User talk:Kero584

"It is impossible for a molecule to have two point group designations. Its point group is D3h"
You sure?--Smokefoot (talk) 01:34, 16 December 2008 (UTC)

Pretty sure, in reference to point groups (http://en.wikipedia.org/wiki/Point_groups_in_three_dimensions) it doesn't make sense for a specific geometry to have two symmetry designations. Each point group is meant to represent one type of symmetry, so every object should have one unique point group. In the case of PCl5 it has a trigonal bipyramidal geometry. I can understand why someone would think that it can have a C3v point group designation. For one, looking back over this wikipedia article, C3v is listed to correspond to trigonal bipyramidal geometry, which I will change as soon as I finish this reply. But if you think of the C3v point group a molecule such as ammonia with trigonal pyramidal structure clearly lacks at least one symmetry element that a trigonal bipyramidal compound would have. The easiest one to explain here, I believe, is the σh. The molecule has a plane of symmetry formed by the plane composed of 3 chlorine atoms and the central phosphorus atom. There is no such symmetry for a C3v point group, so a trigonal bipyramidal structure could not belong to that point group.Kero (talk) 03:56, 17 December 2008 (UTC)Kero584
 * My comment was somewhat playful. And your correction was certainly appropriate.  I was thinking that there are probably cases where species adopt different geometries depending on circumstances, the way that they are crystallized for example.  There is a case, cited in some inorganic texts, of [Ni(CN)5]3- crystallizing in both the C4v and D3h geometries.  The PCl5 case might do the same. Kind of wonky point. Happy editing,--Smokefoot (talk) 04:09, 17 December 2008 (UTC)