Tag: electronic configuration

In ionic bonding, they both want to get full outer shells

Keith S. Taber

Mohammed was a participant in the Understanding Science Project. When interviewed in the first term of his upper secondary (GCSE) science course (in Y10), he told me he had been learning about ionic bonding in one of his science classes. Mohammed had quite a clear idea about ionic bonding, which he described in terms of the interactions of two atoms:

And you said in chemistry you've been doing about electron arrangements [electronic configurations], and ionic bonding.

Yeah.

So what's ionic bonding, then?

Ionic bonding is when, like let's say, a sodium atom and take a chlorine atom, which make salt if they react. What happens is – the sodium atom has one electron on its outer shell, and the chlorine atom has seven, now they both want to get full outer shells, so if I er let's say move the electron from the sodium to the chlorine, then the chlorine would have a full outer shell because it would have eight, and because it's lost that shell the sodium will also have eight.

This account of ionic bonding is a common one, although it is inconsistent with the scientific model. A key problem here is that the driving force for bond formation is seen in terms of atoms wanting to complete their electron shells (the 'full shells explanatory principle'). Mohammed's explanation here uses anthropomorphism, as it treats the individual atoms as though they are alive and sentient, acting to meet their own needs – "they both want to get full outer shells".

When Mohammed was probed, he related a full outer shell to atomic stability (a central feature of the full shells explanatory principle).

Okay. How do you know they want full outer shells?

Because it makes them more stable.

Why does it make them more stable?

(pause, c.1 s)

Erm. (Why do electrons?*) (* sotto voce – apparently said to himself)

(pause, c.2s)

Er, because they don't react as much with other elements if they have a full outer shell.

I see.

They don't react.

There is an interesting contrast here between Mohammed's instant response that full shells "makes them more stable", and the long pause as he thought about why this might be so.

His response reflects something quite common in students' explanations n that a student asked why X is the case may respond by explaining why they think X is the case. (That is, as if an appropriate answer to the question "why is it raining so heavily?" would be "because I got soaked through getting here", i.e. actually responding to the question "how do you know that it is raining heavily?")

Such responses seem to be logically flawed, but of course may be a mis-perception of the question being asked (so the learner is answering the question they thought was asked), or (possibly the case here) substituting a response to a related question as a strategy adopted when aware that one cannot provide a satisfactory response to the actual question posed.

The anthropomorphic aspect of his earlier answer was probed:

How do the atoms know that they need to get a full outer shell, they want to get a full outer shell? Do they know about this stability thing?

Not really.

No?

It's just what happens.

Oh, I see, it's just what happens?

Yeah.

So although Mohammed used an anthropomorphic explanation, it seemed he did not mean this literally. (It may seem strange to suggest a 14 year old might consider atoms alive and sentient, but research suggests this is sometimes so!) This has been described as weak anthropomorphism, where the anthropomorphism is only used as a figure of speech. However, such language can act as a grounded learning impediment because if it becomes habitual it can stand in place of a scientific explanation (thus giving no reason to seek a canonical scientific understanding).

I went on to ask Mohammed about the formation of salt in the process he had described.

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".

Focal figure shown to Brian

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.)

Sodium has one extra electron in its outer shell, and chlorine is minus an electron, so by force pulls they would hold together

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.

Focal figure (Fig. 5) presented to Annie

She was shown a representation of part of a lattice of ions in sodium chloride (see: Sodium and chlorine don't actually overlap or anything), but Annie identified the signified as atoms, not ions, because Annie had an idiosyncratic understanding of what was meant by charge. (Read: Na+ has an extra electron in its outer shell and Cl- is minus an electron and K-plus represents a potassium atom that has an extra electron.)

Annie was asked whether the structure made up of sodium and chlorine 'atoms' would hold together:

Do you think this thing would fall apart? Or would it hold together?

(pause, c.9s)

If you heated it, or reacted it in some way, it would hold together, and it would probably get held together by just forces.

By forces. Any idea what kind of forces would hold it together?

Probably just the attraction.

Uh hm?

The attraction from the plus to the minus because like chlorine's minus an electron and sodium is over an electron. So they could just like hold them together, but not actually combine.

Right, chlorine's, so sodium's, 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. Yet, 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, …

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

And that is what holds them together the fact that this is one short,

yeah,

one over and one short.

One over, and that one's one short.

So the plus means one electron more than an outer, the full shell,

Yeah.

and the minus means one electron

Minus.

less than an outer shell,

Yeah.

and that's what holds them together.

Yeah.

Okay, so there is something holding them together,

right,

and it's to do with these pluses and these minuses,

Yes.

but what we don't have there is chemical bonding like we had before.

No.

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. Whilst Annie's specific deviation charge conception would seem to be rather unusual, alternative conceptions relating to the significance of full shells / octets of electrons seems to be very common among chemistry students. Although Annie's thinking was idiosyncratic it reflected the common full shells explanatory principle that sees electronic configuration as a cause for chemical processes.

So Annie considered that these 'deviation' charges could actually give rise to forces between atoms (see also The force of lack of electrons pulls two hydrogen atoms together*).

Annie did not see ions, but atoms. But she thought that after a reaction, there would be attractions, 'force pulls', holding the product together, but this would not amount to chemical bonding.

Annie's notion of 'charges' on atoms (being extra or missing electrons in the outer shell), that led to her not recognising bonding in the NaCl, was an uncommon alternative conception notion. However, her notion that chemical bonding was something other than 'just forces', and that sometimes structures were held together by 'just forces' when there was no bonding, is a common alternative conception. Indeed it is part of a common 'molecular framework' for conceptualising ionic bonding, that is in turn a part of a common alternative conceptual framework for thinking about chemical bonding, stability and reactions: the octet framework.


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⚗︎).