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Covalent and Ionic Molecules: Why Are BeF2 and AlF3 High Melting Point Solids whereas BF3 and SiF4 Are Gases?
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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.
Keywords (Audience):
Second-Year UndergraduateKeywords (Domain):
Inorganic ChemistryKeywords (Subject):
BoronCiting Articles
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This article has been cited by 9 ACS Journal articles (5 most recent appear below).
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William B. JensenJournal of Chemical Education2004 81 (12), 1772This brief article calls attention to the failure of most introductory textbooks to point out explicitly the fact that nonmolecular solids do not have molecular formulas and suggests some practical remedies for improving textbook coverage of this subject. ...
Electron Densities, Atomic Charges, and Ionic, Covalent, and Polar Bonds
R. J. GillespieJournal of Chemical Education2001 78 (12), 1688Electron Densities, Atomic Charges, and Ionic, Covalent, and Polar Bonds
R. J. GillespieJournal of Chemical Education2001 78 (12), 1688Electron density is a real, observable property of a molecule that can be determined by experiment. In contrast, an orbital is an abstract mathematical concept that, because it is not physically observable, cannot be observed experimentally. Good quality ...
Should Gaseous BF3 and SiF4 Be Described as Ionic Compounds?
Arne Haaland , Trygve Helgaker , Kenneth Ruud and D. J. ShorokhovJournal of Chemical Education2000 77 (8), 1076Should Gaseous BF3 and SiF4 Be Described as Ionic Compounds?
Arne Haaland , Trygve Helgaker , Kenneth Ruud and D. J. ShorokhovJournal of Chemical Education2000 77 (8), 1076The atomic charges in BF3 calculated by the "atoms in molecules" approach are +2.58 and -0.87 for B and F, respectively; in SiF4 the calculated charges are +3.42 and -0.86, and it has therefore been suggested that these molecules should be described as ...
Reply to Coulombic Models in Chemical Bonding
Derek W. SmithJournal of Chemical Education2000 77 (4), 445Reply to Coulombic Models in Chemical Bonding
Derek W. SmithJournal of Chemical Education2000 77 (4), 445Coulombic vs molecular orbital models for explaining the molecular shapes of ionic molecules.
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Lawrence J. SacksJournal of Chemical Education2000 77 (4), 445Coulombic Models in Chemical Bonding
Lawrence J. SacksJournal of Chemical Education2000 77 (4), 445Coulombic vs molecular orbital models for explaining the molecular shapes of ionic molecules.
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- Received: August 03, 2009
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