Molecules in ionic lattices

Ionic lattices contain ions (which may be compound ions, such as SO42- or NH4+, as well as simple ions) but not neutral molecules. Yet it is very common for learners to assume there are molecules, or at least discrete ion pairs, in such lattices.

An aspect of student thinking about ionic bonding

Students commonly develop alternative conceptions of ionic bonding (Taber, 1994). In particular they often understand ionic solid structures in terms of a 'molecular' framework that can be seen as part of a wider 'octet' alternative conceptual framework for thinking about chemistry. The molecular framework includes four related alternative conceptions:-

  • The valency conjecture: each singly charged ion can only form one bond.
  • The history conjecture: a bond exits between a pair of ions that have been involved in an electron transfer event
  • 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")
  • 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 the molecular framework for thinking about ionic bonding

Evidence for student thinking: molecules in ionic substances

The original evidence that students thought in this way derived from interviews with students (but some earlier studies had reported similar comments). This led to a research probe being developed (which was later developed in to a diagnostic instrument for classroom use by teachers). The original instrument had 30 items, but, after feedback from teachers, a shorter, 20 item version was produced.

How do learners conceptualise the interactions in an ionic solid?

Students were presented with a number of statements to judge as true or false. The original sample of students responding to the instrument included upper secondary students (KS4) who were meant to have studied the topic at that ('G.C.S.E.') level; A level students (who had not yet studied the topic beyond KS4 ('G.C.S.E.') level, and A level students who had studied the topic further as part of their A level course.

This work was reported in: Taber, K. S. (1997) Student understanding of ionic bonding: molecular versus electrostatic thinking?, School Science Review, 78 (285), pp.85-95. [Download a copy of the author's manuscript version of the paper]

The shorter, 20-item version of the instrument was later translated to be administered to undergraduate students in Greece and Turkey. (The Greek undergraduates were given a 'forced choice' between 'true' and 'false', without the 'I do not know' option.)

That work was reported in 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 article]

A number of items included in the instrument tested out whether learners perceived molecules n the ionic structure.

(From Taber, Tsaparlis & Nakiboğlu, 2012 [Download article])

In the diagram each molecule of sodium chloride contains one sodium ion and one chloride ion

True or False:

In the diagram each molecule of sodium chloride contains one sodium ion and one chloride ion.

There are exactly fifteen molecules of sodium chloride in the diagram

True or False:

There are exactly fifteen molecules of sodium chloride in the diagram.

There is a bond between the ions in each molecule, but no bonds between the molecules

True or False:

There is a bond between the ions in each molecule, but no bonds between the molecules

There are no molecules in the diagram

True or False:

There are no molecules in the diagram.

Sources:

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