Ionic bonding
Ionic bonding refers to a class of chemical bonding between ions found in some substances (i.e., salts). The bond is understood in simple terms as due to an electrostatic interaction between counter ions – cations (positive ions) and (negative ions) due to their opposite electrical charges.
Strictly, in any stable structure, the components must be in equilibrium – that is, there must be a balance of forces – so, in the crystal a lattice of a salt, there is an equilibrium between attractions and repulsions. We can think of the ions being attracted closer together until the point where repulsion become large enough to balance attractions.
The lattice arrangement means that in general cations are surrounded by anions which are much closer to them than the nearest other cations; and, for that matter, in general anions are surrounded by cations which are much closer to them than the nearest other anions.
The paradigm example often used is NaCl where each Na+ is surrounded by 6 Cl– in an octahedral arrangement (in effect a counter ion to the left of, to the right of, above, below, in front of, and behind, each ion) and similarly each Cl– is surrounded by 6 Na+ in an octahedral arrangement.
Learners are expected to appreciate that the attractive forces between counter-ions (oppositely charged ions, cations with anions) gives rise to the binding of the lattice. However, often learners either do not appreciate the basic electrical principles or, at least, do not bring them to mind in chemical contexts (for example thinking a +1 charged ion can only be attracted to one negative ion {the 'valency conjecture' misconception}).
Read about students' ideas about electrical interactions in the ionic lattice
A common alternative conception of ionic bonding
When secondary or college students who have studied ionic bonding are asked to explain the bonding it is very common for them to explain ionic bonding as being, or at least to be due to, an electron transfer event from a metal atom to a non-metal atom. For example, it may be explained that a sodium atom has an extra electron in its outer shell that it dos not need/want, and it donates it to the chlorine atom which has a gap for such an electron and would like to fill its shell. (Note the anthropomorphic language – with atoms being treated as if sentient actors that know their electronic status and act to change it!)
Of course, there will be substantial variations on how particular students explain this. So, some learners will refer to octets of electrons rather than full shells. The molecular framework can be seen as part of a wider alternative conceptual framework found n chemistry, sometimes called the octet framework.
Read about 'the Octet framework'
Such a description describes how ions could hypothetically be formed from two isolated atoms, but does not describe the bonding. It also refers to an energetically enviable process – although a free electron could spontaneously be captured by a chlorine atom, to give the Cl– ion (which, of course, would still not give a full outer shell), the sodium atom is more stable than a sodium ion and separate electron: it takes energy input to ionise a sodium atom (and not enough energy is released by the capture of the electron by a chlorine atom to cover this). So this process would not happen.
Indeed chemical processes seldom begin with reactants in the form of discrete atoms. The only elements commonly found as comprised of isolated atoms are the noble gases, which do not have much common chemistry.
Despite its irrelevance, this chemical 'just so' story of ion formation seems to appeal, and is commonly found in teaching materials at early secondary level. It is clearly a persuasive story as
a) student will often produce this account of the bonding in NaCl after themselves preparing NaCl by neutralisation (i.e., starting with solutions already containing the ions that will bond to form the salt when the solvent is evaporated).
b) even after being taught the more scientific account in senior classes, students will tend to revert to talk about ionic bond as (or, as due to) electron transfer.
A slice through the NaCl structure. Each ions is bonded into the lattice by equivalent interactions with each of its neighbours.
The alternative conceptual framework
The molecular framework comprises four related alternative conceptions.
(From Taber, Tsaparlis & Nakiboğlu, 2012 [Download article])
In the example of NaCl, these take the form:
The valency conjecture: each singly charged ion can only form one bond
Read about the valency conjecture
The history conjecture: a bond exits between a pair of ions that have been involved in an electron transfer event
Read about the history conjecture
The 'just force' conjecture: oppositely charged ions that have not shared an electron transfer event do not have proper/formal bonds between them, just attractions (cf., "The bond is understood in simple terms as due to an electrostatic interaction between counter ions")
Read about the 'just forces' conjecture
With the result that students commonly consider that there are Na+-Cl– ion pairs which make up sub-units of the structure, ether being molecules, or having a similar role to that played by molecules in simple covalent solids.
Read about student ideas about molecules in ionic lattices
The idea of ion-pairs forming molecules is often reinforced by poorly phrased texts, e.g.,
"When two particles combine to make a molecule, they usually do so in one of two ways: by electrostatic attraction between two oppositely charged ions (ionic bond)…"
Article in the Institute of Physics magazine, Physics World
Some examples of what students think about ionic bonding
The following posts give examples of what students say:
- Ionic bonding – where the electron's transferred to complete the outer shell
- Sodium has one extra electron in its outer shell, and chlorine is minus an electron, so by force pulls they would hold together
- A sodium atom wants to donate its electron to another atom
- In ionic bonding, they both want to get full outer shells
The molecular framework reinforced…
Student conceptions can be encouraged by incorrect or imprecise language in texts and other sources.
Explaining dehydration of body tissue
This example comes form a highly respected scholar, but who – as a physicist – does not distinguish between ions and molecules:
"The excess salt molecules [sic] are not diffused through the vein walls and exert a pressure against it; this extra pressure is balanced by the pressure of water diffusing in the opposite direction."
Bunge, 1998
Sources cited:
- Bunge, M. (1998). Philosophy of Science. Volume 2: From explanation to justification. Routledge.
- Taber, K. S. (1994) Misunderstanding the ionic bond, Education in Chemistry, 31 (4), pp.100-103.
- Taber, K. S. (2013). A common core to chemical conceptions: learners' conceptions of chemical stability, change and bonding. In G. Tsaparlis & H. Sevian (Eds.), Concepts of Matter in Science Education (pp. 391-418). Dordrecht: Springer. [Download the chapter.]
- Taber, K. S., Tsaparlis, G., & Nakiboğlu, C. (2012). Student Conceptions of Ionic Bonding: Patterns of thinking across three European contexts. International Journal of Science Education,34(18), 2843-2873. doi: 10.1080/09500693.2012.656150. [Download this article]
