alternative conception – Page 8 – Science-Education-Research

Carbon electrons will be bigger than chlorine electrons

Carbon electrons will have more mass and charge than chlorine electrons

Keith S. Taber

Annie was a participant in the Understanding Chemical Bonding project. She was interviewed near the start of her college 'A level' course (equivalent to Y12). She was shown a representation of a tetrachlomethane molecule.

Understanding Chemical Bonding project – Focal figure 3

When Annie was asked about the diagram, she was not sure if the differently represented electrons would actually be different from each other, She suggested that perhaps electrons from different atoms would actually contain some of the particular element. Annie seemed unsure where one could tell the difference between electrons from different atoms, but her intuition seemed to tell her they should be different,

Under further questioning, Annie was able to suggests ways in which carbon electrons would be different from chlorine electrons. Most science teachers may expect it would be quite obvious that one electron is much like another one in terms of essential properties (e.g., charge, rest mass). We probably assume students will readily appreciate this, and perhaps that it is not a point that needs to be emphasised. We might expect a student would immediately reject any suggestion that electrons from different atoms should be fundamentally different.

Do you think they would be the same size, electrons from carbon and electrons from chlorine?
No.
Which ones will be bigger, do you think?
The carbon ones.
Do you think they're the same charge? The same electrical charge?
No.
(pause, c.5)
No, which one do you think will have a bigger charge?
(pause, c.2s)
The carbon.
…
Yeah, what about colour. What colour do you think they will be?
Colours. What of the actual electrons?
Mm.
Mm, (pause, c.5s) I don't think they'd really have a colour, but I think if they had to have a colour, then they'd pick out the colour from the element.

A teacher is likely to expect an A level student to appreciate that all electrons are intrinsically the same. Annie seemed to think that the electrons of different atoms were different, somehow reflecting the particular element, and open to the idea they may differ in mass and charge, and possibly even colour.

Whilst Annie's comments are at odds with canonical science, they reflect thinking that is quite common among learners who often fail to appreciate the core principle of sub-microscopic models of matter, i.e., that the emergent properties of matter at macroscopic scale are explained in terms of the different properties of the tiny particles (i.e., quanticles) from which matter is conjectured to be constituted at a much finer scale. She was not keeping clearly distinguished macroscopic properties (such as colour) and properties that sub-atomic particles could have.

Electrons would contain some of the element

Electrons from different elements would be different – perhaps because they would actually contain some of the element in the electron?

Keith S. Taber

When Annie was asked about the diagram, she noted that (following a representational convention) the electrons were represented differently. Using different symbols like this is quite common, but is little more that a bookmaking tool – to help keep count of the number of electrons in the molecule in relation to those that would be present in discrete atoms.

…are there any bonds [shown] in that diagram do you think?
Yes.
How many?
Four.
Four bonds, so we've got four bonds there. Erm, are the bonds actually shown?
Yeah.
So how are they represented on the diagram?
By the circles that overlap, and they're showing it by the electrons, the outer-shell electrons in the chlorine have got black dots and the ones from carbon have got just circles.
Okay. So the carbon electrons and the chlorine electrons are signified in a different way
Yeah.

I followed up this point to check Annie understood that the convention did not imply that there was any inherent difference between the electrons.

So what would be the difference between a carbon electron and a chlorine electron?
(pause, c.5s)

The expected answer here was 'no difference', but the pause suggested Annie was not clear about this. So I set up an imaginary scenario, a kind of thought experiment:

If I gave you a bottle of electrons – which I can't do – how would you be able to tell chlorine electrons from carbon electrons – in what ways would they be different?
They would be different because, erm, I don't know if they would actually contain some of the element in the electron.
Do you think they might have little labels on some with "C"s and some with "Cl"s or
Yeah, I don't know if you got an electron, and you could sort of if you took one single one you could say, right that's chlorine and that one's carbon.
You are not sure, you are not sure if you could, or not?
No.

The idea that an electron might contain some of the element seems to miss the key idea that macroscopic phenomena (samples of element) are considerer to energy from extensive ensembles of submicroscopic particles ('quanticles').

Annie did not seem too sure here – perhaps her intuition was that a carbon electron would be different to a chlorine electron, but she could not suggest how. Electrons have no memories, and there is no way of knowing whether an electron has previously been part of a particular atom (or ion or molecule). A free electron is not meaningfully a chlorine electron or a carbon electron. However, students do not always appreciate this, and may consider that free electrons in some sense belong to an atoms they they derived form, and even that this may later have consequences (as with the 'history' conjecture in thinking about ionic bonding).

Annie went on to suggest that carbon electrons would be bigger than chlorine electrons.

Peter and Patricia Pigeon set up house together

Keith S. Taber

In my work I've spent a lot of time analysing the things learners say about science topics in order to characterise their thinking. Although this work is meant to have an ethnographic feel, and to be ideographic (valuing the thinking of the individual in its own terms), there is always an underlying normative aspect: that is, inevitably there is a question of how well learners' conceptualisations match target curricular knowledge and canonical science. We all have intuitions which are at odd with scientific accounts of the world, and we all develop alternative conceptions – notions which are inconsistent with canonical concepts.

Peter and Patricia started seeing each other at this local fence earlier this year.

Soon passion got too much for them and they (publicly) consummated their relationship on this very fence (some birds have no shame).

It is easier to spot this in others (you think what?!) than it is in ourselves. But occasionally you may reflect on the way you think about a topic and recognise aberrations in your own thinking. One of these examples in my own thinking relates to bird's nests. I know that birds build nests as a place to lay and hatch eggs. Using the ground would be very dangerous due to vulnerability to predators. Simply using branches would be precarious – especially as eggs are hardly best shaped to be balanced on a tree branch. I also know that once the young are fledged have fled the nest, it has outlived its purposes.

They quite liked the area, and decided to look for a place nearby.

Soon they had identified a nice place to build their new home in some nearby ivy.

Yet it was only a few years ago – I think when came across discarded nests in the garden – that I released I have carried around with me since quite young the metaphor that a nest is a bird's home – it is where the bird family lives. Perhaps I made up that idea as a child. More likely I was told that or heard it on a children's programme. If so, perhaps it was not meant to be taken too literally – it was just meant to compare the nest with something that would be familiar to a child. But I think well into adulthood I had this notion of that birds lived in trees – not explicitly, but insidiously in the back of my mind: as if a bird had a home in a tree and that was where it was based – unless and until perhaps it could afford to move upmarket to a better tree!

They decided to do their own build, which involved Peter in the tiring work of going out to get building materials.

Peter set about the serious business of setting up their new dream home.

Peter was quite confident, and would often return which rather large pieces of nesting material.

"Oh, that seems to have got caught up."

Over time Peter started to be more realistic in selecting material he could get through the front door.

Although I was well aware (at one level) that birds do not have permanent family homes to which they return at the end of a hard day's exertions, I also had this nest=home identity at the 'back of my mind' giving the impression that this is how birds live. As humans we take for granted certain kinds of forms of life (perhaps home, work, family, etc.), and these act as default templates for understanding the world. This makes anthropomorphising nature seem quite a natural thing to do.

Peter heading out to work, again.

And getting home with his latest acquisition – landing on his feet.

Watching this process develop was quite entertaining. Peter would spend ages pecking at pieces of plant that were firmly fixed in the ground, ignoring nearby loose material. His early attempts to take material back to the nest were troubled. He would take material that was too large to get through the foliage into the secluded nesting area. He would also fly close to 'home' and then abort as found he could not land with his goods. However, he soon seemed to learn what worked, and developed a technique of first flying onto the fence or the roof the ivy was growing on to, so he would not be flying up to the nesting place from the ground in a single stage.

The sequences below show the pigeon flying out from, and back to, the nest.

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The jumping/diving action is clear in the sequence below:

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The fourth and fifth frames in the sequence below show the 'landing gear' coming into position (reminiscent of a bird of prey taking its prey):

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The landing action is also clear near the end of the sequence below:

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Another take off. catching the first few flaps:

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My favourite sequence – quite extended for my hand-held camera work! – in the 11th frame our pigeon is just entering frame right. But notice a sparrow sitting on top of the foliage to the left. The sparrow has presumably seen/heard the much larger bird comings it way, and in the next frame can be seen to be moving its wings ready to take off. The next three frames have the sparrow heading right as the pigeon moves to the left (the sparrow is a smudge beneath the pigeon's left wing in the third of these frames), and the sparrow appears to have disappeared from view in the next, but must have been obscured by the pigeon as it seen to the right of the next frame. The sequence ends with the pigeon in landing mode.

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Electrons repel each other, keeping them out of the nucleus

Keith S. Taber

Brian was a participant in the Understanding Chemical Bonding project. He was interviewed during the first year of his college 'A level' course (equivalent to Y12 of the English school system). Brian was shown, and asked about, a sequence of images representing atoms, molecules and other sub-microscopic structures of the kinds commonly used in chemistry teaching. He was shown a simple representation of an atom which he identified as showing "electron configuration…of an element, sodium".

Brian identified the electrons and nucleus, and was asked about the arrangement of the electrons:

Can you tell me why the electrons stay there, in these positions, why they don't fly off into space?
'Cause they're held by the nucleus.
In what way does the nucleus hold them, any idea?
It's got a positive charge, and so attracts the electrons, which are negatively charged.
Okay, so, it's got an electrical attraction there.
Yeah.
Why don't they just go into the nucleus then, if they're attracted, why don't they just get pulled into the nucleus?
Because, 'cause there's more than one electron, they repel each other, and keep them out.
Ah, so what about these ones [on opposite sides of the nucleus] though, these repel each other do they, even though they
Yeah.
are drawn on opposite sides?
Yeah.
So that's what stops them actually falling into the nucleus, that they repel each other?
Yeah.

It seems that Brian recognised electrical interaction between the nucleus and the electrons in an atomic structure. He also recognised that electrons would repel each other, but did not seem to have considered that in itself that was an insufficient explanation for the structure of the atom (as, for example, the sole electron in a hydrogen atom does not fall into the nucleus).

Although Brian's explanation was based on sound principles (negative electrons repel each other), it is an alternative conception. Coulombic forces are proportional to charges and diminish with separation – inspection of the figure should suggest that the two inner electrons (tending to be pushed inwards by outer electrons) at least must experience net force towards the nucleus.

The stability of atoms – the failure of electrons to spiral into the nucleus leading to atoms collapsing – was one of the phenomena which led to the development of quantum theory. In classical physics the stability of electron orbits was a puzzle to be solved, as orbiting electrons 'should' have acted as electrical oscillators, and emitted energy as their orbits decayed into the nucleus whilst the atom (very quickly) collapsed. Quantum theory posited limited allowed energy states, rather than a continuum of possibilities – but learners new to the topic do not know about this.

Often learners simply accept atomic structure when presented with planetary-system type representations of the atom. 'Quanticles' such as atoms are so far from direct human experience that they presumably seem strange enough such that questions that might seem obvious to a teacher do not arise for students. (Students also commonly accept the 'atom is like a tiny solar system' teaching analogy, and may map inappropriately between the two systems.)

Dissolving salt is a chemical change as you cannot turn it back

Dissolving salt is a chemical change as you cannot turn it back as it was before

Keith S. Taber

Sandra was a participant in the Understanding Science Project. When I interviewed Sandra about her science lessons in Y7 she told me "I've done changing state, burning, and we're doing electricity at the moment". I asked her about burning:

Well, tell me a bit about burning then. What's burning then?
It's just when something gets set on fire, and turns into ash, or – has a chemical change, whatever.
Has a chemical change: what's a chemical change?
It means something has changed into something else and you can't turn it back.
Oh I see. So burning would be an example of that.
Yeah.

So far this seemed to fit 'target knowledge'. However, Sandra suggested that dissolving would also be a chemical change. Dissolving is not normally considered a chemical change in school science, but a physical change, the distinction is a questionable teaching model. (Chemical change is said to involve bond breaking/making, and of course dissolving a salt does involve breaking up the ionic bonding to form solvent-solute interactions.)

Are there other examples?
Erm – dissolving.
So give me an example of something you might dissolve?
Salt.
Okay, and if you dissolve salt, you can't get it back?
Not really, not as it was before.
No. Can you get it back at all?
Sort of, you can like, erm, make the, boil the water so it turns into gas, and then you have salt, salt, salt on the, left there. Sometimes.
But you think that might not be quite the same as it was before?
No.
No. Different in some way?
Yeah
How might it be different?
Be much smaller.
Oh I see, so do you think you'd have less salt than you started with?
You'd have the same, but there would just be more particles, but they'd be smaller.
Ah, so instead of having quite large grains you might have lots of small grains
Yeah.

So Sandra was clear that one could dissolve salt, and then reclaim the same amount of salt by removing the solvent (water) which from the canonical perspective would mean the change was reversible – a criterion of a physical change.

Yet Sandra also thought that although the amount of salt would be conserved, the salt would be in a different form – it would have different grain size. (Indeed, if the water was boiled off, rather than left to evaporate, it might indeed be produced as very small crystals.)

So, Sandra seemed to have a fairly good understanding of the process, but because of the way she interpreted the criterion of a chemical change, something [salt] has changed into something else [solution] and you can't turn it back [with the same granularity]. Large grains will have changed into small grains – so this would, to Sandra's mind, be a chemical change.

Science teachers deserve a great deal of public appreciation. A teacher can teach something so that a student learns it well – and yet still form an alternative conception – here because of the inherent ambiguity in the ways language is used and understood. Sandra's interpretation – if you start off with large particles and end up with smaller particles then you have not turned it back – was a reasonable interpretation of what she had learnt. (It also transpired there was ambiguity in quite what was meant by particles.)

Puppies that automatically retrieve your stick

Dogs that have been taught over and over to retrieve have puppies that automatically have already got that sense

Keith S. Taber

Bert was a participant in the Understanding Science Project. In Y11 he reported that he had been studying about the environment in biology, and done some work on adaptation. he gave a number of examples of how animals were adapted to their environment. When asked to explain how this occurred he initially used an example of selective breeding in dogs.

…our homework we did about adapting, like how polar bears adapt to their environments, and camels….
And so a polar bear has adapted to the environment?
Yeah.
So how has a polar bear adapted to the environment?
Erm things like it has white fur for camouflage so the prey don't see it coming up. Large feet to spread out its weight when it's going over like ice. Yeah, thick fur to keep the body heat insulated.
What about a camel then?
Well it has long eyelashes to keep the sand out of it. It has pretty much all its fat stored in its hump so that it can erm, so all the body, so that not much body heat is produced from everywhere else. It doesn't have hair on its belly to increase heat loss. And yeah, oh yeah, they're quite big so it has quite a lot of grip on the sand.
…
No, okay. So do you have any other examples of adaption?…
Oh well, well there's humans isn't there. Because like they started off like with an arched back and they went on all-fours and everything. And well their minds obviously have adapted and evolved, yeah. Erm (pause) and dogs, they have different … because people who are actually breeders, they, when they breed dogs they breed them to be like, like Retrievers. Because they've like been taught over and over to retrieve. And so when they have puppies then they automatically have already got that sense. That's not really adapting though is it?
So somebody has trained these dogs to go and, when they shoot birds or something, they're trained to go and get the birds they've shot and bring them back?
Yes.
Okay. And if you do that enough, baby puppies bred from those dogs will just know to do that?
Well they won't know to do that, but they'll already have that kind of sense. And like, well my dog that I have, it's a Chocolate Labrador, and I said look, she had webbed feet which is adapted for swimming, for retrieving, I don't know, retrieving birds from water or something.

Although Bert was aware of how traits could be passed on to offspring he was thinking in terms of the inheritance of acquired characteristics – a Lamarkian model of evolution – rather than the selection of qualities that vary across a population. For some pupils the notion of evolution makes sense, but in terms of changes that occur in an individual in response to environmental challenges being somehow passed on to their offspring. The inheritance of acquired characteristics is a scientific concept, that is a historical (scientific) concept, but not a canonical (current scientific) concept, so Bert's understanding of evolution would be considered an alternative conception.

(Bert then went on to consider an example of a naturally occurring adaptation, the polar bear's fur, however he again considered this in terms of an acquired characteristic being passed on to future generations.)

A chemical bond would have to be made of atoms

Keith S. Taber

Amy was a participant in the Understanding Science Project. When I had talked to Amy when she was in Y10 she had referred to things being bonded: "where one thing is joined on to another thing, and it can be chemically bonded" and how "in a compound, where two or more elements are joined together, that's an example of chemical bonding".

The following year, in Y11, when she was studying fats she talked about "how they're made up and like with all the double bonds and single bonds" where a double bond was "where there are kind of like two bonds between erm carbon atoms instead of like one" and a bond was "how two atoms are joined together". Later in Y11, Amy told be that she did not know how to explain chemical bonding, but "in lessons like we've always been shown these kind of – things – where you kind of, you've got the atom, and then you've got the little, grey stick things which are meant to be the bonds, and you can just – fit them together."

Source: Image by WikimediaImages from Pixabay

As Amy had told me "everything is made up of atoms", I provocatively asked her if the chemical bond was made of atoms. Amy had "absolutely no idea" but she "suppose(d) it would have to be, wouldn't it".

Not only is this an alternative conception, but to a chemist, or science teacher, the idea that chemical bonds are themselves made up of atoms seems incongruous and offers a potential for infinite regress (are those atoms in the bonds, themselves bonded? If so, are those bonds also made of atoms?)

This alternative conception could be considered a kind of associative learning impediment – that is where a learner makes an unintended link and so applies an idea outside of its range of application. All material is considered to be made of atoms – or at least quanticles comprising one of more nuclei bound to electrons (i.e., ions, molecules). Even this is not an absolute: the material formed immediately after the big bang was not of this form, and nor is the matter in a neutron star, but the material we usually engage with is considered to be made of atom-like units (i.e., ions, molecules).

But to suggest that Amy has made an inappropriate association seems a little unfair. Had Amy thought "all matter was made of atoms" and then suggested that chemical bonding was made of atoms this would be inappropriate as chemical bonding is not material but a process – electrical interactions between quanticles. Yet it is hard to see how one can over-extend the range of 'everything', as in "everything is made up of atoms".

There is an inherent problem with the motto everything is made up of atoms. It is probably something that teachers commonly say, and think is entirely clear – that it is obvious what its scope is – but from the perspective of a student there is not the wealth of background knowledge to appreciate the implied limits on 'everything'.

Learners will readily pick up teaching mottos such as "everything is made of atoms" and take them quite literally: if everything is made of atoms then bonds must be made of atoms. So although she was wrong, I think Amy was just applying something she had learnt.

She'd never thought about whether ionic bonding is the same thing as chemical bonding

Keith S. Taber

Amy was a participant in the Understanding Science Project. When I talked to her near the start of her GCSE 'triple science' course in Y10 she told me that ionic bonding was "atoms which have either lost or gained electrons so they are either positively or negatively charged" and that chemical bonding was "like in a compound, where two or more elements are joined together", but she seemed unsure how the two concepts were related.

I followed up on Amy's use of the term 'compound' to explore how she understood the term:

How would you define a compound?
Erm …Something which has erm two or more elements chemically bonded.
… So you give me an example of that, compound?
Erm, sodium oxide.
Sodium oxide, okay, so there are two or more elements chemically bonded in sodium oxide are there?
Uh hm
And what would those two or more elements be?
Sodium and oxygen.
Okay. Erm, so when we say sodium oxide is chemically bonded, what we are saying there is?
[pause, c 2s]
Erm – a sodium atom has been bonded with a oxygen atom to form erm a new substance.

So Amy's example of a compound was sodium oxide, which would normally be considered essentially an ionic compound, that is a compound with ionic bonding. So this gave me an opportunity to test out whether Amy saw the bonding in sodium chloride and sodium oxide as similar.

Okay, so that was chemical bonding,
Mm.
and that occurs with compounds?
Yeah.
And what did you say about ionic bonding?
Erm, it's the outer electrons they are transferred from one element to another.
Now what does that occur in? You gave me one example, didn't you?
Uh huh
Sodium chloride?
Yeah
…
Erm. Would sodium chloride be er an element?
[pause, c.2s]
Sodium chloride, no.
No?
It would be a compound.
You think that would be a compound?
Yeah.
And a compound is two or more elements joined together by chemical bonding?
Yeah.

So Amy had told me that sodium chloride, which had ionic bonding, was (like sodium oxide) a compound, and she had already told me that a compound comprised of "two or more elements chemically bonded", so it should be follow that sodium chloride (which had ionic bonding) had chemical bonding.

Do you think sodium chloride has chemical bonding?
Er – I think so
And it also has ionic bonding, or is that the same thing?
Erm,
[pause, c.2s]
I dunno, I've never thought about it that way, erm,
[pause c.3s]
I'm not sure, erm
[pause, c.2s]
I dunno, it might be.

Clearly, whatever Amy had been taught (and interviewing students reveals they often only recall partial and distorted versions of what was presented in class) she had learnt

(1) that ionic bonding was transfer of electrons (an alternative conception) as in the example of sodium transferring an electron to chlorine; and that
(2) a compounds was where two or more elements chemically bonded together, and an example was sodium oxide where the elements sodium and oxygen were chemical bonded.

Yet these two pieces of learning seemed to have been acquired as isolated ideas without any attempt to link them. Initially Amy seemed to feel ionic bonding and chemical bonding were quite separate concepts.

When taken through an argument that led to her telling me that sodium chloride, that she thought had ionic bonding, was a compound, which therefore had chemical bonding, there should have been a logical imperative to see that ionic bonding was chemical bonding (actually, a kind of chemical bonding – as the logic did not imply that chemical bonding was necessarily ionic bonding). Despite the implied syllogism:

sodium chloride has ionic bonding
sodium chloride is a compound
compounds have elements chemically bonded together
therefore ionic bonding …

Amy was unsure what to deduce, presumably because she had seen the two concepts of ionic bonding and chemical bonding as discrete notions and had had given no thought to a possible relationship between them. However explicit teaching had been on this point, it is very likely that the teacher had expected students to appreciate that ionic bonding was a type of chemical bonding – but Amy had not integrated these ideas into a connected conceptual structure (i.e., there was a learning bug that could be called a fragmentation learning impediment).

Ionic bonding – compared with chemical bonding

Keith S. Taber

Amy was a participant in the Understanding Science Project. The first time I talked to Amy, near the start of her GCSE 'triple science' course in Y10 she told me that "in normal chemistry (i.e., the chemistry part of 'double science', as opposed to the optional additional chemistry lesson as part of 'triple science' that Amy also attended) we're doing about ionic bonding" which she understood in terms of "atoms which have either lost or gained electrons so they are either positively or negatively charged" because "in ionic bonding it's the electrons that are transferred".

When asked other examples of ionic bonding apart from sodium and chlorine Amy told me "That's the one I did".

To a teacher it seems inherently obvious that ionic bonding is type of bonding – in much the way that a snare drum is a kind of drum or a conscientious student is a type of student. However, this may not always be obvious to students (even the conscientious ones).

When I asked Amy about bonding she referred to things being chemically bonded, and when I asked if ionic bonding was the same as chemical bonding, she was not sure how these concepts were related:

So what exactly is bonding?
Erm, where er one thing is joined on to another thing, and it can be chemically bonded or, yeah {laughs}
So we can talk about chemical bonding?
Mm.
Are there other types of bonding then?
Erm, there must be, if there's chemical bonding, I'm not sure, erm
[pause, c.5s]
But we talk about chemical bonding,
Mm.
and we talk about ionic bonding. So is ionic bonding the same thing as chemical bonding or is there a difference?
Erm, in, well in chemical bonding, erm like in a compound, where erm – two or more elements are joined together, that's an example of chemical bonding, but in – erm – ionic bonding it's the erm electrons that are transferred. [pause, c.2s] I think.

It seems Amy had been taught about chemical bonding and had learn about this as "a compound, where two or more elements are joined together", and she had been taught about ionic bonding and had learnt that this was where "the electrons are transferred".

Ionic bonding is not (and need not be associated with) electron transfer. It is not possible form talking to Amy to now exactly what her teacher told her – clearly she could have misunderstood or forgotten material form class. It is possible that it was made clear that ionic bonding was one type of chemical bonding, but Amy either missed that point or did not now recall it. It is also possible is was not made explicit but was assumed to be obvious (especially if ionic bonding had been presented as part of a sequence on chemical bonding. Sadly, what is obvious to teachers is not always obvious to learners, and indeed I've seen in my interviews that students are not always clear when one topic has finished and another has started. There is no sense here that I wish to criticise the teacher (who for all I know gave an exemplary presentation of the chemical bonding), but would simply suggest that when teaching one can never assume what should be obvious is obvious and that it is probably difficult to be too explicit about key ideas, or to reiterate them too often!

So at this point it seemed Amy only knew one example of ionic bonding, sodium chloride, and did not associate this with compounds which had chemical bonding. This could be considered a fragmentation learning impediment – a failure to make a link that was expected from the teaching. I went on to ask her for an example of a compound, and a she told me about sodium oxide I thought this was an opportunity to probe at the association between ionic boding and chemical bonding a little more.

Ionic bonding – where the electron's transferred to complete the outer shell

Keith S. Taber

Amy was a participant in the Understanding Science Project. The first time I talked to Amy, near the start of her GCSE 'triple science' course in Y10 she told me that "in normal chemistry (i.e., the chemistry part of 'double science', as opposed to the optional additional chemistry lesson as part of 'triple science' that Amy also attended) we're doing about ionic bonding" which was "atoms which have either lost or gained electrons so they are either positively or negatively charged" and

"how the outer electron's transferred…to complete the outer shell of the erm chlorine, thing, ion…and the sodium atom loses erm, one electron is it, yeah one electron, erm, which the chlorine atom gains, and that yeah that completes its outer shell and makes the sodium positively charged and the chlorine negatively charged".

Amy told me that "in ionic bonding it's the electrons that are transferred, I think."

So Amy had acquired a common alternative conception, i.e. that ionic bonding involved electron transfer, and that this occurs to atoms to complete their electron shells.

Ionic bonding refers to the forces between ions that hold the structure of an ionic substance together, rather than a mechanism by which such ions might hypothetically be formed – yet often learners come away form learning about ionic bonding identifying it with a process of electron transfer between atoms instead of interactions between ions which can be used to explain the properties of ionic substances.

Moreover, the hypothetical electron transfer is a fiction. In the case of NaCl such an electron transfer between isolated Na and Cl atoms would be energetically unfavourable, even if reactants containing discrete atoms were available (which is unrealistic).

Whether students are taught that ionic bonding is electron transfer is a moot point, but often introductory teaching of the topic focuses not on the nature of the bonding, but on presenting a (flawed) teaching model of how the ions in the ionic structure could form by electron transfer between atoms. As this mechanism is non-viable, and so not an authentic scientific account, it may seem odd that teachers commonly offer it.

One explanation may simply be custom or tradition has made this an insidious alternative conception. Science teachers and textbooks have 'always' offered the image of electron transfer as representing ionic bonding. So, this is what new teachers had themselves been taught at school, is what they often see in textbooks, and so what they learn to teach.

Another possible explanation is in terms of what what is known as the atomic ontology. This is the idea that the starting pint for thinking about chemistry at the submicroscopic level is atoms. Atoms do not need to be explained (as if in nature matter always starts as atoms – which is not the case) and other entities such as ions and molecules do need to be explained in terms of atoms. So, the atomic ontology is a kind of misleading alternative conceptual framework for thinking about chemistry at the submicroscopic level.

Current only slows down at the resistor

Current only slows down at the resistor – by analogy with water flow

Keith S. Taber

Students commonly think that resistance in a circuit has local effects, and in part that is because forming a mental model of what is going on in circuits is very difficult. Often models and analogies can be useful. However when an analogy is used in teaching there is also the potential for it to mislead.

Amy was a participant in the Understanding Science Project. Amy (when in Y10) told me she had been taught to use a water flow analogy for electric current. However, because her visualisation of what happens in water circuits was incorrect, she used the analogy to inform an alternative conception about circuits:

Do you have any kind of imagined sort of idea, any little mental models, about what (the flow of electricity round the circuit) might look like? Do you have a way of imagining that?
Erm, yeah, we've been taught the water tank and pipe running round it. … just imagine the water like flowing through a pipe, and obviously like, if the pipe becomes smaller a one point, erm, the water flow has to slow down, and that's meant to represent the resistance of something.
So, so if I had my water, er, tank and I had a series of pipes, they'd be water flowing through the pipes, and if I had a narrower pipe at one point, what happens then?
The water would have to slow down.
So would it slow down just as it goes through the narrow pipe, or would it slow down all the way round?
Erm – just through that part.

(Amy does not appreciate the implications of conservation of mass {that is, the continuity principle} here – at steady state there cannot be a greater mass flow at different points in the circuit).

…
And so how do you imagine that's got to do with resistance, how does that help you understand resistance?
…well resistance, it slows the current down, but then erm, once it passes a resistor or something it, the current is free to flow through the wire again

Analogies can be very useful teaching tools, but when using them it is important to check that the students already understand the features of the analogue that are meant to be helpful. It is also important to ensure that they understand which features are meant to be mapped onto the target system they are learning about, and which are not relevant.

Analogies are only useful when the learner has a good understand of the analogue. In this case, as Amy did not appreciate that the water flow throughout the system would be limited by the constriction, she could not use that as a useful analogy for why a resistor influences current flow at all points in a series circuit. This is an example of where a teaching model meant to support learning, which actually misleads the learner. That is, for Amy, with her flawed understanding of fluid flow, the teaching model acted as a pedagogic learning impediment – a type of grounded learning impediment.

The electrons come from batteries

Electrons flowing through circuits come from batteries

Keith S. Taber

Bill was a participant in the Understanding Science Project. I was asking him about topics he had studied in science, and I asked about electricity:

…have you done any work on electricity?
Erm, yes, I've done a bit.
Do you remember any of that?
Er we had to use symbols to draw circuits, and then we got to make those circuits.
Ah, so you remember doing the symbols, and you remember making up the circuits?
Yeah.
That's good. So what exactly is electricity?
It's made up of electrons which, erm, flow through wires, and into light bulbs to light them up. And they come from batteries.
Electrons do?
Yeah.
So what are electrons?
Erm {pause, c.4s} really don't know.

Bill here demonstrates a common alternative conception that in a circuit the battery, or other power supply, provides the electrons that flow, rather than providing a the electric field which acts on the electrons already present in the conducting path (e.g., in the wires).