[Ciara]

PF5 belongs to the D3h point group. Determine a convenient set of hybrid orbitals for P.

I know how to answer this if it gave the irreducible representation, but how do you do it without it? please help

[Tilc]

Hey!
I have understood you want to know how to determine D3h group from the molecule. You can do it looking at this tree:

http://csi.chemie.tu-darmstadt.de/ak/immel/tutorials/symmetry/images_flowchart.gif

It's easy solving each question on the tree if you have seen all the simmetry elements (plains, axis,...) at first.

For example water (H2O): there are a C2 axis and 2 C2v plains parallel to C2. On the tree we answer the questions printed:
1)Linear: No
2)Two or more Cn with n >or= 3? no
3)Cn? yes, n=2
4)Select Cn with highest n;then are n C2 perpendicular to Cn? No, Cn=C2 here and there aren't any C2 perpendicular
5)σh? No
6)nσv? Yes, there are 2σv (σ parallel to C2)
7) Grup theory: Cnv =C2v

I hope you like this answer.