Category: learner thinking

Na+ has an extra electron in its outer shell and Cl- is minus an electron

The plus sign shows Na+ has an extra electron in its outer shell; the minus sign shows Cl– has seven electrons in its outer shell so its minus an electron

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 of the English school system). Annie 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.

Focal figure (Fig. 5) presented to Annie in interview

She was shown a representation of part of a lattice of ions in sodium chloride (see: Sodium and chlorine probably get held together by just forces*), but Annie identified the signified as atoms, not ions:

Any idea what that’s meant to be?

(pause, c.6s)

Just sodium and chlorine atoms.

As an A level student, Annie would be expected to understand the differences between atoms, ions and molecules, and to known that there were ions in NaCl, but this could have been a simple slip of the tongue. This was tested by further questioning:

Erm, so if you look at these, I mean you said they were sodium and chlorine

Yes.

because presumably you recognise the Na and the Cl,

Yeah.

but only two of them are labelled with ‘Na’ and ‘Cl’.

Yes.

What about the others – what do you think they are?

They’re probably sodium and chlorine, or else they could be, because of the signs, you’ve got plus and minus signs on them representing the charge, or else it could be similar elements going down the groups.

Okay, so you recognise that these, these things represent charges, and you probably guess it’s just me being lazy that I haven’t labelled them all, [Annie laughs] so I’ve just labelled the first couple, erm, so these are what, so you reckon this little one will be, what will that be do you reckon?

Sodium.

That will be a sodium, molecule?

Atom.

Sodium atom, what about this one here?

Chlorine atom.

That’ll be an atom. But these have got charges on?

Yeah.

So Annie recognised the symbols for positive and negative charges, and thought that the figure signified atoms, with charges. The simplest interpretation here is simply that Annie did not recall that atoms were neutral, and 'charged atoms' are called ions in chemistry.

However, Annie then told me that sodium has like one extra electron in its outer shell, and chlorine is minus an electron, so by force pulls they would hold together, and explained this in terms of her notion of charges:

…say that about the electrons again.

Sodium has like one extra electron, ‘cause it has like an extra electron in its outer shell, and chlorine has seven electrons in its outer shell so its minus an electron so by sort of exchanging, the sodium combining with the chlorine just by force pulls they would hold together.

So Annie saw the plus (+) symbol to mean one electron over a full shell (2.8.1), and the minus (-) symbol to mean one electron short of an octet of electrons (2.8.7). For Annie these charges were not net electrical charges, but deviations from octet configurations. These 'deviation charges', for Annie, provided the basis for the attraction between the 'charged' atoms.

This was checked by asking Annie about the electron configurations.

So we looked at a sodium atom earlier, you recognised it as being a sodium atom, I did not say it was, and that had an electronic configuration of…do you remember what the electron configuration was?

Eleven.

So a total of eleven electrons

Yeah.

So do you know what shells they were going to?

Sorry?

Can you tell me what the configuration is in terms of shells? How many in the first shell, how many in the second shell…

2.8.1

2.8.1?

Yeah.

So this here (indicating a cation on the figure), you are saying that this here is 2.8.1

Yes.

And this is 2.8.7 would it be?

Yeah, 2.8.7

Annie held an alternative conception of the nature of the charges associated with ions: that neutral atoms had charges if they did not have full shells/octets of electrons. That this was a general feature of her thinking became clear when she was asked about the symbols for other ions: such as K+ and F–.

Whilst Annie's specific 'deviation charge' conception (i.e., that (neutral) atoms would be charged when they did not have fill shells/octets of electrons) would seem to be rather idiosyncratic, alternative conceptions relating to the significance of full shells / octets of electrons seems to be very common among chemistry students.

Although species with Annie's deviation charges did not have actual overall electrical charge, Annie considered that these 'deviation' charges could actually give rise to forces between atoms (she thought that as sodium has one extra electron in its outer shell, and chlorine is 'minus an electron', then they would hold together; The force of lack of electrons pulls two hydrogen atoms together⚗︎).

 

Calcium and oxygen would not need to bond, they would just combine…

Calcium and oxygen would not need to bond, they would just combine, joining on to make up full shells

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 of the English school system). Annie 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. Near the end of the interview, she was asked some general questions to recap on points she had made earlier. She suggested that Ca2+ and O2- would combine, but without any chemical bonding.

Could you have a double ionic bond?

(pause, c.3s)

Can you have a double bond that's ionic?

Not really sure.

If you had say, say you had calcium, two-plus (Ca2+), and oxygen two-minus (O2-),

yeah,

could that form a double bond?

(pause, c.4s)

Are you not sure?

It wouldn't need to.

It wouldn't need to?

No.

Why's that?

Because one's lacking two electrons, and one's got two, so, they would just combine without needing to sort of worry about other, other erm elements.

Right so they…

Sort of joining on to make up full shells.

So they combine, but you wouldn't call that a chemical bond?

No.

From what Annie had reported earlier in the interview, she would see Ca2+ as a calcium atom (that's "got two" electrons in its outer shell) and O2- as a oxygen atom (that was "lacking two electrons"), as she held an alternative conception of what was meant by the symbols used to indicate electrical charge plus and minus signs represent the charges on atoms)*.

Annie here suggests that the atoms with their charges (i.e., for Annie, deviations form full shells) would combine, and join up to obtain a full shell. From her perspective, there was no need for ionic bonding. Although Annie's notion of what was signified by the charge symbols would seem to be idiosyncratic, the idea that chemical processes occur to allow atoms to obtain full shells (the 'full shells explanatory principle') is one of the most common alternative conceptions in chemistry.

Atoms evolved so that they could hold on to each other

Bert Suggests Chemical Bonding Evolved 

Keith S. Taber

Bert was a participant in the Understanding Science Project. During one interview he reported that he had just completed a topic of alkanes and alkenes in his chemistry classes. He explained that a carbon atom has "to have four bonds", so if a carbon atom had "only got one two carbons on one side and one hydrogen then it'll make a double bond, to have four bonds". So I asked him what he understood a bond to be:

I: …what's a bond?

B: A bond is erm, it just, it's something to hold, hold two atoms together.

I: So what might you use to hold two atoms together?

B: Erm, So they can be kept, so that they're not too, I think it's just to make, so it can make big lines so it can erm, oh, so they, so not every, so because solids they have erm, I guess a lot of bonds, to keep it all, all together, I'm guessing. And erm like gas has a lot less bonds because it's a lot more free.

I: That makes sense [Bert], I'm just wondering what you would use to bond two atoms together. … I'm just wondering what kind of thing you use to bond two atoms together.

B: Erm • • I'm not sure. I guess, I guess they were just, when er, they're made with it I guess.

I: Yeah. Do you think it's made of adhesive? … is it made of a glue do you think?

B: No, I don't think so. I think it was like, I don't know, it could have been like evolution, like.

I: Ah.

B: Yeah, the atoms evolved so that they could hold on to each other.

I: Oh I love that. • • • The atoms evolved so that they could hold on to each other?

B: I guess so. That's how the world was made.

In this interview segment Bert seems not to have considered the nature of the bonds between atoms, but just to have accepted what he has learnt about valency. When asked about the nature of the bond he could offer no mechanism for bonding, but instead suggested that chemical bonds had evolved as "that's how the world was made". Here Bert is drawing upon a general explanation considered to be universal in the domain of living things, but applying his learning from biology to explain a physical phenomena.

This seems to be a creative association drawing upon prior learning, but the idea of evolution is being used outside is canonical range of application, leading to a potential associative learning impediment. Potentially, Bert's thinking about evolution as explaining how atoms can bond (a potential explanation about origins, though inappropriate if evolution is understood as natural selection) could stand in place of seeking a physical explanation for the nature of bonding.

Guessing what is produced in beta decay

Keith S. Taber

Amy was a participant in the Understanding Science Project. I talked to her after she had just started studying radioactivity in her Y11 physics class. She had been introduced to alpha, beta and gamma radiation, and thought that the teacher had been telling the class crazy things.

Forgetting the source?

Interestingly, in a later interview just before her GCSE examinations, Amy told me: "beta radiation is, I dunno, an electron – thing, which is emitted somehow." When I asked her where's it emitted from, she told me

"that's what I don't know, 'cause I asked about that, and I didn't get an answer because, erm, apparently the neutron is made up of other stuff, and it, that sort of decays to give other things and that's where the electron comes from, apparently, maybe, I don't know, I'm guessing now".

When asked if she knew what else was emitted, Amy suggested that "I'm guessing proton, but I don't know". It seems this had seemed so 'crazy' that Amy was not able to believe what she had been taught, and what had previously been reported as something she had been told, was now considered by her to be just a guess at what was going on. This is an interesting reminder of how human memory works, that we do not always recall the origins of our ideas, so can not always distinguish our own creative ideas from we might have been previously told at some point.

Crazy physics: radioactivity is just mad!

Some crazy thing about a neutron turning into a proton or something

Keith S. Taber

Amy was a participant in the Understanding Science Project. I talked to her after she had just started studying radioactivity in her Y11 physics class. She had been introduced to alpha, beta and gamma radiation, and the teacher had been telling the class crazy things: "he told us some crazy thing about a neutron turning into a proton or something, and losing, two electrons or some crazy thing like that."

This was crazy because "it doesn't make any sense". Amy had only just started the topic, and had not yet sorted out all the details in her own mind, but the teacher had "showed us this little equation thing they [sic] have, and when – is it the electrons that are released or something – the atom, it changes, it changes into something else". This was not something else other than an atom, but "an element will become a different element or some stupid thing like that". Amy concluded that "all of it sounds crazy, physics is crazy. I don't understand it." This seemed a rather harsh judgement.

Amy was generous enough to talk to me regularly about her understanding of the science she was taught in school. She, regularly, told me she knew or understood little about the topics she was taught, although a little probing often revealed considerable learning. However, on this occasion, Amy genuinely seemed to be genuinely mystified by what she was being told.

I spent a little time talking to Amy about radioactivity. I discussed beta decay with Amy, concluding with "an electron comes shooting out of the atom, and that's your beta radiation". Amy maintained "it's crazy". We discussed the different types of radioactivity she would study: "it's crazy". I think Amy believed what I was telling her, but she made it clear what she thought of these nuclear changes and the accompanying transmutation of elements: "it's mad"!

What I find especially interesting here, is that Amy seemed to have strong intuitive views of the way the world should work, even when the focus was something so far from our everyday experience. Atoms, nuclei, electrons, neutrons, protons, are all theoretical constructs we introduce to students that have no obvious and clear link with anything in everyday experience. They also behave in ways unlike objects of common experience. It can only have been a couple of years since Amy was taught that atoms had structure and comprised of protons, neutrons and electrons. But now she readily accepted that neutrons existed. Indeed they had become so 'real' to her, that the idea one must spontaneously change into a proton and an electron seemed quite mad. So Amy had relatively quickly applied her intuitive ideas about the way the world is to form a mental model of a neutron as a discrete, stable entity: not something that could suddenly reveal itself as potentially two other things that were supposed to be quite different. When Amy was introduced to a model of the sub-atomic components of the atom she 'assigned' (i.e., pre-consciously interpreted them to have) them certain ontological qualities (such as stability, permanence) which made sense to her at the time, but which did not capture the way scientists understand these entities. This assumption of the nature of the neutron acted as a grounded learning impediment, because when she was later taught about radioactivity it did not make sense in terms of her prior learning. 

Too crazy to remember?

In a later interview, Amy was able to tell me that the neutron is made up of other stuff, and it decays to give an electron and, she thought, a proton – although (she told me) that was only a guess.

Sharing the same shell and electron makes them more joined together like one

Keith S. Taber

Umar was a participant in the Understanding Chemical Bonding project. When I spoke to him in the first term of his advanced level chemistry course he identified figure 2 (below) as representing a hydrogen molecule, with covalent bonding.

Sharing the same shell and electron makes them more joined together like one

Umar was a participant in the Understanding Chemical Bonding Project. When I spoke to him in the first term of his advanced level chemistry course he identified figure 2 (below) as representing a hydrogen molecule, with covalent bonding.

Figure 2 (Focal image – Understanding Chemical Bonding project)

Umar suggested that covalent bonding is when atoms share electrons to combine into one whole thing. That discussion took place early in the interview, before we then discussed a whole range of other images. Near the very end of the interview I returned to ask about figure 2 again.

Interviewer: And number 2 was what kind of bond?

Umar: Covalent.

I: Now, what holds the molecule together in number 2?

U: The two electrons – shared.

I: And how does that hold them together?

U: 'cause they're sharing the same – shell and electron.

I: And why does that hold them together?

U: Makes them more, together like, makes them more like joined together like one.

After we had first discussed what this image was meant to represent early in the interview, Umar discussed a wide range of other images, and in the context of some of these he discussed bonding in terms of forces and electrical charge. As he had not mentioned such notions in the context of figure 2, even after using the ideas elsewhere, I sought to see if he recognised that forces were acting in the hydrogen molecule.

I: I see. Is there any force there holding them together?

U: It's, erm could be the charges of the electrons and the charge of the nucleus.

I: Would the nucleus have some sort of interaction with the electrons – some sort of attraction or repulsion?

U: Yeah.

I: Would it be attraction or repulsion?

U: Erm, attraction.

I: So which electron does this nucleus attract.

U: Erm, it attracts both of them, and the other one attracts both of them because they are both, like, opposite charges. So that's why they are like, around there. It might be like they move around. Around that part.

I: So they might actually move about?

U: Yeah.

I: I: But you think the two nuclei attract the two electrons?

U: Yeah.

I: Do the two electrons attract the two nuclei?

(Pause, c.3s)

U: Yeah, think so, the – yeah.

I: Yeah? Do the two electrons attract each other?

U: No, they repel.

I: Do the two nuclei attract each other?

U: No they repel.

So it seemed that Umar understood the forces acting in the covalent molecule but that these ideas were not readily cued in that context even though he readily used the idea of forces between charges to explain other kinds of chemical bonding. In the context of covalent bonding however, the notion of the bond as electron 'sharing' was cued instead. Arguably the notion of the covalent bond as sharing of electrons acted as a grounded learning impediment perhaps blocking him bringing to mind alternative ways of thinking about the bond. This could be seen as an example of weak anthropomorphism: the idea that the electrons were 'shared' stood in place of a more scientific explanation of the bonding process.

Covalent bonding is when atoms share electrons to combine into one whole thing

Keith S. Taber

Umar was a participant in the Understanding Chemical Bonding project. When I spoke to him in the first term of his advanced level chemistry course he identified figure 2 (below) as representing a hydrogen molecule, with covalent bonding.

UCB Figure 2 (for interview-about-instances technique)

Can you tell me what you think that's meant to represent?

Er, two hy-, a hydrogen molecule, 'cause it's like they've got one electron, in the only one shell, and they're joined together, a covalent bonding, and they're sharing it.

So what is a covalent bond exactly?

When they share electrons.

When you share electrons?

Yeah.

So when Umar thought of covalent bonding he seemed to primarily associate this with the notion of 'sharing' of electrons. The idea that atoms can 'share' anything could be considered an example of anthropomorphism, but this is a common metaphor that is widely used in discussing bonding.

The 'sharing' notion is however little more than a descriptive label, and has limited explanatory power. Acceptable explanations of the bond would draw upon scientific concepts, such as electrical forces, or atomic orbital overlaps allowing the formation of lower energy molecular orbitals. I probed Umar to see how he understood the nature of the covalent bond.

Or do you think they're stuck together?

I think they're quite strong together, covalent is quite a strong bond.

So that will hold them together will it?

Yeah.

Umar certainly saw the bond as a strong linkage of some kind, but so far my questions had not revealed how he understood the bond to hold the molecule together.

Well how does it do that?

It's like, they're joined together, 'cause first of all they just had two atoms with one electron each, and now they're sharing two electrons between them. So it's quite strong.

Oh, why's that?

Because the the the actual, when they share them they're like combined into like one sort of whole thing, instead of two separate atoms.

Right, so the, so the bond, which is the sharing of two electrons, that holds them together,

Yeah.

to make one thing, which we've called a molecule.

Yeah.

So at this point in Umar's course he seemed to conceptualise the covalent bonding as electron sharing and saw the action of sharing to inherently hold the molecule together, and seemed to be satisfied with that as an explanation for the bond. This discussion took place early in the interview, before we then discussed a whole range of other images. Near the very end of the interview I returned to ask about figure 2 again (see Sharing the same shell and electron makes them more joined together like one)*.

So if someone was stood here, we'd be a solid

Keith S. Taber

Morag was a participant in the Understanding Science Project. During her first term in secondary school, Morag told me she had studies changes of state, which was about "melting things, it's like solid, liquid and gas. Where like an ice cube melts to go to water, it evaporates to go to gas, it then condenses to go to water and then freezes to go to ice".

When I asked her about about the states of matter, Morag gave me a quite polished response. In the middle of this, she stood up and started moving about. It appeared that she had modelled the states of matter in class through a simulation, with the students acting as particles – and this association seemed to now be cued by her recalling the explanations for the different states of matter:

I: So silly question, 'cause I'm sure everybody knows really, but what's a solid, what's a liquid and what's a gas then?

Morag: A solid is an object where the particles are very close together, but still have room to move very slightly, you know like they can only move little bits, er, it has a fixed shape, it cannot be poured – and that's all I can remember.

I: That's quite a bit. And that's different to a liquid, is it?

M: Yeah, 'cause a liquid you can pour, it takes the shape of its container, the particles are spread out more evenly, but still in a, but are still spread in a – yeah they're spread evenly it can be poured, (it takes the shape of its container), the particles are still quite close, but they are further away than they were in a solid, so they can move just a bit more. If you know what I mean, like. So if someone was stood here [indicating next to her], we'd be a solid, 'cause we just move very slightly,

I: all right, yeah

M: and if we were a liquid we would be stood just a bit further away, so we can move a bit more.

I: I see, so if you had brought a friend with you,

M: Yeah, and if we were stood like that, if she was stood there, we'd be a solid, 'cause we were quite close, but we still had room to move about

I: Mm

M: if we were a liquid, we'd be a bit further, but we still, still quite close, but still had move to room, to move about, and I'm not going to tell you about gas until we get onto gas.

I: Okay. So you and your friend could be a liquid? Which means that I could pour you and you would take up the shape of your container?

M: No, I mean like we'd be the particles in liquid.

I: Ah, I see.

M: you know like

I: Moves around!

M: like, so like, like, so we'd be like that, and there would be lots of us, but we could still move about. Yeah? And if we were a liquid we would be like that, and we could still move about. And if we were a gas we'd be further apart, but and then we can, and then we can move around the room freely.

Molecules are like a jigsaw

Keith S. Taber

Tim was a participant in the Understanding Science Project. When Tim was interviewed in the first term of his 'A level' (college level) physics course he had been studying the topic of materials with one of his teachers, and "at the moment we're doing about why some materials are brittle, and some aren't, and about the molecules". When Tim was asked about the molecules, he compared molecules to the pieces in a jigsaw:

Interviewer: So what's a molecule?

Tim: Erm it's like a bit of a particle, so, something that makes up something.

I: Have you got any examples?

T: Of a molecule?

I: Yeah, something that makes up something.

T: Erm, like the wood in the table is made out of wood molecules.

I: I see. So, that's one type of molecule, is it, a wood molecule? And there are other types of molecule?

T: Yes it's a bit like a jigsaw, like when you put all the, like you need to put the…, you put them all together to make – something.

I: I see, yeah. So, if I wanted to be really awkward, in what way is it like a jigsaw?

T: Erm, well they sort of fit together, like in a jigsaw some bits are sort of straight and have nice parallel, a nice parallel microstructure, and some, some jigsaws have funny bits that don't fit together quite as nicely.

I: I see. So are there some ways it's not like a jigsaw?

T: Yeah. (Tim laughs.) Well, erm, I dunno, it's like a jigsaw in the way that the bits fit together to make something, to make something, but then again, I dunno.

I: I mean, I quite like this idea of it being like a jigsaw – I was wondering whether, whether you had got that from somewhere, or that's just something you'd come up with?

T: No I just thought about it, just then.

I: Oh that's really creative.

T: It's quite random actually. (laughs)

Tim's comments about a molecules being a bit of a particle was followed up later in the interview, and it transpired he was not sure if a molecule is a bit of a particle – or vice versa.*

So when asked to explain about molecules in materials, Tim used an apparently spontaneous analogy of this being like a jigsaw, with different types of pieces that fitted together. Moreover, he also seemed to recognise that different materials had molecules that fitted together more or less readily, and materials could also be considered to have similar diversity. Tim described this as being 'random', which seems unfair as the analogy clearly has merit, but presumably saw it this way as the comparison had apparently appeared in his consciousness unexpectedly (i.e., the thought had 'popped into his mind', as a kind of insight.)

Tim seemed a little phased by being asked to explain the negative features of the analogy – and this may reflect the tendency to focus on the positive aspects of an analogy, rather than its limitations. Analogy has the potential to channel student thinking in inappropriate directions (e.g., as associative learning impediments) when not considered critically. However, analogies also have potential to help 'make the unfamiliar familiar' and so can be a powerful learning tool.

Creating an explanation for the soot from Bunsen flames

Letting the dirt out: Creating an explanation for the soot from Bunsen flames in the absence of appreciation the nature of combustion

Keith S. Taber

Jim was a participant in the Understanding Science Project. Jim, a Y7 student, had been studying burning in science. He had been using Bunsen burners, and had been taught about the different flames (i.e., the safety flame, and the 'roaring' blue flame used for heating), and the use of the valve at the base of the burner to select the frame. Not yet appreciating the nature of burning, he was not aware that the soot obtained when interrupting the safety flame was due to incomplete combustion. Rather he had developed his own interpretation of why using the burner with the hole closed off led to a dirty flame:

What is burning, then?

It usually involves a flame. Erm which can either be yellow, orangey-yellow, or …like a, bluey colour, bluey-purple.

I: Oh, so is that significant, the colour of the flame, does that mean something?

J: Well, the yellow one has a lot of …if you touch it with glass or something, …will go black, but if you use the blue flame, it won't, so if you are heating something, you should use the blue flame.

I: Why do you think it goes black, if you use the orangey-yellow flame?

J: Because with the Bunsen burners, if you are twisting the knob, open, the dirt gets out, and you get the nice clear blue flame, but to get the orange flame, you have to have it closed, don't you, and then that doesn't let the dirt out, so it doesn't kind of, when it gets out of the top it doesn't have time.

I: So what happens if the hole is open?

J: You get, a blue flame.

I: Right, and what happens if the hole is closed?

J: Get a yellow flame.

I: And why does the hole make a difference?

J: I don't know, it probably lets the dirt out, or the air get into it or something.

I: So what dirt is this, that might be let out, do you think. Dirt from where?

J: Maybe the excess gas particles that have already been burnt or something. Don't know.

Presumably no one had told Jim that the hole was to let dirt out of the Bunsen so it did not get into the flame. However the hole was presumably letting something in or out (he later suggests, the hole might let air in – perhaps something the teacher had told the class but which had not been readily recalled?) and there was dirt in the flame when it was closed, which was not there when it was open. Jim interpreted his observations in terms of prior knowledge (of what holes do, and of dirt) to construct an explanatory scheme that made some sense of the effect of closing or opening the air hole. This would seem to have potential to be an associative learning impediment of the 'creative' type.

Gas particles like to have a lot of space, so they can expand

Keith S. Taber

Derek was a participant in the Understanding Science Project. I interviewed Derek when he was in Y7 of the English school system. We had been talking about work that Derek has been doing in his science classes on burning. As part of the conversation, Derek defined a solid in particle terms:

what's a solid then, what's a solid?

Lots of particles really close together that can't move a lot.

When I followed this up, Derek explained how a liquid or gas was different to a solid:

And you say solids are made of particles. What are liquids then, they are not made of particles then?

No they are, they are just more spread out particles. And then, you get a gas, which the particles can move a lot more than solid and liquid, they can move wherever they like.

And where do they like to move?

As far away from each other as possible.

Why do you think that is?

'cause they like to have a lot of space, so they can expand.

Why do you think particles like to have a lot of space?

(Pause, c.3s)

Don't know.

Are they unfriendly lot, unsociable?

(Pause, c.2s)

No, they just, they like to have, like be as well away from each other as possible.

The question "where do they like to move" was couched in anthropomorphic terms to reflect the anthropomorphism of Derek's statement that gas particles could "move wherever they like", to see if he would reject the notion of the particles 'liking'. However Derek did not query my use of this language, and indeed suggested that the particles "like to have a lot of space".

When he was asked why, there was a pause, apparently suggesting that for Derek the notion of the particles liking to be far apart seemed to be reasonable enough for him not to have thought about any underlying reason, and his "don't now" was said in a tone suggesting this was a rather uninteresting question. Although Derek rejected the suggestion that the particles were 'unfriendly', 'unsociable' his tone did not suggest he thought this was a silly suggestion: rather it was just that the particles "like" to be as far "away from each other as possible".

The use of anthropomorphism is very common in student talk about particles. Whether or not Derek really believed these gas particles actually had 'likes' in the way that, say, he himself did, cannot be inferred from this exchange. But, in Derek's case, as in that of many other students, the anthropomorphic metaphors seem to offer a satisfactory way of thinking about particle 'behaviour' that is likely to act as a grounded learning impediment because Derek is not open to looking for a different kind (i.e., more scientifically acceptable) type of explanation. Given the common use of his language, it seems likely that it derives from the way teachers use anthropomorphic language metaphorically to communicate abstract ideas to students ('weak anthropomorphism'), but which students accept readily because thinking about particle behaviour in terms of the 'social' models makes sense to them ('strong anthropomorphism').

Do the forces from the outer shells push the protons and the neutrons together?

Keith S. Taber

Annie was a colearner (participant) in the Understanding Chemical Bonding project. In her first interview, during the first year of her two year 'A level' college course, Annie was asked about a (Bohr type) representation of a (sodium) atom. Annie did not know what held the protons and neutrons together in the atomic nucleus, but suggested it might be due to forces from the electrons "pushing":

Interviewer: Can you identify the different parts of that diagram? What's the blob in the centre?

Annie: It's the nucleus.

I: That's the nucleus. Do you know what's in the nucleus?

A: The protons and, no the electrons and the neutrons, no the protons and the neutrons. The electrons are round the outside.

I: There's protons and neutrons in the centre okay.

A: Yeah.

I: Erm, what holds them together, any idea?

A: Is it the forces from the outer ring? Outer rings or outer shells? The electronic forces?

I: What repelling them in? Holding them

A: Yeah.

I: in the centre? It could be.

A: Pushing them.

I: It's not actually, but that's a sensible suggestion. So you haven't actually done anything about what holds the nucleus together?

A: No.

…

The question of why the nucleons should be held together (given the repulsion between positive protons) is not usually considered in school chemistry lesson, and does not seem to be a question which students tend to spontaneously consider. The interview continued…

I: What holds the electrons in place?

(pause, c.4s)

A: Er (pause, c.9s) Not really sure, but I know there's a set pattern of how many can go in each shell, so if its connected with that?

I: Huh hm, do you think, do you think you need anything to hold the electrons in place, or I mean is it just the way the Universe is, or God's will, or, you know, or just aesthetic, you know nature's aesthetic,

A: Yeah.

I: and it looks pretty? I mean do you think there has to be some physical reason why the electrons are there rather than anywhere else?

A: Probably is to do with the structure of it.

I: But you are not, you're not sure why,

A: No.

I: it should be that the electrons should be in orbitals or orbits?

A: No.

I: Rather than just scattered higgledy-piggledy.

A: No, I don't know that.

In this section of the interview, Annie seems to suggest she is not aware of any forces acting on the electrons, and suggests it may be something inherent in the electronic structure which holds the electrons in place. It seems odd that Annie does not invoke a force from the nucleus, given her comment just earlier about a possible pushing from the outer electron ring/shell onto the nucleons. It seems Annie does not know about, or at least does not bring to mind, an electrical force attracting the electrons and nucleus. However, this was tested by a slightly different question…

Okay. So can you tell me why the electrons don't fall out of the atom? I mean if you imagine that this was sort of, er, an atom that's placed vertically, why don't the electron's just fall out of the bottom?

A: The forces hold them together.

I: What kind of forces are they. Do you know?

(pause, c.5s)

A: The attraction from the nucleus, from the protons.

I: So the protons in the nucleus attract the electrons?

A: Yeah.

I: So what kind of attraction is that. What kind of force is that?

A: Er (pause, c.7s) I don't know

So Annie is aware that the electrons are attracted by the nucleus, and specifically by the protons. Despite this, Annie does not suggest the interaction is electronic, or specially refer to charge. Her suggestion that the outer electron shell may push on the nucleus, holding it together, contradicts Newton's third law in that forces between bodies are either attractive or repulsive, not not a mixture of the two. So if the nucleus attracts electrons, then electrons must attract (not push) the nucleus. Annie's suggestion was also inconsistent with the way forces between charges depend upon separation (by an inverse square law): the repulsion between adjacent protons would be far larger than any force due to the more distant electrons.