Define valence shell expansion?

Valence shell expansion can better be explained by the valance shell electron pair repulsion theory. It was first developed by R.J. Gillespie and Ronald Nyholm, who were chemists during the 1950's. This theory is often taught to chemistry students because it is so quick and easy to learn the methods that are involved in guessing the way that atoms are arrange inside a molecule.

• Rules of VSEPR theory.

The most important benefit of teaching the VSEPR theory is that is an extremely simple concept to grasp. This model is based upon the assumption that pairs of electrons replace each other when they are in position around the atom. Pairs of electrons that are bonded together want to be as far away from each other as possible so they repel each other. However, lone pairs of electrons i.e. Electrons that are not bonded together take up more space.

• Applications of the VSEPR theory.

The arrangement of electrons in an atom according to the VSEPR theory depends entirely on how many are present. For example, if there are four electrons present in an atom with no lone electrons then according to VSEPR, the bonds will be arranged in a tetrahedral shape. If an atom has three bonds then they will be arranged in pyramidal shape around the centre of the atom.

• Limitations of VSEPR theory.

Although VSEPR theory seems extremely simple and quick to learn and that it seems to explain the arrangement of molecules in atoms very well, it does have quite a few limitations. First of all, the theory does not apply to all molecules for example, complex ones that have been formed in reactions with transition metals.

Secondly, the VSEPR theory does not allow the chemist to do calculations. The only thing that you can do with this theory is to gain a rough idea of the shape of the molecule.