Trainee Science Teachers’ Conceptions of Chemical Stability
Trainee Science Teachers’ Conceptions of Chemical Stability
Keith S. Taber
Homerton College, University of Cambridge
Royal Society of Chemistry
presented to the symposium
Issues in Science Teacher Education
at the
British Educational Research Association
Annual Conference 2000
University of Cardiff
Saturday, September 9th 2000
Trainee Science Teachers’ Conceptions of Chemical Stability
Keith S. Taber © 2000
presented to the science SIG symposium
Issues in Science Teacher Education
at the
British Educational Research Association
Annual Conference, Cardiff Univesity, September 7-10 2000
Dr. Keith Taber
Senior Lecturer in Science Education
Homerton College, University of Cambridge
Royal Society of Chemistry Teacher Fellow 2000-2001
c/o
Education Dept.
Royal Society of Chemistry
Burlington House
Piccadilly
London W1V 0BN
e-mail:
Abstract
School pupils and college students have been found to commonly use an alternative conceptual framework known as the 'octet framework' to explain chemical phenomena. Many alternative conceptions are considered to derive from children's intuitive theories about the world. However, learners have no direct knowledge of the interactions between chemical species at the molecular level, and it has been claimed that the octet framework derives - at least in part - from the way in which chemistry is presented and taught in schools. It is currently an open question whether some science teachers themselves hold the alternative views, and thus explicitly teach from this viewpoint.
The present study used a simple paper-and-pen instrument to investigate the extent to which there was evidence of trainee science teachers thinking in terms of the octet framework rather than from valid scientific principles. The instrument, previously piloted with A level students, elicits respondents' notions of chemical stability. Evidence was found to suggest that a considerable proportion of a group of trainee science teachers used 'octet thinking'.
In this paper the alternative conceptual framework will be briefly reviewed, and the instrument used described. Results from the trainee teachers will be presented, and the significance of the findings will be explained. The implications of these results for the training of science teachers will be emphasised.
Key words: constructivism, teacher training, learning chemistry
Trainee Science Teachers’ Conceptions of Chemical Stability
1. Introduction: Constructivism and alternative conceptions.
2. An alternative conceptual framework from chemistry education.
3. The origins of the ‘octet’ framework.
4. Notions of chemical stability.
5. The chemical stability probe.
6. Trainee teachers’ responses.
7. The significance of the findings.
1. Introduction: Constructivism and alternative conceptions.
Constructivism is well accepted now as both a mainstream research programme in science education, and as a referent for teaching.
Very briefly, the basic idea is that each individual learner uses information from their environment - including, but by no means only including, teaching - as the raw material to construct their own representations of the world. Although in a classroom context the teacher’s input is significant, the main criterion of what is learnt is what can be understood, or made to make sense; and this means that the main determinant of new learning is the existing structure of knowledge available to a learner (i.e., their cognitive structure).
The role of the teacher is to help support, or scaffold, the learners re-construction of knowledge to give representations that will lead to desired future behaviours: for example we want our pupils’ understandings to match the teacher’s understandings well enough for them to produce the examination answers that are judged ‘correct’!
In fact when pupils and students are asked examination-type questions they often produce answers that are not considered correct. Sometimes this is due to a lack of ability to clearly express their thoughts, or even to comprehend what the question was meant to elicit. In these cases, perhaps, the learners does have a ‘desired’ sort of understanding of the concepts concerned - but the failure to ‘perform’ is due to other ‘deficits’.
On other occasions the pupil concerned does not know the answer, or is confused about the topic, for whatever reasons. In a test, or when put-on-the-spot, some pupils may guess - or produce an answer apparently at random - whilst others will simply report that they do not know.
These are all situations that teachers often meet. But in recent decades a lot of attention has turned to another scenario, when the pupil thinks they do understand the concept and know the answer, but their thinking does not match the acceptable scientific version of knowledge.
In these cases we often refer to the pupil holding alternative conceptions or alternative frameworks for the concept area[1], as the pupils think they do understand - and they are often very successful in making sense (although not always the intended sense) of new information from their alternative perspective. This presents the teacher with a different pedagogical challenge to a pupil who agrees they do not know about a topic, or realises they are confused. Pupils can often develop highly complex alternative frameworks that are internally self-consistent and able to be widely applied: they just do not match the accepted scientific views of the world! These alternative conceptions are often also stable and resilient in the face of new information: pupils can be quite ingenious in reconstructing new knowledge into their existing frameworks!
Before a teacher is likely to be successful in teaching pupils with alternative conceptions the accepted school science models it is important that:
•the teacher is aware of the significance of alternative conceptions;
•the teacher is able to elicit the pupil’s own way of understanding the topic;
• the pupil is made explicitly aware of their own conceptual frameworks.
Once this is achieved the teacher can present exercises and activities designed to persuade the pupil of the greater verisimilitude of the accepted scientific models over the pupils’ alternatives. This may not be an easy task, but at least the teacher is made aware of the nature of the problem and is able to tackle it accordingly.
2. An alternative conceptual framework from chemistry education.
There is a vast literature on children’s ideas in science[2], and although chemistry is somewhat under-represented, many alternative conceptions have been found[3]. Some of these are quite ‘contained’ or discrete. Consider the following example:
research shows that many pupils\students believe that a salt produced by neutralisation must be neutral.[4]
Leaving aside our sympathy for pupils who have somehow got the strange idea -
that: because materials that are neither acidic or basic are called neutral, then the process of neutralisation, by which an acid neutralises a base, will produce something neutral[5]
- this is quite a contained idea that effects just one part of their chemistry.
However, other ideas are more extensive and expansive, and deserve to be called alternative frameworks[6]. One that I feel is particularly significant, that seems to be commonly adopted by pupils and students, is called the ‘octet framework’[7].
Before I outline the framework, I should point out that as all learners are individuals, with somewhat idiosyncratic ideas, any talk of a ‘common alternative framework’ could be considered something of an oxymoron. Research that looks at pupils’ alternative frameworks has to be in-depth detailed investigation, which produces something (the researchers’ representation of the learners’ representations!) that is unique for each learner.
However, there are sometimes striking similarities between the ideas of different learners. (This is to be encouraged when those ideas closely reflect what the teacher is trying to teach{!}, but are often also found with alternative conceptions.) In presenting a common alternative framework one has two basic choices: to discuss a particular learner’s ideas, but point out that these were reflected in other cases; or to produce a type of amalgam or aggregate representation.
The ‘octet framework’ presented here (see appendix A) is of the latter form[8], and was developed through a grounded theory approach, starting with detailed individual case studies, and gradually building a more general model[9]. This, then, is a model of a conceptual structure that represents no one individual learner in all its details, but is matched to varying degrees by many chemistry students. Most of the research leading to this model was undertaken with A level students, but (those parts that are relevant) are also reflected in data from KS4 pupils as well.
The basis of octet thinking is the ‘full shells explanatory principle’:
Atoms form bonds in order to achieve stable electronic configurations - variously referred to as octets, full outer shells or noble gas (electronic) configurations/structures.
This principle is the basis of a complex of related notions[10], such as,
•One way atoms can obtain full outer shells is to donate (give away) electrons (but they can only do this if another atom accepts them).
•One way atoms can obtain full outer shells is to accept (take) electrons form another atom.
•An ionic bond is (/is formed by) the transfer of electrons.
•If atoms overlap their outer shells then electrons in the overlap count towards the outer shells of both.
•An atom can therefore obtain an ‘octet’ by sharing electrons with another atom.
•A covalent bond is a pair of electrons shared between atoms.
A number of logically related features were identified in the research as being associated with ‘octet thinking’[11]:
•an atomic ontology
•use of anthropomorphic language
•significance given to electronic history
•electrovalency as the determinant of the number of ionic bonds formed.
•a dichotomous classification of bonding.
•demarcation between bonds, and ‘just forces’.
Although the research which uncovered this way of thinking had focused on the concept of chemical bonding, the octet framework is more widely significant. For example, it effects students’ thinking about patterns of ionisation energy, so that some students think that electrons can only be stripped from an atom until it has a ‘full shell’. Perhaps most significantly, it is the basis for explanations about why chemical reactions occur.
It has been found, for example, that when chemistry students are asked why hydrogen reacts with fluorine, they will commonly suggest it is because through the reaction the fluorine and hydrogen atoms are able to get full shells:
“Fluorine is a halogen and has 7 outer electrons. To be stable it would like 8 electrons in its outer shell. By covalently bonding with the hydrogen atom which would like 2 electrons in its outer shell they form hydrogen fluoride which is stable”
This response was provided by an A level student from a school sixth form, despite having been given the equation:
H2(g) + F2(g)2HF(g)
and a diagram (figure 1) showing the molecules of reactants.
Figure 1: why do H2 and F2 react?
And this response was not unusual: most of the students in that sixth form chemistry group gave similar responses!
Given that:
1. A level chemistry students commonly explain chemical reactions in terms of arguments about atoms getting full shells, even when the question context shows the atoms already have these electronic structures;
2. chemical bonding and why reactions occur are core concerns of any study of chemistry;
This alternative conceptual framework is clearly very significant to the teaching of the subject.
3. The origins of the ‘octet’ framework.
The octet framework then is potentially a major impediment to the learning of accepted ideas about the subject. Research suggests that this type of ‘octet thinking’ is not only widespread, but tends to be tenaciously held onto despite formal teaching,
It is therefore of some interest to know where the idea originates. Some common alternative conceptual frameworks are believed to arise from early life experience. For example, it is found that children (and adults) commonly have a naive theory of motion along the lines of the pre-Newtonian impetus theories: i.e., when an object is given a push, the push will get it so far before it runs out. This ‘intuitive physics’ is considered to largely derive from early experiences of pushing objects, which then only get so far before they stop. Clearly, in the absence of any knowledge of frictional forces, this is not an unreasonable deduction!
The alternative neutralisation conception referred to earlier, that all products of neutralisation will be neutral, will not have derived from early childhood experience of playing with acids and alkalis! Its origin is, presumably, due to over-interpretation of linguistic clues when the topic is first studied in school. If something that is neither acidic nor basic is called ‘neutral’, and if in a ‘neutralisation’ the acid ‘neutralises’ the base, and vice versa, then it is not unreasonable to expect a neutral product! Indeed this is a very sensible deduction in the absence of a clear understanding of the difference between the strength of an acid/alkali and its concentration.
As neutralisation is commonly taught in school chemistry, but acid strength not discussed until sixth form level, this alternative conception is surely very likely to be adopted unless the original presentation of material on neutralisation is deliberately planned to avoid this.
There is nothing the science teacher can do to stop children coming to class with an impetus framework for forces and movement, but the adoption of the alternative neutralisation conception is largely a result of teaching (of the way in which we order the presentation of topics). I have distinguished these two types of case by referring to them as ontological and pedagogic (or epistemological) learning impediments[12].
As with neutralisation, bonding is a topic which pupils do not (usually) think about until they are taught science formally, so the octet framework must be considered to be a pedagogic learning impediment. This is helpful, as it means it may be avoidedif we plan our teaching differently!
One interesting aspect of the octet framework is that many explanations of chemical bonding given in school text books easily lend themselves to fitting the alternative framework. Indeed, some seem to be explicitly using it! For example, many diagrams supposedly showing bond formation start from isolated atoms, although there are very few chemical reactions where this would be the case. I have suggested some possible reasons for this[13]:
“Three possibilities are:
1:The diagrams are not meant to represent chemical processes of our world, but the primeval formation of molecular matter in some previous cosmological epoch.
2:Diagrams of this form are used because this is the way the authors were taught, and it has not occurred to them that they are misleading.
3:The authors are aware of the inaccuracy of the diagrams, but chose to use them because they are consistent with the (invalid) explanation of chemical processes in terms of achieving full shells.”
I described the first possibility as “rather obscure”, and suggested that “the third possibility would seem to suggest a somewhat cynical attitude on the part of authors who are aware they are presenting misleading information, but chose to develop the deceit rather than find a more intellectually valid approach.”
However the second option was perhaps more worrying - that those who were meant to be in the know also held the alternative conceptions. And if this was possible with textbook authors, who are often teachers, it could also be the case with other classroom practitioners.
Yet, one would imagine, by the time someone has made the transition from pupil through science graduate to teacher, they will have adopted the accepted scientific models? There is some evidence to suggest that this may not be the case.
For example, table 1 gives a comparison of the way the ionic bond is understood in orthodox science, compared with the ‘molecular’ interpretation which is common used when students apply the octet framework[14]:
framework / molecular framework / electrostatic frameworkstatus / alternative framework / curricular science
role of molecules / ion-pairs are implied to act as molecules of an ionic substance / ionic structures do not contain molecules - there are no discrete ion-pairs in the lattice
focus / the electron transfer event through which ions may be formed / the force between adjacent oppositely charged ions in the lattice
valency conjecture / atomic electronic configuration determines the number of ionic bonds formed. (e.g.: a sodium atom can only donate one electron, so it can only form an ionic bond to one chlorine atom.) / the number of bonds formed depends on the co-ordination number, not the valency or ionic charge (e.g.: the co-ordination is 6:6 in NaCl)
history conjecture / bonds are only formed between atoms that donate / accept electrons. (e.g.: in sodium chloride a chloride ion is bonded to the specific sodium ion that donated an electron to that particular anion, and vice versa.) / electrostatic forces depend on charge magnitudes and separations, not prior configurations of the system (e.g.: in sodium chloride a chloride ion is bonded to six neighbouring sodium ions)
‘just forces’ conjecture / ions interact with the counter ions around them, but for those not ionically bonded these interactions are just forces. (e.g.: in sodium chloride, a chloride ion is bonded to one sodium ion, and attracted to a further five sodium ions, but just by forces - not bonds.) / a chemical bond is just the result of electrostatic forces - ionic bonds are nothing more than this (e.g. the forces between a chloride ion and each of the neighbouring sodium ions are equal.)
Table 1: two ways of conceptualising the ionic bond
Oversby asked a small group of trainee chemistry teachers about this ‘molecular framework’ for ionic bonding. A majority (6/11) of the group believed that this alternative framework was “an adequate representation of ionic bonding”[15].
4. Notions of chemical stability.
It is therefore sensible to explore the chemical understandings of teachers and teachers-in-training to find out if they hold, and would therefore explicitly teach, alternative conceptions of chemistry. In this paper an example of such enquiry is given: a consideration of student teachers’ responses to a probe about chemical stability.