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.
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.
