or triple bond) may not be represented accurately.

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Name of the model Information conveyed Drawbacks

Dot-and-cross diagram

• Number of outermost shell electrons involved in chemical bonding.

• Bond order.

• Bond angle and atomic size cannot be shown clearly.

• Inner shell electrons are not shown.

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Name of the model

Information conveyed Drawbacks

3D diagram

• The spatial arrangement of atoms can be shown in 2D. (without the trouble of building space-filling or ball-and-stick models)

• Atomic sizes are not to scale

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Name of the model

Information conveyed Drawbacks

Structural formula

• Bond order (single, double or triple bond).

• the structure of a molecule (i.e. which atoms are bonded to which).

• Bond angles and electronic arrangements cannot be shown clearly.

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Name of the model Information conveyed. Drawbacks

Chemical formula

• The numbers and types of atoms in a molecule.

• A very convenient shorthand to represent a molecule universally, particularly when the molecule is made of many types of atoms.

• It cannot show the actual arrangement of atoms in a molecule.

CH4

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Can we have a true picture of a methane molecule

when we put all the information from these

models together?

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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 in these previous models.

Even seeing all of these models collectively, we still CANNOT construct a complete representation of a real methane molecule.

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ConclusionChemists or scientists use multiple types of

representation or models to convey their ideas.

Each type of model highlights a particular set of information while neglecting other information.

None of these can truly and completely represent molecules.

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Therefore, it is very important to be aware of the scope and limitation of

each model.

A deeper understanding can result from viewing multiple representations or types of

models.

Conclusion

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Conclusion

It may not be appropriate to judge a model that you learn in an early stage of study (e.g.

particles as spheres) to be ‘wrong’.

While these models suffer from their explanatory power, they have the advantage of simplicity.

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Other examples of models in chemistry

The shell and orbital models of atoms Redox reaction

(in terms of oxygen/hydrogen transfer, electron gain/loss & oxidation number)

Arrhenius and Bronsted-Lewis models of acid/base

Prediction of the shift in equilibrium position in terms of Le Chatelier’s principle and reaction quotient.

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Other examples of models

9V

PV = nRTPV = nRT

Ideal Gas Law

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What have you learnt about the use of models in science?What have you learnt about

the use of models in science?

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What are the roles of models in science?

A model can be regarded as a representation of an idea, an event, a process or a system.

A model is created to test ideas.

It will change or be modified to inform or assist the development of those ideas.

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When scientists develop a model, it is most common that they will highlight the more

relevant areas selectively while neglecting some less important aspects.

A model helps us to represent ideas within a defined scope.

What are the roles of models in science?

A model is NOT a direct copy of reality.