teaching and learning the concept of chemical...

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Teaching and Learning the Concept of chemical bonding

Tami Levy Nahum1, Rachel Mamlok-Naaman1, Avi Hofstein1 and Keith Taber2*

1. Department of Science Teaching, The Weizmann Institute of Science, Rehovot, Israel

2. Faculty of Education, University of Cambridge, England

1. Dr. Tami Levy Nahum

Postal address: Department of Science Teaching, Weizmann Institute of Science, Rehovoth 76100, Israel

E-mail: [email protected]

Tel number: 972-8-9342497

Fax number: 972-8-9344174

2. Dr. Rachel Mamlok-Naaman



Tel number: 972-8-93422446

Fax number: 972-8-9344115

3. Prof. Avi Hofstein



Tel number: 972-8-9343811

Fax number: 972-8-9344115

4. Dr. Keith Taber * Corresponding author

Postal address: Science Education Centre, University of Cambridge Faculty of Education, 184 Hills Road, Cambridge CB2 8PQ, United Kingdom


Tel number: +44 (0) 1223 330569

Fax number: +44 (0) 1223 767602

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Word count (including reference list etc): 13 565

Author biographical notes:

Dr. Tami Levy Nahum is Senior Intern in the Department of Science Teaching, at the Weizmann Institute of Science in Israel; Her research interests include inquiry-based laboratories, students' misconceptions and pseudo-conceptions, alternative assessment methodologies, teaching and learning chemical bonding, and students' evaluative thinking capabilities in the Israeli multicultural context. She was involved in the development of a new chemistry curriculum and in scores of teachers' professional developments courses. Currently, she is also involved in a National Project focusing at the professional development of science teachers in Junior High School.

Dr. Rachel Mamlok-Naaman is Senior Staff scientist and coordinator of the Chemistry Group at the Department of Science Teaching at the Weizmann Institute of Science. In addition, head of the National Centre for Chemistry Teachers at the Weizmann Institute of Science, and a senior member of the Science and Technology-for-All Group. Engaged in development, implementation and evaluation of new curricular materials, and research on students' perceptions of chemistry concepts. Publications in the areas of scientific and technological literacy, teachers' professional development, cognitive aspects of students' learning, assessment and curriculum development. In the last three years she has been involved in an educational project in the framework of the European Union.

Prof. Avi Hofstein is head of the Chemistry Group at the Department of Science Teaching at the Weizmann Institute of Science in Israel. His scientific activities focus on all aspects of the curricular process in the context of chemistry education programs as well as in the program "Science for All". This includes curriculum development, implementation, professional development of both regular teachers and leadaing teacher, and research of various aspects related to teaching and learning science (chemistry). In recent years he was involved in several EU projects on technology education and continious professional development of science teachers.

Dr. Keith Taber is Senior Lecturer in Science Education in the Faculty of Education at the University of Cambridge, UK. After completing his first degree in chemistry, he trained as a teacher of chemistry and physics, and taught in secondary schools and a further education college. After completing his doctorate on students’ developing understanding of chemical bonding he moved to Cambridge, initially focusing on teacher education, but increasingly working with higher degree students. He was the Royal Society of Chemistry’s Teacher Fellow in 2000-2001. His main research interests relate to aspects of conceptual learning in science – including issues of conceptual integration; learning about the nature of science and its relationship with religion; and challenging the most able (‘gifted’) learners.

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Teaching and learning the Concept of chemical bonding

Number of words:

Abstract:

Chemical Bonding is one of the key and basic concepts in chemistry. Clearly, many of the concepts taught in chemistry in both secondary schools as well as in the colleges are highly based on understating the fundamental ideas related to this concept. Nevertheless, the concept is perceived both by the teachers as well as by learners as difficult to teach and as causing many misconceptions regarding the students. Many of these misconceptions result from over-simplified models used in text books, by the use of traditional pedagogy that present a rather limited and sometimes incorrect picture of the issues related to chemical bonding and by assessment of students' achievement that influence the way the topic is taught. In addition, there are discrepancies between scientists regarding the definition of the topic and regarding models to teach it. In this review paper we tried to provide science educators, curricula developers and pre-service and in-service professional development providers an up-to-date picture regarding the research and developments of the chemical bonding. We reviewed the external and internal variables that might cause these misconceptions and the problematic issue of using limited teaching/learning models. Finally we review the approaches to teaching the concept that might overcome some of these misconceptions.

Keywords: chemical bonding, misleading factors, misconceptions, alternative frameworks, fundamental principles, models

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Teaching and Learning the Concept of chemical bonding

Tami Levy Nahum1, Rachel Mamlok-Naaman1, Avi Hofstein1 and Keith Taber2

1. Department of Science Teaching, The Weizmann Institute of Science, Rehovot, Israel

2. Faculty of Education, University of Cambridge, England

Introduction

A chemical bond is the physical process responsible for the interactions between atoms and molecules, and that which confers stability to bounded particles. The explanation of these interactions, which are basically electrical forces, is a complex area that is described by the laws of the quantum mechanics theory (Wikipedia, 2010).

In school science teaching, ideas need to be presented in ways that are both authentic

representations of the scientific concepts, and yet simple enough to be meaningfully

understood by the learners. Yet chemical bonding is inherently an abstract and

complex concept. When the authors of this review approached prominent scientists,

worldwide, regarding this issue, they got a variety of complicated definitions.

Very often, when considering the teaching of chemical bonding, the arguments are

focused on pedagogical issues rather than scientifically related disagreements.

Needless to say, seeking a core understanding of chemical bonding becomes more

problematic when we try to translate key ideas to the classroom practice. This review

sets out to integrate research-based knowledge regarding the teaching and learning of

chemical bonding.

Bonding: The nature of the concept

There is no doubt that chemical bonding is one of the key concepts in

chemistry and one of the most fundamental ones. It is also one of the areas in the

physical sciences where understanding is developed through diverse models - which

are in turn built upon range of physical principles - and when learners are expected to

interpret a disparate range of symbolic representations standing for chemical bonds

(Taber & Coll, 2002). The concepts associated with chemical bonding and structure,

such as covalent bonds, molecules, ions, giant lattices, and hydrogen bonds are highly

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abstract and in order to fully understand these concepts, students must be familiar

with mathematical and physical concepts and laws that are associated with the

bonding concept, such as orbital, electro-negativity, electron repulsions, polarity, and

Coulomb's law. In addition, learning about chemical bonding allows the learner to

make predictions, and give explanations, about physical and chemical properties of

substances.

Sánchez Gómez & Martín (2003) have discussed how the most advanced models

available to chemists for understanding the structure of matter, that is those that we

might judge our best approximations to ‘reality’ given the current state of knowledge,

derive from quantum chemistry. However, Sánchez Gómez and Martín also suggest

that the majority of chemists are quite content to work with models that largely pre-

date developments in quantum chemistry, and which have to be considered to be less

well supported by our current best understanding of matter. As this means that most

chemists are using a set of models that are now understood to be limited

representations of the structure of matter, they describe this popular if flawed

conceptual toolkit as the folk molecular theory (FMT). The chemical concepts of

FMT are then retained as useful conceptual tools, and for many chemists continue to

provide a set of explanations of chemical phenomena (that is molar scale phenomena

actually experienced in the laboratory, or in nature) in terms of an understanding of

the submicroscopic composition and structure of matter. Atoms, molecules, ions,

bonds and so forth are therefore available components of explanations that help to

systematize chemical knowledge. In each of the following examples of high

school/college level chemistry explanations, material behaviour that can be directly

investigated is explained in terms of theoretical entities that are part of a conceptual

‘toolkit’:

• copper conducts electricity because it has metallic bonding with delocalised electrons

• diamond has a high melting temperature because its atoms are strongly bound into a lattice by covalent bonds

• sodium chloride is soluble in water, but not in benzene, because the crystal is held together by strong ionic bonds, but the ions are also able to form bonds with water molecules when hydrated

• water evaporates readily because it comprises of small covalent molecules

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• water expands on freezing because it forms a lattice of hydrogen bonded molecules

• a stream of water can be deflected by a charged rod because the molecules have a dipole moment due to the polar bonds

In each of these cases the phenomenon to be explained is something that can be

directly demonstrated on the bench (e.g. copper conducts electricity), and the

explanation refers to entities that are discussed as if real objects (e.g. metallic

bonding, delocalised electrons), but which are theoretical constructs such as types of

bond between atoms.

Understanding particle theory: Quanticles

The notion of chemical bonding is then part of an extensive explanatory framework

that chemists use to make sense of molar scale phenomena in terms of a conjectured

sub-microscopic level of material structure. A key feature of these models is that

matter is not considered to be homogenous and continuous, but rather at a small

enough scale it comprises of myriad components – which are considered to be the

fundamental particles from which macroscopic structures are built.

Particles at the submicroscopic level – atoms, ions, electrons, molecules – are not like

more familiar particles such as salt or sugar grains. These particles of the molecular

world are fuzzy packets of fields without surfaces or definitive volumes, which extend

indefinitely and which can often interpenetrate each other. To use the term ‘particles’

can mislead learners. They are something else – one of us has commonly called them

‘quanticles’ (Taber, 2002b) to emphasise this distinction.

Precisely what level of quanticle is considered has shifted historically. The most

salient notion here is surely that of the atom, which offers a strong link with a

macroscopic notion of fundamentality in matter. That is, all substances and materials

can be understood as derived from a limited number of chemical elements (the

macroscopic level); and each element is composed of (to a first approximation) one

type of atom.

The notions of atoms derives from Democritus, and although the modern chemical

notion of the atom rather different to this ancient notion (modern atoms are not

actually atomos, and are not hard volumes of material with discrete edges), the

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teaching model of atoms introduced to school students (e.g. ‘everything is made of

tiny particles called atoms’) has been considered to retain something of the flavour of

the ancient idea (Taber, 2003b).

Since chemists understand substances in terms of clusters of sub-microscopic

particles, the chemical bonds between those particles are used to explain many of the

chemical and physical properties of substances and chemical phenomena (Hurst,

2002; Levy Nahum, Hofstein, Mamlok-Naaman & Bar-Dov, 2004). Since bonding is

a central concept in teaching chemistry, a thorough appreciation of its nature and

characteristics is essential for understanding almost every other topic in chemistry,

such as carbon compounds, proteins, polymers, acids and bases, chemical

thermodynamics, proteins, carbohydrates, and polymers (Fensham, 1975; Gillespie,

1997; Hurst, 2002; Levy Nahum et al 2004). Gillespie (1997) in his essay titled: Great

ideas in chemistry, included chemical bonding (i.e., what holds atoms together in

molecules and crystals) as one of the six most important key concept that should be

included in every high school and introductory college chemistry courses. In addition,

the concept is very much related to the understanding of many important and

fundamental biological aspects such as molecular biology (e.g. DNA and RNA – i.e.

nucleic acids). However, based on the literature, bonding is considered by teachers,

students, and chemists to be a very complicated concept (Gabel, 1996; Levy Nahum,

Mamlok-Naaman, Hofstein, & Krajcik, 2007; Robinson, 2003; Taber, 1998, 2001,

2002a; Tsaparlis, 1997).

This review is targeted at 10-12th grade (upper-secondary school) and the

beginning of college and undergraduate. Thus, it is mainly focused on the nature of

the ‘chemical bond’, assuming that students at this stage, have already learned about

the periodic table, the particulate nature of matter, elements, compounds, and other

basic concepts that are usually taught in the lower-secondary school. Therefore these

concepts are not included in the current review which is focused on issues related to

the teaching and learning of the chemical bonding concept.

How is bonding being taught?

The problematic traditional approach for teaching the concept

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One of the goals of the chemistry teaching community is to develop more effective,

pedagogically and scientifically sound, strategies to teach high-school students the

concept of chemical bonding. According to Teichert and Stacy (2002), this aim is

motivated by many studies conducted worldwide that clearly revealed that the

traditional approach to teaching bonding is problematic, and misconceived. More

specifically, during the last two decades researchers have found that students

commonly lack a deep conceptual understanding of the key ideas regarding the

bonding concept and often fail to integrate their mental models into a coherent

conceptual framework (Bodner & Domin, 1998; Griffiths & Preston, 1992; Herron,

1996; Peterson & Treagust, 1989; Taber, 2001).

The problematic approach through which this concept is presented in many chemistry

textbooks worldwide has been examined extensively in the last two decades by

researchers of chemistry teaching and learning (Ashkenazi & Kosloff, 2006; Atzmon,

1991; Hurst, 2002; Justi & Gilbert, 2002; Taber, 1998). In these textbooks, elements

are conveniently classified as metals or-nonmetals (although sometimes semimetals

are also mentioned). Very often this dichotomy among elements leads to a

dichotomous classification of bonding related to compounds, namely covalent

(existing between non-metallic elements) or ionic (existing between metallic and non-

metallic elements). Hurst (2002) suggested that a common approach used by

chemistry curriculum developers (and also by textbooks writers) is to present four

different groups of substances: the ionic lattice, the metallic lattice, the molecular

lattice, and the covalent lattice. According to Hurst, these divisions and

characterizations are then (wrongly) used to differentiate between substances.

Furthermore, many chemistry textbooks do not refer to the hydrogen and Van-der-

Walls interactions as bonds, they refer to them just as ‘forces’ presumably since they

do not meet the ‘octet rule’ (the problematic aspect of using this ‘rule’ for molecules

is discussed below). This traditional pedagogical approach (Figure 1) which is

commonly used worldwide for studying bonding can be found not only in high-school

textbooks but also in general chemistry textbooks intended for college freshmen

(Hurst, 2002).

___________________________________

Insert Figure 1 at about here

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___________________________________

Levy Nahum et al (2008) focus on two main problems. Firstly, the presentation of

each bond type as a different entity that belongs to a specific category does not foster

a deeper understanding of chemical bonds. Specifically, it may obscure the important

notion of a unified rationalization of all chemical bonds based on underlying

principles. Secondly, over-emphasis of the four ‘ideal’ bonding categories (mentioned

above) is misleading and may actually hinder the learning process. While the ‘ideal’

classification is not without merit, it is now known (Naaman & Kronik, 2004) that

many important groups of modern materials simply cannot be ‘forced’ into one of the

rigid categories.

In the following section, several limitations of the rigid traditional approach,

which were mentioned above are illustrated, with specific examples from literature.

(1) Covalent versus ionic bonding. Typically, covalent and ionic bonds are

presented dichotomously, as ‘electron sharing’ or ‘electron transferring’ bonds,

respectively. However, in hetero-atomic systems, bonding is better described in terms

of a continuum of a covalent-ionic dimension or ‘scale’ (Levy Nahum et al, 2007;

Sproul, 2005; Taber & Coll, 2002). Furthermore, bonds between two identical atoms

are purely covalent, but purely ionic bonds actually do not exist at all. The

dichotomous presentation impedes the understanding of the more subtle ‘scale’.

(2) Electronegativity and bond polarity. Because within the traditional approach

bond polarity is essentially viewed as an additional characteristic of covalent bonds

(Hurst, 2002; Taber & Coll, 2002), the important concept of electronegativity (EN) is

only introduced in the context of covalent bonding and not as an integral part of bond-

polarity concepts (Weinhold & Landis, 2005). EN differences between atoms are then

used as an indication of whether compounds should be classified as ionic or covalent.

However, EN differences are not the ultimate measure for predicting bond type

(Sproul, 2005). Indeed, cases of bonds between atoms with large EN differences that

nevertheless possess a significant covalent nature are known experimentally (Woicik

et al, 2005).

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(3) The octet ‘rule’. Because it is simple for the learners to visualize and use, the

octet ‘rule’ is often presented as an obligatory condition for ‘proper"’ bonding. Thus,

students often adopt the anthropomorphic notion of atoms ‘wanting’ to possess

‘octets’ or ‘full outer shells’ and consider that chemical reactions occur in order to

‘allow" atoms to achieve this ‘natural desire’ (Taber, 1998; Taber & Watts, 1996).

But this causes some students to have difficulties in accepting anything that is not

clearly explicable in ‘octet’ terms, e.g., hydrogen bonds or even covalent bonds or

transition metal bonding not leading to ‘octets’ (Weinhold & Landis, 2005). It is now

known that many molecules and complexes do not ‘obey’ this rule (e.g. the ‘1998

Nobel Prize molecule’ NO is a good example; Culotta & Koshland, (1992). The octet

rule is certainly a time-honored useful guideline and shall remain so. However, it is

not a valid explanation for bond formation. For these reasons, Taber (2001a), Hurst

(2002) and Taber and Coll (2002) suggested that an over-emphasized ‘octet

framework’ may actually impede higher-level learning process.

(4) Metallic bonds. The traditional discussion of metals and metallic bonds may

involve both an over-generalization and an over-simplification. Over-generalization

occurs because in many general chemistry textbooks metals are characterized by a set

of common physical and chemical properties e.g., malleability, ductility, low

ionization potential, etc (Ben-Zvi, Eylon, & Silberstein, 1986). But the fact is that

there is not even one chemical property common to all metals, and there is a great

variability of parameters in any other quantity, e.g., brittleness, conductivity, boiling

point, etc. Over-simplification occurs because many textbooks introduce a metallic

bond as ‘metal ions floating in a sea of electrons’. This analogy is problematic

because it presents the metallic bond as a bonding entity that is entirely different from

the covalent one, whereas a more modern description views both types of bonds as

involving ‘electron sharing’. The difference is again explained in terms of a

continuum scale, this time involving the degree of electron delocalization (Myers,

1979).

(5) Hydrogen and van der Waals bonds. In most textbooks, covalent and ionic

bonds are described as ‘real’ chemical bonds, whereas hydrogen and van der Waals

bonds are often presented as ‘just forces’ (Taber, 1998). Again this distinction is far

too rigid and misleading. While the relative strengths of different types of bonds are,

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of course, very important, even ‘weak’ bonds do indeed bond together different

chemical units and sub-units and can have profound chemical consequences (Levy

Nahum et al, 2007; Taber & Coll, 2002), e.g., hydrogen bonding in biochemistry (for

example in the double helix structure of DNA). Therefore, a continuum scale is a

more appropriate scientific description. Two related over-simplifications are the

classification of hydrogen bonds as strictly inter-molecular, whereas they are often

intra-molecular as well (in proteins for instance), and the discussion of such bonds

only when N, O, or F atoms are involved, whereas hydrogen bonds, albeit weaker or

non-conventional, may occur with other atoms or groups as well (e.g., Naaman &

Vager, 1999; Nakanaga & Buchhold, 2002).

Despite this, the traditional general chemistry curriculum as a whole has by and

large been taken for granted by science educators for over a century and only little

was done to develop valid teaching models and pedagogical remedies to overcome

these problematic approaches (de Vos & Pilot, 2001).

In the following sections these problematic issues are being reviewed and elaborated

with specific research data as examples.

Chemical Bond: Learning difficulties, misconceptions, and alternative conceptions

The nature of chemical bonds

The curricular models of bonds presented in school science are based on the fact that

there are physical forces between different quanticles. At school level these forces are

normally understood as being electrical in nature.

Wightman, Green and Scott (1986) described a UK lower secondary class learning

about the particle model of matter. They found that the children considered the

linkage between particles in material terms: to be like elastic, or based on shapes

interlocking. When the interactions between particles were compared to a magnetic

attraction (i.e. a type of force familiar to the pupils), pupils’ conceptions tended to be

vague and to retain material aspects: e.g. there was “like string between the atoms sort

of holding it all together” (p.291). Part of the children’s difficulties seemed to be

related to finding a model of bonding which allowed melting and vaporization, yet

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still enabled bonds to form when particles were in close contact. As one of the

students commented, “the point is, how do we get the bonding back?” (p.292). It was

suggested that when the particles slowed down the bonding was then able to “get to

grips” with the particles as it is “a bit easier to keep slower things together” (p.292).

In this context, it has been suggested by Papageorgiou and Johnson (2005) that in

introducing the particle model of matter the notion of bonding might be introduced

“through the idea of the particles having an ‘ability to hold on to each other’, rather

than the more commonly employed talk of attraction”.

Griffiths and Preston (1992) asked grade-12 students (16-18 years) in Canada to

sketch molecules of ice. They report that typical diagrams showed the molecules

touching each other without spaces between, and they conjectured for these learners

the concept of bonding might have little to do with forces of attraction. For some

students in their sample, molecules were not bound due to inherent interactions

between atoms, but were held together by “something external to the molecules”.

In a French study of first year undergraduate science students Cros and colleagues

(1986) carried out 40 unstructured interviews; 50 semi-structured interviews;

followed by a survey of 400 students at two Universities before starting their lecture

courses. They found that the interactions between atoms in molecules were often

unknown or poorly known and that often students were not even aware that such

interactions existed:

A study conducted in the US by Nicoll (2001) was a first attempt to elucidate and

detail the types of misconceptions relating to bonding, electronegativity, electrons,

and molecular geometry that undergraduate chemistry majors hold. Nicoll reported

that quite a few students tried to explain chemical bonding in terms of electrons

attracting one another. She reported a category of incorrect explanations for bonding

phenomena or incorrect explanations for why bonding occurs and quoted one student

who explained “When you have two electrons, they’re negatively charged ions. They

don’t want to come together. But sometimes they do, and that’s a chemical bond”

(p.717).

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A common alternative conceptual framework for chemical bonding

Taber (1998) undertook a study of students’ thinking about chemical bonding and

related concepts among UK students. The core of the research was an interview-based

study to explore developing understanding of chemical bonding among students

studying chemistry at A level in a further education college (i.e. 16-19 year old

students). Whilst the in-depth nature of the study demonstrated diversity in student

thinking, Taber identified common tendencies and conceptions that were reported as

an alternative conceptual framework – that is a model representing the common

features found across a range of students. Taber characterised students as entering

study at this level holding a notion of chemical bonding deriving from school study,

and tied to the octet rule. For these students bonds would form because atoms wanted

to obtain full electron shells. They would do this by sharing electrons (covalent

bonding) or through an electron transfer (ionic bonding). It was found that students

tended to retain and use the ‘octet rule explanatory principle’ even after they had

studied bond energies and thermodynamic considerations on more advanced courses.

So students would commonly explain the reaction between fluorine and hydrogen in

terms of the atoms wanting to obtain full shells even in response to a probe question

that presented the reaction equation showing that the reactants were already

molecular, and so satisfying the octet rule (Taber, 2002a).

The anthropomorphic nature of many student explanations of bonding, based upon

what atoms 'wanted' or 'needed' to do, seemed to be a key feature for these

explanations seeming authentic to the students themselves (Taber & Watts, 1996).

Taber (2000, 2001b) argued that progression in student thinking during the A level

course could be seen as a shift from understanding bonding in terms of the octet rule

framework to adopting electrostatic and orbital ideas. A detailed study of one

student’s progress demonstrated that this could be a slow process that might not be

complete during the two-year college course. The model students brought from school

level study appeared to be tenacious. In part this seemed to be because the octet rule

explanatory principle was not an isolated conception, but part of a well integrated

framework of ideas.

Taber suggested that in the absence of a viable rationale for bond formation being

offered in school courses, the octet rule was adopted and extended to fill the

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‘explanatory vacuum’. It was not clear to what extent this was encouraged by teachers

– that is whether in the absence of a suitable curriculum model to explain bonding

teachers’ own explanations suggested the octet rule offered a rationale for bond

formation. However, in a study aimed at exploring New Zealand students’ mental

model of bonding, Coll and Treagust (2003) undertook an examination of curriculum

material and interviews with instructors. They identified “two target models” for ionic

bonding, one of which “termed the electrostatic model is based on the octet rule;

another model, termed the theoretical electrostatic model, is based on the computation

of the alternating attractive and repulsive forces within an ordered ionic lattice”. In

that context they found that “the former model is introduced at secondary school; the

latter model is introduced in the second year of the undergraduate program”. That is,

for school level learners, ionic bonding was taught in terms of a curriculum model

based upon the octet rule.

The covalent bond.

From a test instrument to diagnose senior secondary students' understanding of the

topic covalent bonding and structure, Peterson et al (1986) identified common

misconceptions as those selected by at least 20% of their sample . They listed the 13

misconceptions identified under 6 categories in Table 1.

___________________________________

Insert Table 1 at about here

___________________________________

Some of these misconceptions could be seen as partial understanding or application of

curricular models, as when considering molecular shape to be determined by only the

bonds or only by the non-bonding electron pairs; or when considering the polarity of a

molecule to depend upon bond polarity without consideration of the arrangement of

bonds. The authors acknowledged that the confusion between intermolecular and

intramolecular interactions “may be a case of mistaken terminology rather than a

conceptual misunderstanding” (p.311).

This type of instrument used, offers limited insight into student thinking. A common

feature of work in this area is the necessary trade-off between large samples offering

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indications representative of student populations, and in-depth studies that can explore

the nuances of students’ reasoning (Taber, 2009a). However, we would identify three

aspects of the study worthy of further note:

• Firstly, although the research concerned ‘understanding of the topic covalent

bonding and structure’ the authors clearly considered that polar bonds fell

within this topic. That is, for the purposes of the study, polar bonds are

treated as a type of covalent bond (a point to which we return later).

• Secondly, one of the misconceptions identified (‘nitrogen atoms can share 5

electron pairs in bonding’, see Table 1) relates to the failure to apply the octet

rule. This misconception is worthy of note, in light of the work reviewed

above that suggests that the octet rule often takes on particular (and

unwarranted) significance in student thinking about bonding. Overall then,

Peterson and colleagues suggest that knowledge of the octet rule increases

from grade-11 to grade-12, but that during grade-12 a new misconception

appears. Although the authors offer no explanation, a feasible possibility is

that during grade-12 students meet compounds of period 3 where elements

such as phosphorus ‘expand their octets’, and they simply apply this to

nitrogen as well.

• Finally, the authors draw attention to the notion of a covalent bond as the

sharing of a pair of electrons, and acknowledge that this linguistic formulation

may be interpreted differently by students than intended by teachers.

Coll and Treagust (2001) undertook an in-depth interviews with six Australian

learners found that these “learners view covalent bonding as the sharing of electrons,

with the secondary school and undergraduate learners relating this specifically to the

octet rule of full shell stability”. They reported how the two postgraduate students

they interviewed tended to supplement their octet rule based model, that “covalent

bonding results from the sharing of electrons” with elements from other models (e.g.

orbital, electron clouds). They reported that all the learners struggled to apply their

model of covalent bonding to explain chemical phenomena.

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The ionic bond.

Levy Nahum et al (2004) explored misconceptions among Israeli students, based on

their responses in the national matriculation examinations (final examinations set by

the government at the end of the 12th grade) over a period of twelve-years. One of the

common patterns they revealed was that in general students confuse ionic compounds

and molecular compounds. Similar pattern was also found by Butts and Smith (1987)

who undertook research to follow up a survey finding that the difference in properties

between ionic compounds and molecular compounds had been rated as a difficult

topic by 29% of students who were asked. Butts and Smith reported that there were

some who believed that sodium ions and chloride ions were released only when the

solid dissolved in water. In addition, they reported that the understanding of a

common form of a model used to illustrate NaCl structure in chemistry teaching (a

‘ball and stick’ model):

…was confused, many interpreting the six wires attached to each ball (‘ion‘) as each representing a bend of some sort (’there is one ionic bond and five physical bonds’ and ‘I would have expected seven wires not six, because chlorine has seven electrons in its outer shell’). Very few were able to explicitly state that the wires did not represent bonds but merely held the wooden balls in place (p.196).

It should be noted that ionic bonding can be considered a description of the overall

interactions between the various ions in an ionic crystal; however, it is also common

to consider that in the NaCl lattice, each ion forms six ionic bonds with surrounding

counter-ions.

In a UK interview study, Taber (1994) found that 16-19 year old students

demonstrated ideas about ionic bonding similar to those found by Butts and Smith.

Taber proposed an alternative conceptual framework for making sense of ionic

bonding (the ‘molecular’ framework), that as well as including molecular or pseudo-

molecular entities, encompassed three particular alternative conceptions (Taber,

1994).

• that an ionic bond only existed where there had been an electron transfer between atoms to form ions;

• that electrovalency (charge) limited the number of ionic bonds formed;

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• that the ionic lattice comprised of ions that were ionically bonded to some counter-ions, and attracted to other by ‘just forces’.

Based on the above, regarding NaCl, a student would consider a Na+ ion to be bonded

to a single Cl- ion by an ionic bond, and then attracted to another 5 because of the

electrical forces between oppositely charged ions. That is, the ionic bond was

considered to be something more than the electrical attraction, and this was because it

was associated in the student’s mind with a conjectured electron transfer event during

which ions were formed to allow the atoms to obtain octet structures/full outer shells

(that is, the molecular framework for thinking about ionic bonding was subsumed

within the broader octet framework for understanding bonding phenomena). For the

students the focus of ionic bonding is the electron transfer event. Indeed students

asked about precipitation reactions have been found to explain ionic charge in a

precipitate as due to electron transfer between the species, even after acknowledging

those same ions were already present in the reagent solutions (Taber, 2002a).

A survey of students near the end of school science (14-16 year olds) and undertaking

A level study (16-19 year olds) suggested that students were commonly supportive of

statements reflecting the molecular framework, but often also agreed with statements

reflecting the curriculum model (i.e. that all adjacent oppositely charged ions are

ionically bonded together). Taber suggested that students often operated with a

mixture of the alternative conceptual framework and the curriculum model (Taber,

1997).

The survey instrument has recently been used in translation to survey samples of

undergraduate students in Greece and Turkey, and it was found that students in these

countries also commonly agree with statements based on the alternative conceptions

of ionic bonding elicited from UK students (Nakiboğlu, Tsaparlis & Taber, 2009).

Coll and Treagust (2003) reported on a study where students from New Zealand at

three educational levels - secondary school, undergraduate and graduate - were

interviewed about their mental models of bonding. They reported that “the secondary

school learners identified ionic bonding as an attraction between charged species” but

“related their mental models directly to the octet rule of full-shell stability” (p. 472).

So although the bond was understood in terms of “attraction between oppositely

18

charged species”, again, the octet rule was seen as the “sole driving force” for

bonding, leading to a transfer of electrons as an “automatic consequence” (p.478).

Coll and Treagust reported that the undergraduate students placed greater emphasis on

lattice structure and saw the attraction between oppositely charged species as the

principal driving force for formation of ionic bonding. However, even the graduates

related the formation of ions to the octet rule, and they quoted one of these graduate

students describing in anthropomorphic terms how “the sodium prefers to have a one

plus and the chlorine prefers to have a one minus” (p.474). At this level, Coll and

Treagust found that the octet rule was “routinely mentioned, but not seen as principal

driving force” (p.478).

The metallic bond

Taber reported that students commencing their A level studies in the UK generally

had limited knowledge of metallic bonding (Taber, 1997b, 2003a). He suggested that

their school level learning had focused on covalent bonding (as electron sharing) and

ionic bonding (in terms of electron transfer), and metallic bonding seemed to have

been given little attention. He found that students adopted the model of metallic

bonding in terms of a ‘sea of electrons’, but this seemed to often be learnt as a

‘slogan’ without deep understanding (student diagrams suggesting that the ‘sea’

metaphor implied a vast excess of electrons despite the lack of charge conservation

this implied).

For these students a chemical bond was understood to be a means of allowing atoms

to fill their shells, and students interpreted metallic bonding in terms that fitted this

framework: either considering metallic bonds not to be ‘proper’ bonds but just forces,

or as some variation on, or hybrid of, the ionic and covalent cases. The delocalised

electrons could be seen as shared around to fill electron shells, or as donated to the

lattice to allow ion formation.

Acar and Tarhan (2008) undertook a comparative study of learning about metallic

bonding in two classes of 15 year old students in Turkey. One class was taught by

traditional teacher and text-book led instruction; the other by cooperative group-work

guided by enquiry questions and supported by a teacher said to be acting as a

‘facilitator’. Distracters on the multiple choice post-test that referred to the bonding in

19

metals as being ionic, as being a type of ionic bonding, or looking like ionic bonding

were selected by fair proportions of the control group (approximately two-fifths, half,

and three-fifths respectively). Almost a third of this group selected a statement

referring to the interactions as ‘weak forces’. It should be noted that the students in

Acar and Tarhan’s research studied ionic and covalent bonding prior to being

introduced to metallic bonding. In this regard, Taber (2001a) has argued that students

will tend to interpret new classes of bonding in terms of previously learnt ones. Acar

and Tarhan report interview comments that support their view that a large proportion

of the students confused metallic bonding and ionic bonding in this study. These

comments suggest that it may be difficult to distinguish between students confusing

ionic and metallic bonding, and simply making sense of the new family of bonding in

terms of the more familiar category.

Polar bonding

It is clearly agreed by scientists that most bonds are polar, in the sense that the

electron density is neither equally shared (as in the covalent model) nor considered to

reside entirely around one atomic core (as in the ionic model).

Taber (1998) found in his interview study that when students learnt about polar bonds

they tended to consider them in terms of their existing model of covalent (sharing

between two atoms of non metallic elements) and ionic bonding (electron transfer

from atom of metallic element to atom of non-metallic element). Learning about

degrees of electronegativity should break down the metal/non-metal dichotomy of

element type that provides the basis for the covalent/ionic dichotomy of bond type.

However, as students understood the covalent and ionic bond types in terms of

distinct ways to obtain full outer shells, they did not readily adopt polar bonding

(which could not readily be explained in terms of the octet explanatory principle) as a

primary category of bond. Instead they tended to see polarisation as a secondary effect

imposed upon the more familiar pattern of covalent and ionic bonds. Indeed, although

it is possible to conceptualise polar bonds as distortions in either covalent or ionic

bonding, Taber found that his student generally saw polar bonds as examples of

covalent bonds where sharing was uneven. Although they learnt explanations of bond

polarity in terms of electrical interactions, this was superimposed upon their well-

20

established notion of a covalent bond as being the sharing of electrons to allow atoms

to have full shells.

Intermolecular bonding.

Most of the research studies that have been conducted regarding inter- and intra-

molecular forces clearly showed that in general students tend to confuse these two

categories. In his research Taber (1988) operated from a more inclusive notion of the

chemical bond, such that hydrogen bonds, van der Waals’ forces, solvation

interactions etc were all considered to be types of chemical bonding. He accordingly

considered student thinking deficient when they excluded these types of interactions

from their notion of the chemical bond. Once again, Taber found that students’

school-level learning about bonding in terms of ways of allowing atoms to fill their

outer electron shells acted as a barrier to accepting forms of bonding that could not be

explained in these terms. Taber characterized students as seeing interactions such as

van der Waals’ forces as not being ‘proper’ bonds.

Henderleiter, Smart, Anderson and Elian (2001) interviewed 22 undergraduate

students in a US public university who had completed the second organic chemistry

course of a two-semester sequence. They reported how “four students confused

hydrogen bonding with a covalent bond that exists between hydrogen and some other

atom”; five students thought that “hydrogen bonds can be induced”; and that five

students “confused intramolecular hydrogen bonding with a chemical reaction” and

thought that “intramolecular hydrogen bonding resulting in formation of new covalent

bonds”. They commented that “these results illustrate that some students completing

what is typically their second year of college-level chemistry still possess

misconceptions found in younger, less experienced students. Similar findings were

found by Levy Nahum et al, (2004), who reported that students in Israel confused

inter-molecular and intra-molecular bonding.

External sources that might cause learning difficulties and misconceptions

regarding learning the chemical bonding concept.

According to Gilbert (1982) and to Kind (2005), a key perspective will be that it is

often less useful to think about the problems mentioned above in terms of curriculum

science (the representations of scientific knowledge prescribed in the curriculum) as

21

being ‘correct’ or ‘true’, and alternative ideas presented by students as simply being

‘incorrect’ or ‘false’. Rather, when considering understanding of chemical bonding, it

is important to recognize that curriculum science comprises a set of models which are

intended to provide an authentic representation of the models used by scientists, at a

level accessible to learners (Gilbert, 2004). It is suggested that this complicates

judgments of the correctness of students’ conceptions, and that both the nature of the

scientific models, and the way the topic is taught, contribute to student learning

difficulties in the topic.

There are several reasons for dissatisfaction among the chemistry teaching community

regarding the current teaching and learning of this concept; we will refer to two main

components based on the literature: (1) the traditional pedagogical approach, as it

appears mainly in chemistry textbooks worldwide, and (2) the assessment methods

used worldwide (high stakes testing). For example, the Israeli Matriculation

Examination (ME) influences teachers' instruction and students' learning regarding the

bonding concept, as the teachers' main objective is in preparing their students for the

ME questions and answers, leading to superficial teaching which results in

misconceptions and pseudo-conceptions (Levy Nahum et al, 2004, 2007). This is

shown in Figure 2.

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___________________________________

Gilbert, Osborne and Fensham (1982) pointed out the importance of avoiding

confusion between what they called ‘children’s science’ and ‘scientist’s science’, or

for that matter ‘teacher’s science’. Certainly in terms of school science teaching, it

would often be quite inappropriate to consider current ‘state of the art’ scientific

models as suitable target knowledge to inform the planning of teaching and the

evaluation of student learning. Rather there is at least a two-stage process of

transformation between ‘scientific knowledge’ and the learning material set out for

students to learn in classes.

The first stage of transformation generally occurs at a system-wide level, beyond

particular schools or classrooms – at least in educational systems where there is a

22

prescribed curriculum, or a set of reference ‘standards’. In this process the models of

science inform the development of curricula models: representations of scientific

knowledge considered to be suitable for teaching and learning for a particular age

range. These curricula models are intended to be authentic representations of the

science, but at a suitable level of simplification. This process of curricula model

development is commonly undertaken by committees, which are often likely to be

dominated by educationalists who are not themselves the scientists who develop and

apply the scientific models in their professional work. There is inevitably a process of

interpretation therefore which may lead to aspects of the scientific models being

misconceiving and distorted.

An example of the type of argument that informs the development of curriculum

science is provided by Gillespie (1997) suggesting treatment of chemical bonding in

introductory general chemistry courses (at undergraduate level):

All chemical bonds are formed by electrostatic attractions between positively charged cores and negatively charged valence electrons. Electrostatic forces are the only important force in chemistry. Bonds are not formed by the overlap of orbitals, as we read not infrequently; this is just a model—admittedly a very useful one and essential for the chemistry major, but I don’t think it is essential for students at the introductory level. We can obtain a very good understanding of chemistry without it; indeed, many chemists make little use of it. It distracts attention from the real reason for bond formation: the electrostatic attraction between electrons and nuclei. … We can simply describe ionic bonds as resulting from the electrostatic attraction between ions, and covalent bonds as resulting from a shared pair of electrons’ attraction for the two atomic cores. The corresponding Lewis structures tell us how many bonds an atom will form. In my opinion, these concepts are all we need to discuss chemical bonding at the introductory level (p.862).

In this passage Gillespie rightly labels the notion of bonds as overlap of orbitals as

‘just a model’. However, the notion of bonds as ‘the electrostatic attraction between

electrons and nuclei’ is presented as thought this description is something more than a

model - the way things really are – where it might be better to consider this also as a

model. Whilst Gillespie’s argument about overcomplicating chemistry for novices has

merits, it is important to realize that it is not possible to explain all we want students

to understand purely in electrostatic terms. So to “simply describe …covalent bonds

as resulting from a shared pair of electrons’ attraction for the two atomic cores” asks

23

students to simply accept that negatively charged electrons can often be considered to

occur in pairs (something a purely electrostatic approach would suggest is unlikely),

and the adoption of Lewis models relies upon the heuristic of the octet rule, unless

quantum considerations are acknowledged to offer a theoretical basis for why

electrons appear to be found in somewhat discrete ‘shells’ of specific capacity (2, 8,

18 etc). This issue is revisited below.

Coll and Treagust (2001) reported an in-depth study of six Australian students’

preferred mental models of bonding, based upon the target knowledge according to

their “examination of curriculum material and interviews with instructors” (p.360).

They reported across three levels of learning (secondary school, undergraduate and

graduate). They wrote:

This inquiry resulted in the identification of what we have termed target models for each of the target systems… For metallic bonding there are two target models – the sea of electrons model and the band theory… For ionic bonding there are two models – a model based in part on the octet rule of full shell stability termed the electrostatic model, and a theoretical model based on the calculation of the forces present in an ionic lattice, termed the theoretical electrostatic model… For covalent bonding, a total of four models were identified – the octet rule, the valence bond approach, the molecular orbital theory, and the ligand field theory (pp.360-361).

Pedagogical content knowledge (PCK), since its inception as teacher-specific

professional knowledge, has been researched extensively (Kind, 2009). As shown in

Figure 2, the combination of the traditional pedagogical approach of curriculum

developers worldwide and the demands imposed by the common ME questions

generated a growing body of PCK with regard to this concept, which is overly

simplistic; it is not aligned with the up-to-date scientific knowledge and fails in

developing conceptual understanding. According to Magnusson, Krajcik and Borko

(1999):

PCK is a teacher's understanding of how to help students understand specific subject matter. It includes knowledge of how particular subject matter topics, problems, and issues can be organized, represented, and adapted to the diverse interests and abilities of learners, and then presented for instruction (p. 96).

The traditional PCK regarding the bonding concept (also termed content-specific

pedagogical knowledge, by Magnusson et al. (1999) has been developed during the

24

last decades within the high-school chemistry education community. This content-

specific pedagogical knowledge guides many teachers in their classrooms. According

to Taber (2005), one of the professional capabilities of the teacher is to find ways to

make complex ideas seem accessible, but this must be balanced by the need to present

material in a way that is scientifically valid, and provides a suitable platform for

future learning. In other words, the teacher needs to find the "optimal level of

simplification": simplifying sufficiently to suit the learners’ present purposes, but not

oversimplifying to undermine their future needs. Over the years, the traditional

pedagogical approach became increasingly simplistic and adopted clear-cut

definitions in order to facilitate students' learning. Unfortunately, the consequent

superficial teaching results in meaningless learning (see Figure 2); students often do

not understand these concepts and this is reflected in their misconceptions and in their

pseudo-conceptions (Vinner, 1997). In discussing the teaching of the chemical

bonding topic in Israel, Levy Nahum et al., (2004) wrote that:

In Israel, chemistry is taught and learned based on the same syllabus and curricular materials. The syllabus and the materials are centrally developed by the Ministry of Education. Thus, all the students study the same curriculum and teachers teach using the same books.

Moreover, they argue that:

the way that this topic is taught is highly based on the definitions of the key concepts. Based on the syllabus and the textbooks, and according to the Matriculation Examinations demands, teachers tend to define these concepts in certain ways and to use “rules” and classifications that are not in alignment with the scientific theoretical models (p.315).

The traditional approach is characterized by clear-cut definitions and rigid

distinctions and is an insufficient basis for rationalization of current chemical

knowledge. This problem might be amplified by traditional assessment methods, in

which the superficial study of classification and ‘rules’ by rote is rewarding to both

students and teachers because it allows for an efficient evaluation process based on

clear-cut answers to well-defined questions (Levy Nahum et al., 2004; Levy Nahum et

al., 2007).

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New directions for teaching the chemical bonding concept

A new bottom-up framework

Based on a long-term collaboration between prominent scientists, researchers in

chemistry education, and expert teachers, an innovative program aimed at teaching

the chemical bonding concept, which follows a holistic approach to curriculum (Levy

Nahum et al., 2007), was developed and implemented in 11th-grade chemistry classes

in Israel. Its general approach relies on basic concepts such as columbic forces and

energy at the atomic level to build a coherent and consistent perspective for dealing

with all types of chemical bonds.

As described by Levy Nahum et al. (2008, p. 1680): “It is possible to show how

this diversity [of bond types] arises from a small number of fundamental principles

instead of presenting it as a large number of disparate concepts”. The framework

proposed by Levy Nahum et. al (2008) (see Figure 3) introduces the elemental

principles of an isolated atom (stage 1); it then follows by discussions of general

principles of chemical bonding between two atoms (stage 2), and the general

principles are then used to present the different traditional categories of chemical

bonding as extreme cases of various continuum scales (stage 3). Equipped with this

knowledge, students can then construct a coherent understanding of different

molecular structures (stage 4) and properties (stage 5).

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___________________________________

The primary purpose of the first stage is to provide a qualitative description that is

conceptually consistent with quantum mechanics but gives a very clear, intuitive

answer to the question which puzzles many students, ‘what really causes atoms to

interact and form a chemical bond?’

In order to provide evidence to students that there is nothing ‘mysterious’ about

chemical bond formation, this stage begins by introducing the concepts of energy and

force and the interrelation between them. The understanding that nuclei are held

together because of nucleus-electron attraction, which is a simple consequence of

26

Coulomb's law, is the first step towards a rational view of chemistry which is not

based on rules of thumb, anthropomorphic concepts, etc. A crucial concept is that

stability, in general, is obtained by minimizing energy. The above principles are best

explained by considering the energy curve for any two isolated atoms (figure 4).

Once this is understood, all chemical bonds, of any type, can be rationalized in

terms of energy stabilization (i.e., bond energy) and all equilibrium inter-atomic

distances (i.e., bond lengths) reflect positions where there is no net force on the

nuclei, i.e., attraction balances repulsion.

___________________________________


___________________________________

Importantly, Figure. 4 is general. It describes the relation between energy and

inter-nuclear distance for the H2 dimer as well as it describes the Li2 dimer, the NaCl

hetero-dimer, or even the He2 dimer! Obviously, there is very much that separates H2

and He2. One of the key goals of the proposed framework is to emphasize that a

continuum scale exists between extreme cases of qualitatively different bonding

scenarios. Having understood, in stage 2, the common denominator of all bonds, some

distinct bonding categories can now be rationalized, as shown in Fig. 5. The example

of a chemical bond in a diatomic molecule may be the most instructive because of the

focus on one bonding entity only. In this context, the concept of electronegativity can

be introduced, naturally, as one way of quantifying the covalence/ionicity balance.

This continuum follows Pauling (1967), who recognized that bonds between unlike

atoms typically have greater bond energy than that of the average of the

corresponding homo-atomic bonds. However, bond strength is not only a function of

the degree of ionicity but also a function of atomic size and other factors.

__________________________________


___________________________________

Similar arguments are also given and discussed in relation to hydrogen and van-

der Walls interactions (for more details see Levy Nahum et al, 2008). For instance,

27

the He2 dimer - a large-bond-length, very weakly bound molecule – can be used for

introducing the concept of induced-dipole induced-dipole interactions, which can be

rationalized as yet another manifestation of Coulomb’ law.

In the preliminary results of her research, Levy Nahum (2007) found several

practical advantages of the new framework. Generally, the proposed new framework

overcomes a major difficulty in the traditional ‘top-down’ approach by removing the

artificial division between different types of bonding. Instead, a variety of bonds are

introduced to the students from a continuum point of view. Furthermore, a gradual

exposure of the main concepts and ideas, in five stages, allow overcoming the

dichotomous classification without falling into the trap of over-simplifications and

over-generalizations. Most importantly, such a framework facilitates the attainment of

two important objectives: First, preventing pedagogical impediments for further

studies. Second, fostering the understanding that molecular species and bonding

scenarios that textbooks often designate as ‘exceptions’ can, in fact, be rationalized by

the same small number of principles used to rationalize the ‘regular cases’. Levy

Nahum et al report that at present, this unit has been tested in ten 11th-grades classes

and more research is clearly needed. The following quotes strongly enhance Levy

Nahum’s faith in the presented approach, suggesting that the new approach

encourages teachers and students to ask relevant questions and be interested of

understanding bonding principles, whereas the consequence of the traditional

presentation reduces students' reasoning and questioning, which is essential for a

deeper understanding of the main ideas. Both students and teachers acquired a much

deeper understanding of the underlying key concepts regarding bonding.

Teacher: I loved the idea that I can teach and explain all bonds based on a uniform

model. Starting from sub-microscopic ideas and moving up to the material world improved

the students' learning and thinking.

Student: The continuum scale of bonds helped me to understand…last year the teacher

said: 'it's one of the two' (covalent/ionic).

In the academic year 2010, the new program will be implemented in all 11th-grade

chemistry classes in Israel, which is possible because the educational system is

centralized. In due course, a full scale procedure, assessing both the teaching and

learning will be conducted, aiming at reporting broader and statistically sound field

28

results. In this system framework, building off a knowledge-in-pieces perspective,

Yayon, Mamlok-Naaman and Fortus (2010) describe the development and testing of a

matrix that represents a systematic organization of the canonical knowledge on

chemical bonding required at high-school level and a tool for representing students’

knowledge of bonding. The matrix contain three strands: the structure of matter at the

nano-scopic level, electrostatic interactions between charged entities, and energy

aspects related to bonding.

However, Levy Nahum (2007) is also aware of the weakness of such a

framework – using abstract theoretical ideas right from the start may prove difficult

for some students. According to Loucks-Horsley and Matsumoto (1999), in the

framework of reform in science education an extensive, dynamic, and long-term

professional development of science teachers should take place. Teachers need to

receive guidance and support throughout the various teaching and implementation

stages involving changes in the curriculum. It has been noted by Joyce and Showers

(1983) that teachers, in general, are excellent learners, and are interested in trying to

teach a new curriculum, as well as in improving and enriching their teaching methods.

However, in the context of introducing and implementing of this new unit, the

psychological process that teachers have to undergo is unique due to two main

reasons: (1) the importance of the topic - many chemistry teachers (and chemists) feel

that bonding is the "heart of chemistry", and (2) in Israel, this topic was taught for

decades in a very definite way, and, since the new curriculum will be implemented

starting from 2010, all the teachers have to adopt it and apply it.

Nevertheless, the teachers understood that the oversimplified teaching

approach they used routinely, for so many years, encouraged their students to learn by

rote-memorization rather than by using their thinking skills and scientific tools.

Teaching models: Finding ways to help students access curriculum science

A further stage of interpretation occurs when an individual teacher, or textbook

author, uses the curricular models as the starting point for developing teaching.

Teachers, in particular, will need to consider how they help particular groups of

students access the curricular models. They will often use further simplifications and

various forms of teaching models, designed to link with students’ current knowledge,

29

understanding and interests, and inevitably giving opportunities further potential for

distorting the original science. Again, the transformation will be undertaken by

someone distant from the development and application of the original scientific

models. Clearly, through these transformations processes there will be distortions of

the original scientific models: some intentional and principled, but possibly also due

to the limitations of the scientific understanding of those in charge with shaping

curricular and classroom knowledge. One particular concern raised by Justi and

Gilbert (2000) was the adoption of ‘hybrid’ models as curricular models. During the

historical development of scientific models, there may be many discrete models

proposed. Each of these will represent a certain stage of knowledge (the response to

combining the strengths of previous models with new features that add further

explanatory power in the light of anomalies or new data or a desire to expand the

range of application of a model). Within such a context, a scientific model will

usually be consistent and logical: but appreciating the features of the model may be

difficult without awareness of that context. According to Justi and Gilbert, who

considered the case of representation in the curriculum of models of the atom,

curriculum models can often be ‘hybrid models’ that are true to no specific historical

model but are rather “composites drawn from several distinct historical models”

(p.993). Such hybrids cannot readily be understood in terms of logical development of

the subject.

This characteristic of curricular models adds to the already significant burden for

students who may well be introduced to a range of models (some curricula models

intended as target knowledge; some teaching models intended as scaffolding towards

such knowledge; perhaps some awareness of formal scientific models from the media

or personal reading) with different but overlapping ranges of application. To the

teacher, there may be a logical sequence to support developing understanding (as in

the different models of oxidation or acidity), but for the students there is a very real

risk of ‘model confusion’ (Carr, 1984).

An example of this would seem to be basing teaching about chemical reactions and

bonding around the octet rule. Taber characterised students as entering study at

college/senior high school level holding a notion of chemical bonding deriving from

school study, and tied to the octet rule. He also identified common tendencies and

30

conceptions which were reported as an alternative conceptual framework (see the

section on students’ conceptions of bonding, above) – that is a model representing the

common features found across a range of students (Taber, 1998).

Instructional techniques

Chemical structure and bonding is a topic in which understanding is developed

through diverse models, which, in turn, are built upon range of physical principles;

and where students are expected to interpret a disparate range of symbolic

representations standing for chemical bonds (Taber & Coll, 2002). According to

Johnstone (1991) and Gabel (1996), matter can be represented on three levels,

macroscopic (physical phenomena), microscopic (particles), and symbolic-

representational (chemical and mathematical language). The symbolic level can be

seen as having the key role of mediating between the phenomenological-descriptive

level of what students can directly perceive and the abstract conceptual level of

theoretical entities such as quanticles (e.g. ‘H2’ can stand for both the substance and

the molecule, and so acts as means of linking one to the other). Yet in the teaching of

chemistry, teachers repeatedly (‘instinctively’) move from one level to another in their

teaching without making this explicit to their students each time they are doing it:

missing an opportunity to help the students integrate the levels and potentially

confusing them in terms of the current referent for a symbol (Taber, 2009b).

It is assumed by both curriculum developers and those involved in research on

learning that the models used can enhance scientific learning (Lunetta & Hofstein,

1981). It is however suggested that students are poorer at modeling than commonly

expected by teachers. Moreover, Gilbert (1998) argues that students tend to think that

models are scale replicas or incomplete copies of actual objects, and therefore they do

not look for ideas or seek purposes in the model form. Harrison and Treagust (2000)

however claim that models are more than communicative tools: rather that they are

also important links in the methods and product of science. Moreover they suggest

that students who participated in negotiating the shared and unshared attributes of

common analogical models for atoms, molecules, and chemical bonds actually used

these models more consistently.

31

The use of visual models for teaching the bonding concept

In recent years computer technologies in general, and web-based teaching and

learning in particular, have gained momentum in teaching and learning the sciences.

Several studies have noted the benefits of web-based learning and its vast potential to

empower learning and teaching in terms of its visualization, accessibility, and

dynamicity (Capri, 2001; Clark, 2004; Linn, Clark, & Slotta, 2003; Mistler-Jackson,

2000). More specifically, in alignment with the idea of visualization to support

students' learning the chemical bonding concept, Clark, (2004) noted the importance

of integrating computer-based visualizations in learning abstract concept and

phenomena. Kozma and Russel (2005) suggested that molecular models, simulations,

and animations have the potential to contribute to the learning of chemistry in general

and to better understanding of the chemical bonding concept in particular. Gilbert and

Justi (2002) wrote that the use of computerized models has been advocated as a way

to improve students' understanding of chemical phenomena by translating information

expressed in different representations (see also Kozma & Russel, 1997). Moreover (as

Justi & Gilbert (2002) wrote), studies on students' use of computer-based models have

shown that they can also improve students' visualization in chemistry (Barnea & Dori,

2000).

A recent study conducted in Israel by Frailich, Kesner, and Hofstein (2009)

investigated the effectiveness of a web-based environment in enhancing 10th grade

students’ understanding of the concept chemical bonding. Computer-based visual

models were developed and implemented for the purpose of demonstrating bonding

and the structure of matter. These were substantially based on student-centered

learning. Drawing on a combined quantitative and qualitative research study Frailich

et al. (2009) were able to conclude that the web-based learning activities which

integrated visualization tools with active cooperative learning strategies provided

students with opportunities to construct their knowledge regarding the abstract aspects

concept of chemical bonding.

32

Summary

In this review we have considered teaching and learning of one of the central concepts

in chemistry, bonding. We have considered the nature of the concept, and the reasons

why it might present a challenge when included in the upper secondary/senior high

school or college curriculum. We have suggested that some aspects of traditional

ways of teaching the topic may have contributed to the learning difficulties faced by

many learners. This was reflected in the literature discussing learners' conceptions of

chemical bonding where some key issues (over-reliance on the covalent-ionic bonding

dichotomy; adopting the octet rule as the basis of explanations of bonding; seeing

bonds as ontologically quite different from physical forces) seem to reflect common

teaching approaches.

A key theme here is the central involvement of modeling at all stages. Chemists use

models of chemical bonding, which are considered useful in supporting their work. It

has been suggested that for many chemists, the conceptual bonding ‘tools’ they

regularly apply may be somewhat different from the most advanced models being

developed by those chemists who actually research into ways of understanding

bonding itself. This can be understood as a pragmatic matter, which many chemists

behaving as instrumentalists, being content to apply bonding models that are ‘fit for

purpose’ in their own areas of research, despite having been shown to be deficient in

terms of not giving a full and authentic account of bonding in some contexts.

Traditional science curricula have included accounts of bonding models selected for a

mixture of their perceived importance in the subject and their judged suitability for

the students concerned. However, such an approach is based on the assumption that

school science and general cognitive development will allow learners to meet and

mater a sufficient sequence of curriculum models to reach the desired end-point at the

end of their upper secondary / senior high school courses. However, based on the

research evidence discussed in this review paper, for students this might be an

unrealistic assumption. Matters are even more complicated in such cases in which

chemistry (physical science) teachers may come to consider, or at least to teach as if

they consider, the curriculum models, which themselves are simplifications of the

chemical concepts, as something more than limited models that are partial

representations of the ‘real’ chemistry. If what is meant to be an education transition

33

state is understood instead as the desired pedagogic product, it becomes clear why

students commonly ascribe limited ideas (bonding dichotomies, the significance of

octets or full electron shells) the status of the way chemists have found the world to

be. Something that was meant to have transitional epistemological value is instead

understood as being absolute ontological reality.

We have shown in this review that the research literature is rather rich in information

resulting from diagnostic studies that were conducted over the years in many

countries. However, we have observed that although vast amount of information

exists it has had only limited influence on curriculum development (development of

learning materials and pedagogical remedies), teachers' professional development,

and classroom practice. In other words there is a huge gap between what we know

about learning the chemical bonding concept and what has been implemented in

educational systems. We have shown that the traditional approach to the teaching of

the concept is strongly embedded in the teachers' knowledge and in the various

curricular approaches around the world.

The chemical bond is one of the key concepts that should be taught in every upper

secondary school, college and undergraduate chemistry course. As an epilogue to our

review, we sincerely hope that the conglomerate of information presented in this

paper will serve as a vehicle for new, more scientifically and pedagogically sound,

curricula that will reach the teachers in the school system.

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Figure 1: A schematic illustration of the traditional approach for teaching chemical bonding (Levy Nahum et al 2008)

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Figure 2: The curricular process and outcomes regarding the traditional teaching of the bonding concept in Israel

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Figure 3: A schematic illustration of a new "bottom-up" framework for teaching

chemical bonding

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Figure 4: A schematic energy curve for any two atoms that interact

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Figure 5: A schematic continuous scale of bond strengths

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Category Misconception

Bond polarity:

• Equal sharing of electron pairs occurs in all covalent bonds.

• The polarity of a bond is dependent on the number of valence electrons in each atom involved in the bond.

• Ionic charge determines the polarity of the bond.

Molecular shape:

• The shape of a molecule is due to equal repulsion between the bonds.

• Bond polarity determines the shape of a molecule.

• The V-shape in a molecule of the type SCl2 is due to repulsion between the non-bonding electron pairs [only].

Intermolecular forces:

• Intermolecular forces are the forces within a molecule.

• Strong intermolecular forces exist in a continuous covalent solid.

• Covalent bonds are broken when a substance changes shape.

Polarity of molecules:

• Non-polar molecules [only] form when the atoms in the molecule have similar electronegativities.

• Molecules of the type OF2 are polar as the non-bonding electrons on the oxygen form a partial negative charge.

Octet rule: • Nitrogen atoms can share 5 electron pairs in bonding.

Lattices: • High viscosity of some molecular solids is due to strong bonds in the continuous covalent lattice.

Table 1: Misconceptions related to covalent bonding reported by Peterson and colleagues

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