The Kimball Free-Cloud Model: A Failed Innovation in Chemical Education?

Abstract

This historical review traces the origins of the Kimball free-cloud model of the chemical bond, otherwise known as the charge-cloud or tangent-sphere model, and the central role it played in attempts to reform the introductory chemical curriculum at both the high school and college levels in the 1960s. It also critically evaluates the limitations of the model, its current implicit role in the teaching of VSEPR theory, and its pedagogical implications, as well as providing a resource paper for those chemical educators interested in exploring its present-day applications.

Orbitals in General Chemistry, Part I: The Great Debate

“…"By the 1990s Gillespie was making the further dubious claim that the so-called 'valence-shell electron domain' or VSED model was in fact an alternative to orbital models, rather than a crude method for approximating localized MO, and that the VSEPR approach itself did not require the use of any orbitals whatsoever for its theoretical justification… However, if it specifically refers to use of the VSED model as an alternative to orbitals, then there is a problem, as the division of the valence-electron density of molecules into spherical, nonoverlapping domains with integral populations consisting of pairs of electrons of opposite spins is identicalhowever you may choose to relabel it -to a use of the Kimball free-cloud model and definitely corresponds, despite claims to the contrary, to the use of an orbital model." 27 Bond order is not an observable property; although one can delineate bonds based on a procedure of counting electrons assigned to bonding and antibonding orbitals, it does not greatly add to an understanding of chemistry, especially when bond orders of fractional quantities, resonance structures with varying bond character or hypervalent compounds are discussed. More directly practically, the bond distance and electron density, which are observables, can be used to discuss bonding.…”

General Chemistry Without Orbitals

“…A student who is unable to understand or to apply the model to solve problems through prediction resorts to memorization, which is the state of teaching chemistry today. 11 Post described these models as 'floating' models, as neither are they derived strictly from theory nor are they based on experimental observation. 12 There is no experimental evidence for any explicit atomic orbital, and, as we show here, the theoretical basis of the use of orbitals is suspect.…”

“…"By the 1990s Gillespie was making the further dubious claim that the so-called 'valence-shell electron domain' or VSED model was in fact an alternative to orbital models, rather than a crude method for approximating localized MOs, and that the VSEPR approach itself did not require the use of any orbitals whatsoever for its theoretical justification… However, if it specifically refers to use of the VSED model as an alternative to orbitals, then there is a problem, since the division of the valenceelectron density of molecules into spherical, nonoverlapping domains with integral populations consisting of pairs of electrons of opposite spins is identical -however you may choose to relabel it -to a use of the Kimball free-cloud model and definitely corresponds, despite claims to the contrary, to the use of an orbital model." 11 There are other and less known uses of orbitals in undergraduate chemical education. For instance, orbitals have been used to describe the shape of isolated, gas-phase atoms, i.e.…”

Wave mechanics fundaments for quantum chemistry in secondary education

“…Die Behandlung der quantenmechanischen Betrachtung der chemischen Bindung in der Schule sollte nicht auf verdünnten Klonen von Hochschultexten [4] zu dem Thema basieren, sondern ein auf die Schule zugeschnittenes Konzept sein, das fachlich konsistent mit dem aktuellen Wissenstand ist. Es ist also erforderlich die Komplexität erheblich zu reduzieren, unter Vermeidung von Widersprüchen zu dem aktuellen Kenntnisstand der Wissenschaft.…”