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  Home > JCE Print > Journal of Chemical Education > Issues > 1998  > July  >
Information • Textbooks • Media • Resources
Covalent and Ionic Molecules: Why Are BeF2 and AlF3 High Melting Point Solids whereas BF3 and SiF4 Are Gases?
Ronald J. Gillespie
McMaster University, Department of Chemistry, 1280 Main Street West, Hamilton, ONT L8S 4M1, Canada
Cover
July 1998
Vol. 75 No. 7
p. 923
Abstract
Calculated ionic charges show that BF3 and SiF4 are predominately ionic molecules yet in contrast to BeF2 and AlF3 they exist as gases at room temperature and form molecular solids rather than infinite three-dimensional "ionic" solids at low temperature. Whether or not ionic molecules form a three-dimensional infinite ionic lattice or a molecular solid depends more on relative atomic (ionic) sizes than on the nature of the bonding in the isolated molecule. The ionic model for BF3 and BF4- provides a simple explanation of their bond lengths and for the constancy of interligand nonbonding distances. BF3 and SiF4 should be represented by ionic structures rather than by the conventional structures with bond lines that are normally assumed to indicate covalent bonds.

See Letter re: this article.
More Information
*  Citation
 Gillespie, Ronald J. J. Chem. Educ. 1998 75 923.
*  Keywords
 Inorganic Chemistry, Bonding Theory, Molecular Properties/Structure, Boron, Silicon
*  History
 Created:
Last Updated:		 June 22, 1999
June 24, 2005
Link to Letter added (April 2004).
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