Prediction of Molecular Polarity for Simple Chemical Species

Copyright © 1997 by William L. Lockhart

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The general principle in predicting molecular polarity is the comparison of similar regions of the molecule.

I. If all similar regions are not the same, the chemical species is polar unless symmetry takes preccedence. A general idea of the polarity direction (towards the negative region) may be obtained from electronegativity values and/or formal charge.

II. If all similar regions are the same, the chemical species is nonpolar.

It is important to draw a distinction between bond polarity (as determined by electronegativity differences) and molecular polarity (as determined by the shape of the molecule). It is unfortunate that the same words (polar and nonpolar) have been used in both situations.

You must be able to construct a correct Lewis structure for a given molecule in order to determine its geometry.

Linear Examples

Trigonal Planar Examples

BCl3		all triangle corners are the same. Nonpolar.
BCl2Br		all triangle corners are not the same. Polar. Cl more electronegative than Br. since there are two Cl, polarity direction towards Cl.
BClBr2		all triangle corners are not the same. Polar. with two Br versus one Cl, the polarity direction is expected towards the Br.
SO3		all triangle corners are the same. Nonpolar.
SO2		all triangle corners are not the same. Polar. the lone pair on the S versus two O would make a reasonable polarity direction difficult even with O more electronegative than S

Tetrahedral Examples

CH4		all tetrahedron corners are the same. Nonpolar.
CH3F		all tetrahedron corners are not the same. Polar. Even with three H versus one F, the polarity direction would likely be toward F
NH3		all tetrahedron corners are not the same. Polar. with threeH versus one N, but N more electronegative than H and a N lone pair, polarity direction would be toward the lone pair
H2O		all tetrahedron corners are not the same. Polar. with twoH versus one O, but O more electronegative than H and two O lone pairs, polarity direction would be toward the lone pairs

Trigonal Bipyramidal Examples

PCl5		two axial positions are the same, and three equitorial positions are the same. Nonpolar.
PCl4F		two axial positions are not the same. three equitorial positions are the same. Polar. Since the three equitorial positions cancel each other for polarity, the polarity direction would be toward the axial F.
PCl3F2		the two axial positions are the same the three equitorial positions are the same. Nonpolar.
SF4		the two axial positions are the same the three equitorial positions are not the same. Polar. Polarity direction would be quite difficult to predict with two F versus a lone pair.
ClF3		the two axial positions are the same the three equitorial positions are not the same. Polar. With two lone pairs versus one F, polarity direction would likely be toward the lone pairs.

Octahedral Examples

SF6		all octahedron corners are the same. Nonpolar.
XeF4		all octahedron corners are not the same. All opposite corners are the same; symmetry takes precedence. Thus, there is no preferred polarity direction. Nonpolar.

Pentagonal Bipyramidal Examples

IF7

The two axial positions are the same. the five equitorial positions are the same. Nonpolar.

Additional Examples to Consider

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