Abstract
The atomic charges in BF3 calculated by the atoms in 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 fully ionic (R. J. Gillespie, J. Chem. Educ. 1998, 75, 923). However, atomic charges calculated by an alternative approach (from the atomic polar tensors) are lower by more than 30%. Calculations based on a spherical ion model show that this model underestimates the mean bond energies of BF3 and SiF4 by more than 40%, and calculations based on a polarizable ion model indicate that the F- ions are considerably distorted in the direction of polar covalency. It is concluded, therefore, that a description in terms of a completely ionic model may be misleading.
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