Umar was a participant in the Understanding Chemical Bonding project. When I spoke to him in the first term of his course he was unsure whether tetrachloromethane (CCl4) would have ionic or covalent bonding.
When I spoke to him near the start of his second term, I asked him again about this. Umar then thought this compound would have polar bonding, however he seemed to have difficulty explaining what this meant ⚗︎ . Given his apparently confused notion about the C-Cl bond I decided to turn the conversation to a covalent bond which I knew, well certainly believed, was more familiar to him.
Is it possible for chlorine to form a bond with another chlorine?
[Pause, c.2s]
Yeah.
What substance would you get if two chlorine atoms formed a bond?
[Pause, c.2s]
You get, it still, you get, if you had like two chlorines it depends what groups are attached to it, to see how electronegative or electropositive they are.
What about if you just had two chlorine atoms joined together and nothing else, is that possible?
[Pause, c.3s]
No.
No?
On their own.
Not on their own?
No.
Umar's response here rather surprised me, as I was pretty confident that Umar had met chlorine as an element, and would know it was comprised of diatomic molecules: Cl2.
So you couldn’t have sort of Cl2, a molecule of Cl2?
[Pause, c.1s]
Yeah, you could do.
Could you?
[Pause, c.2s]
They might be just, they might be like, be covalently bonded.
Perhaps the earlier context of talking about polar bonds and the trichloroethane molecule somehow acted as a kind of impediment to Umar remembering about the chlorine molecule. It seemed that my explicit reference to the formula, Cl2, (eventually) activated his knowledge of the molecule bringing to mind something he had forgotten. Although he suggested the bond was (actually "might be") covalent, this seemed less something that he confidently recalled, than something he was inferring from what he could remember – or perhaps even guessing at what seemed reasonable: "they might be just, they might be like, be covalently bonded".
As often happens in talking to learners in depth about their ideas it becomes clear that thinking of students 'knowing' or 'not knowing' particular things is a fairly inadequate way of conceptualising their cognition, which is often nuanced and context-dependent. This suggests that what students respond in written tests should be considered only as what they were triggered to write on that day in response to those particular questions, and may not fully reflect their knowledge and understanding of science topics. Other slightly different questions may well have cued the elicitation of different knowledge. Now Umar had recalled that chlorine comprises of covalent molecules, I asked him about the nature of the bond:
So what would that be, covalently bonded?
They share the electrons.
So how many electrons would they have then?
They’ll have
[Pause, c.7s – n.b., quite a long pause]
like the one on it, the one of the chlorines shares electrons with the other chlorine to fill in its shell on the other one, and the same does it with the other.
In thinking about covalent bonding, Umar (in common with many students) drew upon the full shells explanatory principle that considered bonding to be driven by the needs of atoms to 'fill' their outer electron shells. (The outer shell of chlorine would only actually be 'full' with 18 electrons, but that complication is seldom recognised, as octets and full shells are usually considered synonymous by students).
So how many electrons does each chlorine have to start with?
In the outer shell, seven.
And how many have they got after this?
They’ve got seven, but they share one.
[Pause, c.1s]
Maybe.
So that’s a covalent bond, is it?
Yeah.
So how many electrons are involved in a covalent bond?
[Pause, c.3s]
Erm,
[Pause, c.3s]
Two.
Two electrons.
So where do those two electrons come from?
They like, one that fills up the gap, fills up the – last electron needed in one of the chlorine shells, and the other chlorine shell fills it up in the other one.
So where do they come from?
Each chlorine. Outer shell.
One from each chlorine?
Yeah.
Okay, and that’d be a covalent bond?
Yeah.
Here, again, Umar is using the full shells explanatory principle as the basis for explaining the bond in terms of electrons 'filling up the gaps' in the electron shells, rather than considering how electrical interactions can hold the structure together. Umar's suggestion that the sharing of electrons "fills up the – last electron needed in one of the chlorine shells" demonstrates the anthropomorphic language (e.g., what an atom wants or needs) commonly used when learners have acquired aspects of the common octet rule framework that is developed from the full shells explanatory principle and used by many learners to explain bonding reactions, chemical reactions, patterns in ionisation energy, and chemical stability.
