alternative conceptions of chemical bonding

Research in Science & Technological Education, Vol. 19, No. 2, 2001

Alternative Conceptions of ChemicalBonding Held by Upper Secondaryand Tertiary StudentsRICHARD K. COLL, School of Science & Technology, The University of Waikato,New Zealand

NEIL TAYLOR, School of Education, The University of Leicester, UK

ABSTRACT Examination of senior secondary and tertiary level chemistry students’ descriptions of their mentalmodels for chemical bonding revealed prevalent alternative conceptions. In addition to some common alternativeconceptions previously reported in the literature, such as misunderstandings about intermolecular forces andmolecularity of continuous lattices, the inquiry found a surprising number of alternative conceptions about simple ideaslike ion size and shape. Some 20 alternative conceptions were revealed, the most common being belief that continuousionic or metallic lattices were molecular in nature, and confusion over ionic size and charge. It is posited that themass of curriculum material students encounter during their undergraduate and postgraduate studies may have somein�uence on the formation of alternative conceptions. Hence, it is recommended that tertiary level teachers in particularconsider the advisability of limiting the teaching of some abstract models for chemical bonding until an advanced stageof the undergraduate degree.

Introduction

There has been something of a revolution in science education since the 1960s. Changesin the way science education researchers, philosophers and others see the world hasresulted in a major rethinking of how science should be taught. Prior to the 1970s,science teaching was dominated by a transmissive approach. Implicit in this approachwas a view that the learning of science was passive and that knowledge could be ‘piped’from the full container of the teacher’s head to the empty vessel of the student’s head(Tobin et al., 1990). Little account was given to students’ alternative conceptions ofscience, which were considered to be easily extinguished or replaced by the teacherthrough persuasive argument. This resulted, in general, in a very didactic approach tothe teaching and learning of science.

However, in the 1970s new cognitive theories began to emerge which challenged thispassive view of learning. An emphasis was placed upon the student as an activeindividual reaching out to make sense of events and constructing knowledge throughsocial interaction and experiences with the physical environment (Driver & Easley, 1978).These new theories acknowledged that, contrary to the view that students have blankminds, they bring to their school learning in science ideas, expectations and beliefsconcerning natural phenomena which they have developed to make sense of their own

ISSN 0263-5143 print; 1470-1138 online/01/010171-21 Ó 2001 Taylor & Francis LtdDOI: 10.1080/0263514012005771 3

172 R. K. Coll & N. Taylor

past experiences. Furthermore, these ideas could differ from the currently acceptedscienti� c view, and from the intended learning outcome, and could be extremely resistantto change (Driver, 1981).

Such thinking led to the development of the constructivist paradigm which has as abasic premise that knowledge is created in the mind of the individual rather thanabsorbed or transmitted from an expert or teacher to a student (Driver, 1989). Theconstructivist view of knowledge acquisition led to changes in the nature of teaching andresearch. Much research into science teaching revealed that students hold many viewsthat are at variance with commonly accepted scienti� c views. In fact so proli� c has thisresearch been that there are now substantial bibliographies, with over 1000 references,into investigations of students’ conceptions in science (e.g. Pfundt & Duit, 1997).

Despite the proliferation of studies into students’ understanding of various aspects ofscience, there has been relatively little investigation of their understanding of chemicalbonding, particularly amongst senior secondary and tertiary level students. In this articlewe address this issue by identifying a range of alternative mental constructs of chemicalbonding and consider their origins and possible implications for teaching at this level.

Dif�culties in the Teaching of Abstract Chemistry Concepts

There seems to be a widespread perception amongst researchers and teachers that manystudents � nd chemistry dif� cult (Carter & Brickhouse, 1987; Nakhleh, 1992; Barrow,1994; Kirkwood & Symington, 1996). The reason suggested is that chemistry is acomplex subject possessing many abstract, frequently counter-intuitive concepts (Gabel,1998). Furthermore, Hawkes (1996) and others (e.g. Fensham & Kass, 1988; Taber,1995a) point out that there are many alternative conceptions in commonly usedchemistry textbooks. Remarkably, Hawkes stated that ‘after writing an article ontextbook errors I received a letter from a Nobel Laureate expressing disbelief in mystatement that only 2% of aqueous CdI2 exists as Co2 1 (aq.)’ (p. 421). One of the essentialcharacteristics of chemistry is the constant interplay between the macroscopic andmicroscopic levels of thought, and it is this aspect of chemistry (and physics) learning thatrepresents a signi� cant challenge to novices (Bradley & Brand, 1985). Numerous reportssupport the view that the interplay between macroscopic and microscopic worlds is asource of dif� culty for many chemistry students. Examples include the mole concept(Gilbert & Watts, 1983), atomic structure (Zoller, 1990; Harrison & Treagust, 1996),kinetic theory (Abraham et al., 1992; Stavy, 1995; Taylor & Coll, 1997), thermodynamics(Abraham et al., 1992), electrochemistry (Garnett & Treagust, 1992; Sanger & Green-bowe, 1997), chemical change and reactivity (Zoller, 1990; Abraham et al., 1992),balancing redox equations (Zoller, 1990) and stereochemistry (Zoller, 1990).

Student Alternative Conceptions of Chemical Bonding

The research that exists on students’ view of chemical bonding has revealed prevalentand consistent alternative conceptions across a range of ages and cultural settings.

Work on the understanding of intermolecular bonding has provided some evidencethat students appreciate the relationship between intermolecular bonding and physicalproperties such as boiling point (e.g. Peterson & Treagust, 1989; Peterson et al., 1989;Taber, 1995b, 1998; De Posada, 1997; Taylor, & Lucas, 1997). However, other researchreveals that students believe intermolecular bonding is stronger than intramolecularbonding (Peterson et al., 1989; Goh et al., 1993), and that they invoke intramolecular

Alternative Conceptions of Chemical Bonding Held by Students 173

bonding in inappropriate circumstances (e.g. in ionic compounds) (Taber, 1995b, 1998),or believe it is absent in polar molecular substances such as water (Grif� ths & Preston,1989; Birk & Kurtz, 1999).

A highly prevalent alternative conception for chemical bonding is that continuouscovalent or ionic lattices contain molecular species (De Posada, 1997; Taber, 1998; Birk& Kurtz 1999). Butts and Smith (1987) suggest that the ubiquitous use of ball-and-stickmodels used to model ionic lattices may be instrumental in the generation of thisalternative conception because students mistake sticks for individual chemical bonds. Thefact that other research revealed that students believed ionic substances such as sodiumchloride possessed covalent bonds adds credence to this suggestion (Peterson et al., 1989;Taber, 1994, 1997). A related alternative conception, reported by Boo (1998), is thatsome students believe that a chemical bond is a physical entity. Boo suggests that thisarises from a worldview that building a structure requires energy input, whereasdestruction involves release of energy—that is, students believed that bond breakingreleases energy and bond making involves energy input.

Confusion about the concept of electronegativity is also widespread, resulting in anumber of alternative conceptions for chemical bonding; inability to establish the correctpolarity of polar covalent bonds, the view that non-polar molecules are only formedbetween atoms of similar electronegativity and that the number of valence electrons, thepresence of lone pairs of electrons, or ionic charge determine molecular polarity(Peterson et al., 1989; Harrison & Treagust, 1996; Boo 1998; Birk & Kurtz, 1999).

Students appear to have little appreciation of the underlying electrostatic nature ofchemical bonding (Taber, 1995b; De Posada, 1997; Boo, 1998). For example, attractionbetween two oppositely charged species was thought to result in neutralisation ratherthan bond formation, the likely source of confusion being the parallel with acid–basechemistry (Schmidt, 1997; Boo, 1998). Similarly, students have a poor understanding ofthe bonding in metals, seeing metallic bonding as unimportant or in some way inferiorto other forms of bonding, despite being able to use the common sea of electrons modelto explain the properties in metals (Taber, 1995a, 1998; De Posada, 1997). There are anumber of other alternative conceptions about covalent bonding reported in theliterature. Some students believe that the number of valence electrons and the numberof covalent bonds are one and the same; other conceptions include confusing resonanceforms with molecular structures and believing that covalent bond formation involves thetransfer of electrons (Taber, 1994, 1997, 1998).

Much of the research cited above was undertaken with students of school age. Theconceptions which tertiary level chemistry students hold about chemical bonding have,in comparison, been researched infrequently. Perhaps by this stage in their academiccareers, students may be perceived as having a sound understanding of scienti� cconcepts, and are therefore less prone to developing naive mental models. In this articlewe describe a study of senior secondary school and tertiary level chemistry students fromNew Zealand and Australia. The study revealed a number of alternative conceptions forchemical bonding and these, along with the implications of the study for the teaching andlearning of senior level chemistry are discussed.

Methodology

Theoretical Framework

The work reported here represents part of a larger study into students’ mental modelsfor chemical bonding (Coll & Treagust, in press). This inquiry is a naturalistic inquiry


conducted within a constructivist paradigm. The authors believe inquiries into scienceeducation such as that described in this work are best addressed using a methodology inwhich the individual constructions of interest are elicited by interactive dialogue betweenresearchers and participants. Speci� cally the authors subscribe to a social and contextualconstructivist belief system. Contextual constructivists assert that a crucial feature ofknowledge creation is that it is not carried out in isolation, but is subject to in� uence byan individual’s context; that is, his or her prior knowledge and experiential world(Wheatley, 1991; Cobern, 1993; Good et al., 1993). Contextual constructivism and therelated social constructivism hold that personal constructivism is too limited as humansare social beings, and knowledge creation is in� uenced by the prior experiences andsocial environment of the student. Wheatley (1991, p. 49) summarises the position byclaiming that ‘we continually negotiate the meaning of events in our lives so that we canbene� t from the experiences of others as well as our own’. Social constructivists believethat an important part of construction is social interaction through which we come to acommon understanding of knowledge, including scienti� c concepts (Wheatley, 1991; vonGlasersfeld, 1993; Solomon, 1994). Tobin and Tippins (1993) put it this way ‘theindividual and social components [are seen as] being parts of a dialectical relationshipwhere knowing is seen dualistically as both individual and social, never one alone, butalways both’ (p. 20). However, we may appear to have the same view of concepts asothers, but our understanding is commonly discrepant, for example, when there is anundetected communication breakdown (Johnson & Gott, 1996).

Data Collection

The researchers began the larger study by conducting an in-depth analysis of thecurriculum material that these students had encountered during their studies. Thisentailed a thorough examination of lesson plans, textbooks, lecture notes and otherrelevant documentation such as topic tests and � nal examinations. These data weresynthesised into a summary of the mental models for chemical bonding. This summarycomprised some 40 pages and a total of eight models along with extensions andmodi� cations of the models. The models identi� ed, were, the sea of electrons model andband theory for metals, a model based on electron transfer and a further model involvingthe calculation of electrostatic charges for ionic substances, and the octet rule, themolecular orbital theory, the valence bond approach, and ligand � eld theory for covalentsubstances. Students’ views were elicited from semi-structured interviews according to theprotocol detailed in Table I.

This comprised a description of a given model chosen by the students during theinterview, followed by probing of their understanding by the use of focus cards thatdepicted model use in some way (Table I). For example, for metals, this involved showingthe participants samples of metallic substances (aluminum foil and steel wool), along withfocus cards depicting the conductivity and malleability of a metal (examples of the cardsfor metallic bonding are provided in the Appendix). The student models revealed ininterviews were then compared with the models found in the curriculum material; thisrevealed the alternative conceptions that form the focus of the present paper.

Alternative conceptions were compiled into inventories and expert-validated by theteachers involved in this study. The expert’s principal contribution consisted of validationof the description of the models and of the researchers’ constructions and interpretationof the students’ alternative conceptions for chemical bonding reported here. It is


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important to note that the research question in this work is subordinate to the aim of thelarger study; namely, to establish the students’ preferred mental models for chemicalbonding. Consequently, the results reported herein should not be deemed as anexhaustive catalogue of the students’ alternative conceptions for chemical bonding.Rather, these data represent a compilation of alternative conceptions revealed when thestudents discussed their mental models for chemical bonding.

De�nition of Alternative Conceptions

In this paper, a conception has been classi� ed as an alternative conception if it meets thefollowing two criteria: the view was in disagreement with the scienti� c view and theconception was related to some aspect of chemical bonding, including speci� c details ofa particular model for chemical bonding. For example, to describe the bonding in ioniccompounds, the students typically described the size and shape of ions as well as thepacking. Hence views at odds with the scienti� c view about ion size, shape or nature ofpacking, have been classi� ed as alternative conceptions However, during discussionscentred on a focus card which compared the electrical conductivity of metallic copperwith that of glass, there was evidence that some of the students held alternativeconceptions about electrical conductivity. Because such alternative conceptions did notpertain to chemical bonding, they have not been included in the report of this work.

Sample Description

The study involved 30 participants from three academic levels; Year 12 secondary schoolstudents (age range 17–18 years), second and third year undergraduates (age range 19–20years) and postgraduates (age range 23–26 years). The secondary school students formedtwo cohorts—two females (Natalie and Linda) from a private school in a middle classarea of an Australian city, and four females (Anne, Anita, Claire and Frances) and fourmales (Neil, Keith, David and Richard) from single-sex schools in a middle class suburbof a small New Zealand city. The male secondary school students were, in general, lesscon� dent and outspoken than their female counterparts, although all students spokefreely during interviews. The participants were interested to pursue science-based careersand stated that they enjoyed chemistry. Eight of the undergraduate participants wereintending BSc chemistry majors from a New Zealand university. Because the interviewsfor these undergraduates were conducted late in the year after the completion of lectures,undergraduate students had, at a minimum, completed 2 years of tertiary chemistryinstruction. There were two male (Bob and Steve) and two female (Renee and Kim)second year undergraduate participants and two male (Alan and Mike) and two female(Jane and Mary) third year undergraduate participants. In addition to the New Zealandundergraduates there was one Australian male (Mike) and one female (Rosaline), bothsecond year students from an Australian university. There were 10 postgraduate students,four New Zealand PhD candidates two being male (Jason and Kevin) and two female(Grace and Christine), and four MSc level candidates, two male (James and Brian) andtwo female (Jenny and Rose), from the same New Zealand university as the undergrad-uates. In addition there were two Australian PhD candidates, both male (John andNigel). All the postgraduates were high academic achievers—a re� ection of the entryrequirements for postgraduate studies. In spite of this, there was a considerable spreadin academic ability even within this cohort, with some students possessing outstandingacademic records. All MSc candidates were purposefully chosen from the second year


class. The intention was to distinguish these postgraduates from � nal year BSc students:these selection criteria ensured that the masterate students had completed all of theirMSc courses.

Research Findings

The alternative conceptions identi� ed in this study are detailed in Table II. Many ofthese alternative conceptions were not widely evident, several alternative conceptionsbeing evidenced by a sole individual. Because the inquiry does not comprise a systematicattempt to uncover students’ alternative conceptions, a given alternative conception mayhave been identi� ed for only one student, but this does not necessarily mean only onestudent held this alternative conception.

Strength and Weakness in Chemical Bonding

The alternative conception that chemical bonding in metallic, ionic and covalentsubstances is weak was prevalent across all levels of the students (AC 1–3, Table II) whoseemed unaware that this view con� icted with direct physical evidence such as thehardness of metallic copper (Table II) and it is noteworthy that the students were showna sample of thick solid copper metal at this point. Keith, for example, attributed themalleability of copper to weak bonding in the metal (AC 1), stating, ‘I think as it goesthrough rollers the copper atoms are just spread out. The bonding between them isn’tso strong’. David likewise reported, ‘I’d see the bonding as pretty loose, not strongbonding for copper’. Similar views were expressed about covalent bonding in some cases(AC 2), with David, for example, when discussing the bonding in chloroform (CHC13)stating, ‘it must be letting off some hydrogens and stuff, so I don’t think the bondingwould be that strong’.

The view that the electrostatic forces holding ionic compounds together are weak alsowas prevalent (AC 3, Table II). This view was the most common explanation offered bythe students for the friability of ionic salts such as sodium chloride that was depicted infocus cards and was evident across all levels of student from secondary school topostgraduate. Neil stated ‘in the sodium chloride the force can just break it, the bondsare not very strong’, Steve said ‘the ionic bonding that is holding this crystal togetherhere is weaker than is the case of the covalent’, and Brian ‘it’s much harder to breakbonds, actual direct covalent bonds, than it is to break weak ionic bonds’. The otherstudents, whilst not explicitly stating that such forces were weak, made statements thatseemed to imply this. For example, Bob stated ‘the sodium chloride is only relying on anelectrostatic force to hold together, whereas the other one’s got a lot of covalent bondslocking it rigidly together making it strong’. Christine apparently also believed ionicelectrostatic forces in ionic substances such as sodium chloride are weak

I see that bonding in sodium chloride can’t be as strong as in SiO2 althoughI see it; I guess I see the sodium chloride as different type of bonding. But ifit’s going to crack it must have like little sections or something. That’s the onlyway I can explain it away, like if it’s the same type of bonding the silica onemust be stronger otherwise the same thing would happen to it.

Overall the data suggest that the students saw metallic and ionic bonding as weak orinferior to covalent bonding, although in a few instances covalent bonding was seen asweak also.


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Molecularity in Continuous Ionic and Metallic Structures

Previous work revealed that the undergraduate and postgraduate students seemed topossess a greater appreciation of the continuous nature of ionic and metallic lattices thansecondary school students (Coll & Treagust, in press). Interestingly, some of the studentsacross all levels possessed alternative conceptions about the nature of metallic and ioniclattices, seeming to believe that such lattices were molecular in nature or containeddistinct molecular species (AC 4, Table II). For example, the students used the termmolecules to describe particles in metals and ionic substances. Frances, attempted toexplain the lack of conductivity of molten sodium chloride thus, ‘the molecules can onlymove a little bit side to side so they can’t move and make the bulb glow’ and Keith stated‘the bonds are rigid and aren’t allowing the sodium chloride to ionise and this holds theelectrons from � owing on through the sodium because the molecules are held tightlytogether by the bonds in the sodium chloride, so they can’t move around’. Keith alsointroduced the concept of molecules when describing the conductivity of metallic copperstating, ‘the copper molecule, it’s � owing. Like it’s going from positive to negative so theelectrons can � ow along’ and Bob when describing a ball-and-stick depiction of bondingin the metal lithium stating ‘I like the way you can see the packing together of themolecules in a regular fashion’.

Some caution is necessary in the interpretation of these data as it is possible that thesestatements merely represent inappropriate or careless use of nomenclature. However,some of the students identi� ed speci� c molecular species in metals and ionic substances,suggesting that their use of the term molecule is purposeful, as for example, in thedescription of the structure of sodium chloride and caesium chloride by Alan, ‘thecaesium atom makes contact with all these chlorides in the middle. That sort ofinteraction between the caesium and chloride, like how you are taught NaCl, the NaClmolecule’.

A related alternative conception, that metals and ionic compounds possess intermolec-ular bonds, was evident for three students (AC 5, Table II). David stated ‘I suppose thereis always the van der Waals things with just the attraction of the electrostatics and stuff’,and as seen in a drawing (Fig. 1) and writing used by Keith to describe the bonding inthe ionic substance LiCl.

Then there is bonding between another, and another [draws wavy linesbetween units of LiCl, writes van der Waals next to wavy line].

The notion of molecularity may have led to this alternative conception, in that thestudents felt it necessary to explain why the structure was held together, and drew uponthe concept of van der Waals forces to do so.

Alternative Conceptions of Ion Size

There were two remarkably prevalent alternative conceptions related to the size of ionsidenti� ed; one in which the sodium ion was viewed as being larger in size than thechloride (AC 6, Table II), the other that the lithium ion was larger than the sodium ion(AC 7, Table II). Again these views were held across all three academic levels. Many ofthe students stated that sodium was the larger ion of the two in sodium chloride. Anitastated ‘the sodium would be bigger than chlorine’, Neil ‘lithium’s smaller than sodiumand, and chloride is smaller than sodium’ and Richard ‘I would say the sodium is thelarge one and then the smaller ones would be the chloride’. Likewise, Neil said ‘lithiumwould be bigger it’d have more protons and electrons it’d be bigger’. Alan stated ‘I would


FIG. 1. Keith’s drawing illustrating the bonding in lithium chloride (LiCl).

consider lithium to slightly larger than the sodium’, and Mary said ‘the lithium maybea little bit bigger than sodium’. It is noteworthy that the respondents were in possessionof a Periodic Table at the time. The responses above reveal the tentative nature of thestudents’ views when the size difference is described at being ‘slightly larger’ and ‘maybea little bit bigger’.

Views expressed by some of the secondary school students offered clues to the originof the alternative conception that the chloride ion is smaller than the sodium ion. Itseems that students at this level at least, may have confused ionic size with the Periodictrend in atomic size.

David: I would say the Na would be bigger and the Cl would be smaller, Iimagine they would be.

Interviewer: Why do you see it that way?

David: The explanation I have had of that is that as you move across thePeriodic Table, well there’s more electrons in the same shell with an increasingnumber of protons as well, and that like attracts them closer which just meansit is smaller.

It is common to illustrate the structure of ionic substances such as sodium chloride inlectures and curriculum material using space-� lling and ball-and-stick models; informalinterviews of all of the instructors in this inquiry indicated that this formed part of theirteaching strategy; likewise, the textbooks used by the students contained diagrams thatshowed the ions to be similar in size. The ball-and-stick models often have the ion sizedepicted as similar; whereas there is a signi� cant difference in ion size in space-� llingmodels.

Alternative Conceptions for the Ionic and Covalent Character of Chemical Bonds

As mentioned above the inappropriate use of nomenclature may be instrumental in theformation of some alternative conceptions, and this is particularly evident in the case ofthe rather esoteric notion of the ionic–covalent continuum. Jason, one of the mostacademically able and most erudite of the postgraduate participants in the study,exempli� es the situation. His academic ability coupled with his extensive tutoring ofundergraduate chemistry suggest that he is likely to be aware of the importance ofnomenclature. Upon describing the formation of the chloride ion from neutral chlorine,he clearly showed that he understands the difference between an ion and a neutral atom,


that is, chlorine and chloride. However, he frequently made inappropriate use ofnomenclature in subsequent explanations.

I see the chlorine as being not a chlorine atom but a chlorine ion, so a chlorinewhich has gained an electron from the sodium atom so that the chlorine atomhas a negative charge. The sodium has a positive charge. It’s like this structureis essentially held together by electrostatic interactions. Now there’s alsorepulsions as well because you also have another chlorine atom which isreasonably close to that chlorine atom so there’s repulsions between the two.

Because of his expertise and experience, it seems that Jason is clear in his own mind ofthe difference between the terms chlorine and chloride; thus interchanging the terms maynot be particularly detrimental for his understanding of chemical bonding. However, itis possible that the interchanging of such terms for novices may lead to alternativeconceptions. To illustrate, consider another highly prevalent alternative conceptionfound—that polar covalent compounds contain charged species (AC 8, Table II). Francesindicated that chloroform contains a proton, and Anita identi� ed Cl minus in the samecompound.

Frances: Because these two have Hs in them and H with the Cl, and the highlyelectronegative ones like chlorides and things like that that gives the hydrogenbonding whereas they can readily donate a proton away to the hydrogen.

Anita: Yeah because H is plus and Cl is minus, but I don’t know why, I justknow it is different but I don’t know why.

Similarly, Mary, an undergraduate, stated explicitly that she viewed the bonding inhydrogen halides as ionic in nature, rather than polar covalent, or possessing of someionic character, stating ‘I think of them as being ionic’. Jenny, a postgraduate, articulatedher views about the bonding in chloroform in greater detail drawing on previousexperience of the bonding in carboxylic acids; part of her MSc research project.However, the value of doing this seems dubious, as she confused the donation of protonsin carboxylic acids with a perception of lability of the chlorine atom in chloroform(CHCl3). The fact that she used the term chloride, rather than chlorine, may indicatethat inappropriate nomenclature has in part contributed to her alternative conception.

I know that to remove an atom from chloroform is quite dif� cult. But if youlook at other organic molecules will, um … I mean if you think, like yourcarboxylic acid it’s quite easy to remove the proton and the same for thechloride, like a halide. It’s quite easy to remove chlorine from a long sort ofcarbon chain.

The fact that Jenny described the chlorine in chloroform as chloride, indicative of anionic species, may have led her to make an inappropriate link to her own experience withthe ionisation of carboxylic acids. An examination of interview transcripts revealed thatit was routine across all academic levels for the students to interchange terms pertainingto halogens with the charged halide. Students used the terminology for neutral atomswhen describing the bonding in ionic compounds, and similarly described halides ashalogens, as seen in Keith’s description of the bonding in sodium chloride and Steve’sfor lithium chloride.

Keith: OK the sodium has got one electron in its outer shell, and the chlorinehas got seven. So the chlorine requires one more electron. The bonds in thesodium chloride I don’t think are as strong between the sodium and the


chlorine so they are not attracted to each other, the sodium and the chlorideions.

Steve: Well basically it’s going to be easier for the chlorine’s wait a sec., umbecause the chlorine’s are still, the chloride’s have still got the same size.

An underlying origin of this view may be misunderstandings about the notion of theionic–covalent continuum. The students are told that there is no such thing as pure ionicbonding and that atoms in polar covalent compounds hold partial charges. It is possiblethat this notion reinforces the view in the students’ minds that chlorine is charged,causing confusion between the notion of a partially charged chlorine atom in a polarcovalent compound and the negative chloride ion. In support of this proposition,inappropriate use of terminology seems to be in� uential in the formation of alternativeconceptions about the bonding in pure molecular covalent substances like moleculariodine (I2) (AC 9, Table II). It is perhaps more likely that the students confuse chlorideand chlorine in a substance like chloroform which they are told is polar covalent andwhere the chlorine atom carries a partial charge. However, this is not the case fornon-polar molecular covalent species like homonuclear diatomics such as moleculariodine. The fact that the same alternative conception is seen for such substances addscredence to this view. Mary and Rose seemed to believe that the iodine atoms in I2 carrya negative charge, although they both subsequently stated that the bonding comprisedsharing of electrons, that is, a covalent bond.

Mary: Iodine well I know iodine as being I2 that’s how I remember it. Iodineis, um, I minus so therefore it’s got one extra electron I guess sitting there,that’s not doing anything [respondent laughs] and it’s able to pair up withanother iodine which also has a spare, and those two electrons come togetherand form a bond.

Rose: In iodine, well it’s like two atoms of iodine, they would be equallycontributing from the covalent bonds because each I minus, each iodine wouldbe like lacking one electron. So they join together to form a more stable I2,donating an electron each and they are shared between the two atoms.

Interviewer: So that’s a combination of two I minuses is it?

Rose: Yeah.In a similar way students seemed to confuse ions with neutral species and nuclei inmetallic bonding (AC 10–11, Table II). Two alternative conceptions of this nature wereidenti� ed, namely, that metallic lattices contain neutral atoms and that the positive ionspresent are nuclei.

Interviewer: Do you see this here as being lithium plus here?

Steve: No I don’t. I just see that as just being a lithium.

Mary: I guess that’s the nucleus [indicating the 1 symbols in the sea ofelectrons model, see focus card MB01 in the Appendix].

Kevin: Well my conception anyway is the fact that the charges of all the nucleiare very positively charged. The electrons are negatively charged. That willmake the whole thing sort of stay together.

Another possible origin of this alternative conception maybe the nature of visual cluesused in the diagrams depicting the sea of electrons model, in which spheres enclose apositive sign (focus card MB01, Appendix).


Alternative Conceptions about Electronegativity

It is possible that another alternative conception, that electronegativity comprisesattraction for a sole electron (AC 12, Table II) may also be related to this.

Steve: It’s the electron attracting ability of a species. So in the case of � uorinefor instance being the most electronegative element, because it has only onespace left to � ll in its 2p orbital, then it would very strongly attract an electronto � ll that orbital, to attain a stable � lled valance orbital.

Claire: It’s still ionic because the lithium has a stronger attraction.

Interviewer: Stronger attraction to what?

Claire: To the lithium. So the one electron they are sharing it will still spendmost of the time around the chlorine or chloride.

Although a de� nition of electronegativity was not explicitly elicited, if a given studentintroduced the term, he or she was asked to explain what the term meant to them.Electronegativity is not an aspect of chemical bonding per se. However, students’understanding of electronegativity impacts upon their understanding of bonding. Most ofthe students produced a de� nition of electronegativity that was in general agreement withthe scienti� c view. However, Steve seemed to believe that the attraction was for a singleelectron rather than a greater attraction for the shared pair. His statement that there is‘only one space left’ resulting in greater attraction suggests that the � lled valence shellconcept of the octet rule is in� uential in his view of electronegativity.

Alternative Conceptions about Metal and Non-metal Nature

An interesting alternative conception revealed in the study was that molecular iodine (I2)is metallic in nature (AC 13, Table II). Christine and Alan stated that they viewed iodineas metallic and Alan also related the bonding to that of lithium which he previouslyencountered in a focus card, Christine stating ‘I de� nitely think of it as a metal’ and Alan‘perhaps even more like the structure of that lithium. I sort of see lithium–lithium bonds.It’s the same in the case of the iodine’.

Jason clearly considered that I2 contains covalent bonds:

Iodine is kind of metallic, so I guess iodine certainly is a gas. Iodine willsublime when you heat it up and you will see the purple colouring in the airand that’s iodine gas and iodine is I2. So you have one iodine covalentlybonded to another iodine [drawing two Is inside circles linked together, Fig. 2]so you have I—I like that. So my guess it’s kind of metallic.

Jason stated that ‘iodine has some sort of metallic properties’. The origins of this viewmay lie in the lustrous, rather metallic, appearance of the crystalline appearance of thesample used during the interviews. Upon probing it seemed that Jason did not believemolecular iodine is a metal, yet he persisted with his statement that it will possess metallicproperties.

Interviewer: What makes you say it has metallic qualities?

Jason: Just looking at it. It looks metallic it’s somewhat shiny. It’s even grey in


FIG. 2. Jason’s drawing illustrating the bonding in molecular iodine (I2).

colour and you have sort of discrete little crystals of it. I don’t know. When Isay it looks like a metal, I don’t expect it to behave as a metal.

Interviewer: So you are saying it is metallic in appearance?

Jason: Yeah. It obviously has some sort of metallic characteristics to it.

Jason seems oblivious to the contradiction in his account. On one hand he states ‘I don’texpect it to behave as a metal’ and then ‘it obviously has some sort of metalliccharacteristics’. His view may be in� uenced by notions of Periodic trends. Although thishas not been articulated by Jason, it is common to state that there is an increase inmetallic character as one goes down a group in the Periodic Table (Chang, 1991, p. 167).This, along with the metallic appearance of I2 may be the cause of the alternativeconception.

An interesting alternative conception was related to the bonding in ionic compoundslike sodium chloride (AC 14, Table II). A number of the students seemed to becomeconfused between ionic bonding and covalent bonding. Two secondary school students,Anne and Frances, identi� ed the bonding in ionic compounds as covalent, despiteinitially describing a process of electron transfer. Anne stated ‘the electrons are sharingbetween the caesium and the chloride’ and Frances likewise stated ‘the chloride iselectronegative which means it can give away an electron and Na plus means it canaccept the electron’.

The comments made by Frances in particular provide clues to the origins of thisalternative conception. Frances mentioned electron transfer, but went on to state that thebonding is covalent in nature. Such a description possesses elements of the theoryemployed to explain the ionic–covalent continuum.

Other Alternative Conceptions about Chemical Bonding

In addition to the more prevalent alternative conceptions described above, there were anumber of conceptions that were identi� ed for a sole participant. Jason seemed confusedabout the number of nearest neighbours in copper and stated that the malleability ofmetallic copper involved changes to the number of nearest neighbours; he saw 12, thensix, followed by four neighbours, instead of the eight that are actually present in coppermetal. Steve seemed confused about the nature of bonding in the alloy steel (AC 17,Table II), viewed the bonding between the non-metal carbon and iron as electrostatic innature rather than covalent.

Well you have still got the retention of the electrostatic forces between the ironcation and the electrons. But, I am just trying to think back. With theintroduction of the carbon, there’s actually going to be new centres locatedinside the metal structure. I presume that they would be held by some sort ofelectrostatic interaction.

Rose, a postgraduate, seemed to believe that the shape of ions could be in� uenced bymacroscopic factors such as pressure (AC 18, Table II) ‘like it’s maybe the conditions it’s


under. The pressure might make a difference’. Neil possessed a novel view of intermolec-ular forces, believing that they were affected by gravity (AC 19, Table II) ‘it moves a littlebit, but it must be that the charges on each side would not be as strong I would imagine.It may be also because it’s affected more by gravity that the others’. In addition, Steveseemed to believe glass was a crystalline substance stating ‘glass is basically a silicastructure, that’s silicon covalently bonded in a silica structure with SiO4 units’. Althoughthese views were only expressed by one individual, as mentioned previously, it is possiblethat these views are not idiosyncratic and were also held by the other students.

Summary

The interview data revealed prevalent alternative conceptions for chemical bondingacross three levels of learning. In addition to some common alternative conceptionspreviously reported in the literature, such as misunderstandings about intermolecularforces and molecularity of continuous lattices, the inquiry found a surprising number ofalternative conceptions about simple ideas like ion size and shape. Some 20 alternativeconceptions were revealed, the most common being belief that continuous ionic ormetallic lattices were molecular in nature, and confusion over ionic size and charge.

Implications for Teaching and Learning

It is surprising that the undergraduate and postgraduate students, especially given theirgood academic records, held such alternative conceptions and it is interesting to considerwhy, in particular, the undergraduate and postgraduate students became confused abouta number of fairly straightforward concepts. In one statement John, one of the PhDstudents, described his views about the purpose of tertiary education. He made it plainthat he had a great desire to gain what he perceived as highly practical skills from hisuniversity studies. He stated that he chose his particular tertiary institution deliberatelybecause he felt it provided ‘a more hands-on approach, something that is a lot morepractical’. He reinforced this view later when questioned about the concept of antibond-ing which he had introduced during a discussion regarding the bonding in benzene.

All I remember is that they are silly things that stick out either side. I couldn’treally understand or fathom them. That’s just how it is. That’s how Iremember the antibonding things. I mean just drawing your diagrams you’vejust got to remember your antibonding. You have your bonding and you haveantibonding orbitals. Antibonding orbitals are just there because they are there.That’s all it is. I didn’t really understand why they are there, just there so thenumbers balance.

It seems that, John at least, is interest in theoretical aspects of bonding models when theyhave a practical application. The fact that he dismisses antibonding orbitals as ‘sillythings’ suggests it is possible that students ascribe complex conceptions low status unlessthey can see their relevance to their work and subsequent study. However, the originsof alternative conceptions are many fold. Of prime importance are students’ existingviews about abstract concepts. It is common for tertiary teachers to consider that studentshave little or no knowledge of such concepts, other than rudimentary grounding insimple theories such as the sea of electrons model or the octet rule. Certainly at thetertiary institutions involved in this study, the teachers assume no foreknowledge of, forexample, molecular orbital theory or the valence bond approach. However, although


students may not have encountered such theories explicitly, they may harbour otheralternative conceptions that impact upon their ability to understand such complex andabstract models.

The students in this study seemed to � nd great dif� culty remembering details ofmodels and concepts despite, in some cases, having encountered them in relatively recentinstruction. The amount of material that the students encounter in an undergraduatescience course is formidable. This, coupled with the mass of advanced material encoun-tered during their postgraduate studies, may offer some explanation for the surprisingnumber of alternative conceptions revealed for the advanced level students in this study.It would seem reasonable to conclude that confusion and careless use of terminology isin� uenced by the sheer mass of material the students encounter at the tertiary level. Wehave argued previously that it is not feasible to consider removing the teaching of thesemodels from the undergraduate chemistry curriculum (Coll & Treagust, in press).Modern chemistry teachers at the senior secondary and tertiary level face a tension inthe competing aims of teaching, that is, the desire to provide adequate chemistryknowledge for those seeking specialist careers as chemists, typically chemistry majors, andyet avoiding overloading non-specialists with unnecessary material that will be of limitedvalue in their own studies and subsequent careers (Fensham, 1980). However, tertiarychemistry teachers may wish to consider the advisability of limiting the teaching of suchmodels until an advanced stage of the undergraduate degree, say the third year of aconventional 3 year bachelors degree. This proposition is offered as chemistry non-ma-jors will have little need for models in their subsequent studies. Hence, the value ofteaching, for example, biology majors, sophisticated complex and highly abstract mentalmodels is in our view dubious.

Correspondence: Dr Richard K. Coll, School of Science and Technology. The Universityof Waikato, Private Bag 3105, Hamilton, New Zealand. E-mail: [email protected].

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Appendix

alternative conceptions of chemical bonding

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