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486 14MOLECULAR INTERACTIONS
δ+
δ-
δ-
δ-
S
O
O
O
δ+
δ+
δ-
Xe
F
F
F F
δ-
δ-
δ-
δ+δ+
δ+ δ+
δ-
δ-
δ-
δ+
S
F
F
F
F
δ+
δ-
δ+
δ+
Figure 14.1
An alternative approach is use a symmetry argument. As explained in Sec-
tion 10A.3(a) on page 394, only molecules belonging to point groups Cn , Cnv
or Cs may have a permanent electric dipole moment. SO3 and XeF4 belong to
point groups D3h and D4h respectively, so they are not polar, while SF4 belongs
to point group C2v so is polar.
E14A.2(b) �e magnitude of the resultant dipole moment, µres, is given by [14A.3a–586],
µres = (µ21 + µ22 + 2µ1µ2 cos θ)1/2.
µres = ((2.5 D)2 + (0.5 D)2 + 2 × (2.5 D) × (0.5 D) × cos 120○)1/2 = 2.3 D
E14A.3(b) �e arrangement of charges is shown on the le� of Fig. 14.2
-
6
x/nm
y/
nm
143
"
"
"
"
"
"
"
30○4e −2e
−2e
u
u
u
162
�
?
�������9
µx
µyθµ
Figure 14.2
�e x component of the dipole are given by [14A.4a–586], µx = ∑J QJxJ, and
similarly for the y and z components; note that in this case µz = 0 because
all of the point charges have a z coordinate of zero. �e components are then
combined using [14A.4b–586], µ = (µ2x + µ2y + µ2z)1/2, which is represented
graphically on the right of Fig. 14.2.
µx =∑
J
QJxJ = [ − 2 × (1.6022 × 10−19 C) × (162 × 10−12 m)
− 2 × (1.6022 × 10−19 C) × (143 × 10−12 m) × cos 30○]
× 1 D
3.3356 × 10−30 Cm
= −27.4... D