The molecular framework: an alternative conceptual framework for thinking about ionic bonding.

Learners commonly develop alternative conceptions about ionic bonding. Indeed, some of the most common of these alternative conceptions can be understood to make up a coherent (alternative) conceptual framework - the 'molecular framework':-

• The history conjecture: An ionic bond only exists where there has been the transfer of an electron between two atoms (indeed for some learns, the bond is the transfer of an electron);
• The valency conjecture: An element can only form as many ionic bonds as its electrovalency, i.e. Na forms Na⁺ and so each Na⁺ ion is only bonded to one counterion;
• The just forces conjecture: Adjacent counter-ions are only bonded if they have had an electron transferred between them: otherwise they are attracted just by forces, not bonds *.

(* Note: A bond is seen as something other than the actions of forces.)

Ion-pairs in a substance like NaC that are considered bonded (i.e. to have been formed by an electron transfer event between them) are often considered to be molecules, or at least to act as molecular-like units (e.g. remaining as a unit when the salt dissolves).

The molecular framework is linked to the full shells explanatory principle, an alternative conception adopted by students which explains chemical bonding and chemical change as driven by the 'need' of atoms to acquire octets/full electron shells. As such, the molecular framework may be held as part of the more extensive octet rule framework for making sense of chemistry in terms of sub-atomic quantiles such as atoms, molecules, ions.

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Dr Keith S Taber kst24@cam.ac.uk

University of Cambridge Faculty of Education

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