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 where "they both want to get full outer shells"*.
Some learners have been found to see the electron transfer process described by Mohammed (which is purely a way of conceptualising ion formation, and has little connection with what actually happens when ionic substances form) as being the bond, and sufficient to hold species together. However, Mohammed did recognise the role of electrical forces in holding the species together:
And did you say that if you take a sodium atom and a chlorine atom, you get salt?
Yeah… Sodium chloride. And the way they bond together, is because now one, the sodium has lost an electron. And they start off neutral because the protons and electrons balance each other out, because the same number of them, but when you lose one you get plus one in the sodium, and like when you have a chlorine, you add an electron so you get minus one. In the end the whole compound is neutral, but because erm, like they, they're differently charged they attract together, and they bond together. I think.
So if I could imagine a sodium atom and chlorine atom, and then they form salt, what would it look like afterwards? How could I imagine it afterwards.
Oh it's like two atoms joined together.
So here Mohammed is clear (if somewhat tentative) that a bond has been formed. Yet his focus is on the iteration of two atoms, forming ions, whereas ionic bonding needs to be understood as the various interactions at work in a lattice. The process of ion formation described by Mohammed:
- would not actually be energetically viable for two atoms (as the electron affinity of chlorine is 364 kJ mol-1, whereas the first ionisation energy of sodium is 494 kJ mol-1, so the electron capture of a chlorine atom would not release enough energy to remove the electron from the sodium atom);
- neither sodium nor chlorine atoms are stable under normal chemical conditions: neither sodium nor chlorine used in a binary synthesis would be in the form of discrete atoms; and sodium chloride is more likely to be formed by neutralisation using substances where the sodium and chloride ions are already present, e.g. sodium hydroxide and hydrochloric acid.)
Mohammed's explanation conflates two levels – the macroscopic level of bench phenomena (such as the substance sodium chloride – common salt) and the level of models at the submicroscopic scale of molecules, ions and atoms. Even if one atom of sodium could interact ('react' is better kept as a term for what occurs at the level of substances) with one atom of chlorine in the manner Mohammed envisages, to give the "two atoms joined together", that entity could not meaningfully be identified as salt as many of the properties of salts emerge from the ionic lattice of myriad ions.
[If sodium-chloride ion pairs could be formed then we might consider these as the component quanticles of a form of sodium chloride, but this would need to be considered a different allotrope to table salt, just as ozone molecules are not the basis of the oxygen in air that supports respiration.]
The "two atoms joined together" sounds much like a molecule (and it is very common for students to identify molecule like ion-pairs even in representations of extensive ionic lattices), so I asked Mohammed about this.
