Problem 9.35

Predict whether the following molecules are polar or nonpolar:

To predict whether or not a polyatomic molecule will have a dipole moment (i.e, be a polar molecule), we note that molecules in which the central atom is symmetrically surrounded by identical atoms are nonpolar. For molecules of the general formula AB_n, certain geometries - linear (AB₂), trigonal planar (AB₃), tetrahedral and square planar AB₄, trigonal bipyramidal AB₅, and octahedral (AB₆) - will lead to nonpolar molecules no matter how polar the individual bonds are. In these cases, the individual bond dipoles will cancel each other out.

(a)IF

This species is polar - there are only two atoms in this molecule, and they differ in electronegativity, so by definition the molecule is polar.

(b)CS₂

Draw out the Lewis structure; there are two double bonds on the central C atom, and we count these as two electron pairs. The molecular geometry is linear (two pairs, both bonding) - this is the case of linear AB₂, and the molecule is nonpolar.

(c)SO₃

Draw the Lewis structure - the electron pair geometry is trigonal planar (the S-O double bond counts as one pair.) The molecular geometry is trigonal planar - we therefore have an AB₃molecule with a trigonal planar geometry. This species is nonpolar.

(d)PCl₃

This molecule is of the general formula AB₃; its molecular geometry is trigonal pyramidal (the only geometry for AB₃ which leads to a nonpolar molecule is trigonal planar.) This species is polar.

(e)SF₆

Here we have an AB₆-type molecule with an octahedral molecular geometry. It is nonpolar.

(f)IF₅

Draw the Lewis structure of this species; its electron pair geometry is octahedral, but its molecular geometry is square pyramidal (5 bonding/1 nonbonding.) The only geometry for formula AB₅ which leads to a nonpolar molecule is trigonal bipyramidal; this species is therefore polar.