*Achary* Housecroft-9780131755536_04, 10/5/7, 23, 23:39, page: 97 Chapter 4 . Point groups Fig. 4.10 Scheme for assigning point groups of molecules and molecular ions. Apart from the cases of n ¼ 1 or 1, n most commonly has values of 2, 3, 4, 5 or 6. Apply the strategy shown in Figure 4.10: Are there three C2 axes perpendicular to the principal axis? Is there a !h plane (perpendicular to the principal axis)? Worked example 4.6 Point group assignments: 3 To what point group does POCl3 belong? No START Is the molecule linear? Does PF5 have Td , Oh or Ih symmetry? Is there a Cn axis? No Yes; a C3 axis containing the P and two axial F atoms Yes; each lies along an equatorial P!F bond Yes; it contains the P and three equatorial F atoms. STOP The point group is D3h . Self-study exercises 1. Show that BF3 belongs to the D3h point group. 2. Show that OF2 belongs to the C2v point group. 3. Show that BF2Br belongs to the C 2v point group. The structure of POCl3 is: 97