Modelling

Modelling in chemistry
John Oversby: University of Reading A significant feature of the discipline of chemistry is that chemical explanations are based on entities existing at the nanometre level and below. Chemical explanation is also based fundamentally on electron rearrangements in ions, atoms and molecules, and these are not generally amenable to direct observation, even with instruments. Modelling of a wide variety is extensively employed in chemistry to make sense of the processes, which take place, and of the features of these entities, which affect the likelihood of chemical change. // 2. //**// Existing school chemical curricula teach chemical models as content to be learned //** // Science is based on "experimental evidence and models" to evaluate 'phenomena and events". // // 1. Pupils should be taught: // // a) that atoms consist of nuclei and electrons // // b) about a model of the way electrons are arranged in atoms // // c) how the reactions of elements depend upon the arrangement of electrons in their atoms // // j) how the rates of many reactions depend on the frequency or energy of collisions between particles // The quotes above are drawn from the consultation materials for the revised National Curriculum (Science) in England (DfEE, 1999), The revision, while maintaining in one section that learners should be taught that chemists use models, treats chemical models as static. They are also seen as equivalent to content in the chemical content section of the curriculum. The emphasis in the text on the phrase // 'accepted models' // indicates this approach. // 3. //**// Existing school chemical curricula omit the process of modelling //** // ... theories do not come automatically from evidence collected, but may involve creative thought or take the form of models // Implicit in this quote from the revised National Curriculum referred to above is that models simply exist, that they are there to be taught. Other sections from this document over‑emphasise the need to teach the provisional nature of models, by focussing mainly on those which have changed, and neglecting to mention that most models are durable. While not changing the character of models as provisional, it is important to recognise that many chemical models have a long lifetime in an atmosphere of rigorous testing and use. Although it is hypothetically possible to imagine that a non‑particulate model of chemical change could prove useful, in practice anyone who seriously criticised the general idea that chemical change involved particle entities would be gently led to one side and his or her sanity questioned. The quote also demonstrates an amazing view that models and creative thought are not related! In examination questions about bonding, especially ionic bonding, or about electrolysis, these are either aimed at testing whether students can use accepted models in a familiar context, or in a slightly modified context. The focus is on application of familiar knowledge or on rehearsal in the examination. I have not seen any question at secondary school level, which refers to the creation of the model itself. It may be felt that this is beyond the intellectual level of the learners at this stage but it seems not to be taught in higher education either. Clearly more careful work needs to be done to verify the generality of this claim but assessment of graduates on initial teacher education courses in England provides some corroborating evidence. Oversby (1998a) has investigated this for pre‑service primary student teachers and found this to be the case. Oversby (1998) has made a case for the prime intellectual activity of chemical researchers being modelling. If this is accepted, then chemical modelling is authentic. The evidence for it being intrinsically satisfying is provided by the vast number of research articles based on chemical modelling which are published in the chemical literature. It is also intrinsically satisfying for some novices too (Oversby 1998b) A case for this is made above. As with any new idea, some clarification of its meaning and extent is called for. The following is an attempt to put forward some of the major components and boundaries. Gilbert ( 1993) made a well‑accepted delineation of the term model as a form of representation. The range of representation is broad and inclusive. The examples provided are not simply representative but include the major classes. 2. **// Recognition of characteristics of good models //** (based on Oversby, 1998b) In this paper only a very brief indication of the issues is attempted. a) Representational features ‑ points of correspondence and non‑correspondence. Models do not exactly correspond to the original, otherwise it would be the original. Explicit recognition of those aspects that correspond to the original is not always recognised. There will be many aspects that do not correspond but here is meant those aspects that might give rise to confusion or misunderstandings. b) Analogical mapping ‑ drawing on features expected to be in common between the model and the original. c) Role in explanations ‑ models are a common and essential component of chemical explanations d) Human creation ‑ models are the product of creativity, of synthesis and are often aesthetic. e) Types of models ‑ avoiding the tendency to think only of computer‑based molecular modelling and ignoring non‑traditional forms such as role‑play and poetry. In chemistry, models include word equations, many drawn forms and prose descriptions. f) Progression in modelling ‑ based on both cognitive psychology thinking (eg progression from concrete to abstract) and natural thinking in chemistry about comparing qualitative and quantitative approaches. g) Coherence with related models ‑ idiosyncratic models tend to be shunned in chemistry. h) Clear and systematic failure in explanations ‑ for example, the ideal gas law fails not in a random way but in a manner which promotes modifications which possess rationality. The models created to modify the ideal gas law indicate significant features of the entities, such as their finite size and the existence of attractive forces. i) Fruitfulness in exploring data from phenomena ‑ the models for dissolution of ionic solutes have promoted the use of data such as measurement of energy changes, entropy changes, and dielectric constants in order to develop the hydration model further. j) Predictive power in novel situations ‑ the development of generalised kinetic models of organic substitution reactions has lead to greater confidence in predicting products from chemical reactions. k) Simplicity ‑ the simple ligand model for inorganic complexes has extended the range of data that can be explained using this model. l) Quantitative if possible ‑ ‑ the transition state model of chemical kinetics has proved useful in explaining why the relation between absolute temperature and rate of chemical reaction is exponential. Such an approach leads to stronger tests of underlying models. 3. **// Contexts for demonstrating modelling capability //** a) Use of existing models in familiar contexts b) Use of existing models in novel contexts c) Creation and use of new models in familiar contexts d) Creation and use of new models in novel contexts These are self‑explanatory but as yet there is no understanding of whether learners find it easier to appreciate modelling if working within their personally created models, or if working with the accepted models of scientists.
 * Rationale for developing modelling capability **
 * // 1. //****// Most chemical explanations of macroscopic phenomena are based on submicroscopic models //**
 * // 4. //****// Existing assessment methods tend to ask for rehearsal of the taught use of models in explanations, or in slightly modified novel situations //**
 * // 5. //****// Chemical modelling, as a process, can promote commitment to and confidence in chemistry where other methods have failed //**
 * 6. ****// Chemical modelling is an authentic chemical activity, and is intrinsically satisfying //**
 * 7. ****// Modelling is an intrinsic element in some National Curricula (eg //****// England //****// ) //**
 * Chemical modelling capability **
 * // 1. //****// Recognition of models ‑ representations of ideas, processes, events, systems, objects //**