Wednesday, June 3, 2009

Configuration Numbers Greater Than N=7

Quite possible the geekiest string of words I've ever put together.

Anyway, an introduction. For determining the molecular geometry ('shape' for those not freaks like me) of simple molecules, a system called VSEPR (Valence Shell Electron Pair Repulsion) theory can be used. Basically, the idea is that which a group of atoms is clustered around the central atom, the available electron pairs will move to the farthest points on the sphere. When there's four electon pairs, for example, as in methane (CH4), ammonia (NH3), and water (OH2), the four pairs become the vertices of a tetrahedron. However, not all those elctron pairs are always taken - ammonia only uses 3, so it's in the shape of a pyramid, while water only uses two and is simply bent (the famous mickey-mouse shape).

For two available pairs, the electrons are simply in a line; for 3 they form the vertices of a triangle; for 4 a tetrahedron, for 5 a trigonal bipyramid; for 6 an octahedron; and for 7 a pentagonal bipyramid. For above 7, though, there's very few compounds / ions that fit. This Dartmouth site, though, has the 8-pair ion XeF8-2 and the 9-pair ion ReH9-2.

While discussing this in class with my chem teacher (I ask him questions that are way above the scope of the class, just for fun) I figured out three possible configurations for the 8-pair vertices. One was a cube, which I rejected because it put the electrons awfully close together. The second I can't remember, but the third, which is correct, is the square antiprism - sort of a cube, but with one face twisted 45°.

Here is another good site, this one with more information on all the hybridized electron orbitals, including the 8-pair, 9-pair, and weird cerium-nitrate 12-pair groups.

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