Tag: ions

Sodium and chlorine don't actually overlap or anything

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. She was shown a representation of part of a lattice in sodium chloride.

Focal figure (Fig. 5) presented to Annie

Any idea what that's meant to be?

(pause, c.6s)

Just sodium and chlorine atoms

That's sodium and chlorine atoms, erm would you say that there was any kind of bonding there?

No.

Although the image included the standard '+' and '-' symbols to signify that ions were shown, Annie referred to "atoms". It transpired that 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 had already identified chemical bonding in representations of molecules of hydrogen , tetrachloromethane , and oxygen, so she was asked why she though there was no bonding in this example:

No bonding. Why do you say that? What is the difference between that and the ones we've seen before?

Well the other ones electrons were shown, and these no electrons are shown and they don't actually overlap or anything they just go in rows.

They go in rows. Okay. … but unlike (the images) we've seen previously they've had bonds in,

Yeah.

chemical bonds, whereas this, we don't have chemical bonds?

No.

So Annie did not interpret the representation of NaCl as portraying bonding. However, on further probing she did recognise that the structure could get held together by forces.

When Annie was asked if what was shown in the figure would would fall apart or hold together, Annie suggested that If you heated it, or reacted it in some way, it would hold together, and it would probably get held together by just forces. However, she did not consider that (i.e., even after reacting) amounted to chemical bonding. (Read: Sodium has one extra electron in its outer shell, and chlorine is minus an electron, so by force pulls they would hold together.)

The canonical interpretation of the figure is that it is a slice through a three dimensions structure of ions, where the attractive forces between cations pull the ions into a bound structure (to the point where attraction and repulsions are in equilibrium), and that this kind of binding is called ionic bonding.

Annie did not see ions, but atoms. She thought there was no bonding because no overlap was shown. In chemistry a wide range of different types of representation are used to show structures at the submicroscopic level – bonds may sometimes be shown by lines or sometimes by overlap or (in the case of ionic structures) neither. This is a potential source of confusion for learners who may not appreciate why different conventions may be used to represent different, or even the same, structures.

K-plus represents a potassium atom that has an extra electron

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.

Earlier in her interview she had suggested that plus and minus signs represent the charges on neutral atoms when discussing the Na-plus (Na+) and Cl-minus (Cl–) symbols, suggesting an alternative conception of electrical charge in relation to atoms, ions and molecules She gave similar interpretations in the case of K-pus (K+) and F-minus (F–):

Right, okay, so this one here where it's got a K and a plus, what does that represent?

Potassium…An atom that has an extra electron.

Potassium atom, and it's got one extra electron over a full shell

Yeah.

and that's what the plus means, one more electron than it wants?

Yeah.

And what about the F minus?

Represents fluorine which has one, it has an outer shell of seven which has one less electron.

So, for Annie:

K+ referred to the potassium atom (2.8.8.1), not the cation (2.8.8)

and

F– referred to the fluorine atom (2.7), not the fluoride anion (2.8)

Students often present incorrect responses in class (or in interviews with researchers) and sometimes these are simply slips of the tongue or memory, or 'romanced' answer guessed to provide some kind of answer.

When a student repeats the same answer at different times it suggests the response reflects a stable aspect of their underlying 'cognitive structure'. In Annie's case she not only provided repeated answers with the same examples, bit was consistent in the way she interpreted plus and minus symbols across a range of different examples, suggested this was a stable aspect of her thinking.

If you take all of the electrons off an atom, then it would not be matter

Keith S. Taber

Mohammed was a participant in the Understanding Science Project. When Mohammed was near the end of his first term of upper secondary science (in Y10) he told me that in his chemistry lessons he had been studying atoms and ionic bonding. When I asked him what an atom was, he suggested that an atom is the smallest amount of matter you can get [*] as well as being "it's the building block of all matter".

The notion that atoms are the smallest components of matter has a strong historical pedigree – but the modern idea of the atom is unlike the solid and indivisible (= atomos: uncuttable) elementary particles imagined by some Greek philosophers. Modern atoms are considered complex structures, and may be dismantled.

It is not unusual for students to suggest that atom is the smallest thing that one can get, and then go on to describe atomic structure in terms of smaller components! The idea that the atom is the smallest thing possible (a kind of motto or slogan) is commonly adopted and then retained despite learning about subatomic particles.

Mohammed, however, justified his suggestion that an atom was "the smallest amount of matter you can get" by arguing that "matter is something that is built out of protons, neutrons and electrons". So Mohammed's notion of what counted as 'matter' (an ontological question) was at odds with the scientific account

Mohammed did not suggest that matter had to have overall neutrality, and his suggestion that matter is something that is built out of protons, neutrons and electrons had to be amended when he realised it would exclude hydrogen atoms as being matter:

So what if I had a balloon full of hydrogen gas, would that, would the hydrogen be matter?

Yeah.

So would that consist of protons, neutrons and electrons?

No it wouldn't. Sorry, can I take away the neutrons

Okay, so matter's what then? What's our new definition of matter?

Protons, electrons.

Mohammed presented his responses with confidence and without hesitation, which seemed to suggest he was offering well established ideas. However, he did not seem to have fully thought through these ideas, and perhaps was constructing a rationale in situ in the interview. The logical consequences of Mohammed's new definition was that atoms and ions would be considered matter but not nuclei or electrons.

What if I had sodium. Do you think that would be matter?… if I had a lump of sodium, would that be matter?

Yeah

And why is that matter?

Because it has, it has a full atom, it has protons, neutrons, electrons, even though you can have no neutrons.

Okay, but it has to have the protons and the electrons?

Yeah.

Now what if I just had one atom of sodium, would that still be matter?

Yeah.

…so let's say I've got my atom, with my eleven protons, and my probably twelve neutrons I think usually. And I've got eleven electrons round the outside. If I take take one of the electrons off this atom, it's not an atom any more is it?

It's an ion.

Now is it still matter?

Yeah.

Because I've still got protons and electrons. What if I took a second electron off, could I take a take second electron off?

Yeah.

What have I got then, then?

You've still got matter.

What if I took a third one off?

Well if you, if you just take all of them off, then you'd stop having matter.

So if I've got eleven electrons, can I take ten of them off?

Yeah.

And I'd still have matter?

Yeah.

The idea of what counts as matter here seems a rather idiosyncratic alternative conception (rather than being a common alternative conception that is widely shared). Science teachers would probably consider that all material (sic) particles are matter, and – perhaps – that this should be obvious to students. However, the submicroscopic realm is far from everyday experience so perhaps it is not surprising that students often form their own alternative conceptions.

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⚗︎).