Chemical Education Today
Letters Elastic Sphere Model of Hydrocarbon Molecules In a previous article, rows of balloons were used for molecular modeling (1). Twenty hydrocarbon molecules, from methane to adamantane and acetylene, were modeled (2). To obtain a longer lifetime and easier use, balloons were replaced by elastic polyurethane sponge balls. Space filling and valence orientation in molecules are automatically achieved by using elastic balls, each sphere representing an electron-pair cloud (orbital), either bonding or nonbonding. Nuclei of atoms are located at the junction of several balls, except for the hydrogen nuclei that lie inside the balls. Elastic forces of these balls are modeling the forces in the molecules. Automatic fixing of molecular geometry, as opposed to fixed-valence orientation, easy rotation around single or triple bonds, resistance to rotation around double bonds, and tensions in molecules felt while making cyclic structures of 3 or 4 carbon atoms are easily demonstrated. Geometric orientation of the orbitals can be achieved by rigid spheres, such as the Bent model (3) or can be found even in nature, such as the nut model (4), but feeling tensions inside the molecule requires elastic spheres. Examples of hydrocarbon and fullerene models are shown in the online material. Atoms in molecules do not retain the shape of the free atoms. The space filling of a molecule is due to the bonding and nonbonding clouds of electron pairs rather than the van der Waals radii and internuclear distances (as in most physical models). Therefore modeling molecules by means of clouds of electron pairs, physically represented by elastic spheres, can offer
an alternative description of space filling and some properties of molecules. Rows of elastic spheres can be assembled easily to molecular models. The proposed physical model fits closely the VSEPR-LCP model of Gillespie and Robinson (5). Literature Cited 1. Niac, G. J. Chem. Educ. 1978, 55, 303. 2. Niac, G. Leonardo Electronic Journal of Practices and Technologies 2008, 12, 175–192. http://lejpt.utcluj.ro/A12/175_192.pdf (accesssed Sep 2009). 3. Bent, H. A. J. Chem. Educ. 1968, 45, 768–778. 4. Niac, G.; Florea, C. J. Chem. Educ. 1980, 57, 429. 5. Gillespie, R. J.; Robinson, E. A. J. Chem. Soc. Rev. 2005, 34, 396–407.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Nov/abs1271_2.html Full text (HTML and PDF) with links to cited URL and JCE articles Supplement Detailed description of the elastic sphere model, including 10 photographs Gavril Niac Universitatea Tehnica ˘ Catedra de Chimie Cluj-Napoca, Romania
[email protected] © Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 11 November 2009 • Journal of Chemical Education
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