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.
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⚗︎).
