VSEPR Theory?

Question

Hi,

In 1940 two chemists named Sidgwick and Powell came up with a theory on the structures of molecules and were the first to introduce the linear, trigonal planar, tetrahedral, trigonal bypyramid, and octahedral geometries. Although they developed the key molecular geometries, their theory was incorrectly stated: "Bonding pairs and lone pairs are of equal importance and distribute themselves to minimize interelectron repulsion." What fundamental modification did Gillespie and Nyholm add to Sidgwick and Powell's theory to develop the VSEPR theory that we use today? Give an example of two different molecules that violate the incorrect statement in the Sidgwic/Powell theory and give evidence as to why they violate the old theory?

Are bonding pairs more important and they try to get as far from one another as possible? It does seem like the bonding pairs and the lone pairs are of equal importance so I don't know what's wrong.

Thanks!

Answer 1

You have put your finger on the crucial feature. Lone pairs are on average closer to the central atom than the shared the electrons used in bonds. So they are, in effect, bulkier because of electron-electron repulsion.

Answer 2

Sidgwick Powell Theory

Answer 3

It depends on the molecule in question. If it is just two atoms, such as an oxygen - oxygen bond. A double bond will still form a linear complex. However, a double bond (present in an oxygen molecule) is stronger and shorter than a single bond between the same atoms; furthermore, a triple bond, such as in Nitrogen - Nitrogen is stronger and shorter than both a double and single bond between the same atoms.