The valency conjecture

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: the valency conjecture

The original evidence that students thought in this way derived from interviews with students. 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 thinking in relation to the 'valency' conjecture.

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

Each chloride ion in the diagram is bonded to only one sodium ion

True or False:

Each chloride ion in the diagram is bonded to only one sodium ion

A sodium atom can only form one ionic bond, because it only has one electron in its outer shell to donate

True or False:

A sodium atom can only form one ionic bond, because it only has one electron in its outer shell to donate

Each sodium ion in the diagram is bonded to only one chloride ion

True or False:

Each sodium ion in the diagram is bonded to only one chloride ion

A chlorine atom can only form one ionic bond, because it can only accept one more electron into its outer shell

True or False:

A chlorine atom can only form one ionic bond, because it can only accept one more electron into its outer shell

Sources:

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