VSEPR theory

The VSEPR model ( VSEPR is the abbreviation of valence shell electron pair repulsion, German valence-shell electron pair repulsion ), and EPA model ( electron pair repulsion model) or originally Vepr - theory (English valence electron pair repulsion theory ), leads the spatial form a molecule back to the repulsive forces between the pairs of electrons of the valence shell.

The model was developed by Ronald Gillespie and Ronald Nyholm and is therefore also called the Gillespie - Nyholm theory.

The derived rules

From the VSEPR model are obtained for molecules of the type AXn following rules:

• The electron pairs on the central atom (A), ie the atom in the center of the molecule, arrange themselves so that the distance between them is as large as possible.
• The lone pairs of electrons ( here symbolized by E) in a molecule of type AXnEm require more space than the bonding electron pairs and thus lead to an increase in the angle XAE and a decrease in the angle XAX.
• Larger electronegativity between A and X, thus reducing the space required by the corresponding binding.
• Multiple bonds occupy more space than single bonds. Here, the space requirement increases with the bond order. Individual free electrons in radicals, however, take up less space than free electron pairs. However, only the sigma- bonds are used to determine the coarse molecular structure. That is, multiple compounds are treated in the determination of the structure, such as single bonds.
• Smaller central atoms or more negatively polarized ligands cause strong steric and electronic repulsion force that can surpass that of a lone pair.

Predictions according to VSEPR with lone pairs on the central atom

Molecular structures can quite easily be predicted by counting the "leftovers" when no lone pairs on the central atom are present. Nevertheless, the approximate consideration of compounds can be schematized with one or more lone pairs of electrons by free electron pairs as binding partners are treated as pseudo- ligands: One arrives here on the pseudo structure of each molecule, the stereochemically active lone pair of electrons with the Greek letter " ψ " be characterized as pseudo- ligands.

The oxygen atom of the water molecule, in which two hydrogen atoms are covalently bonded, has two lone pairs of electrons. Accordingly, results from X = 2 ( hydrogen atoms ) and E = 2 ( lone pairs ) 2 2 = 4 and thus a pseudo tetrahedral molecular shape, which can be described as ψ2 tetrahedra. The molecular shape itself but is described only by the atomic nuclei. By the lone pairs will now be " thought away ", the real structure remains: angled. An example of a ψ1 tetrahedra, ie with only a free electron pair, the ammonia molecule NH3.

Notes:

Limits of applicability

The VSEPR model can be applied to molecules in which the bound to the central atom radicals (atoms or groups of atoms ) are not too large and exert no specific interactions with each other. Not applicable or restricted she is on transition metal compounds. Often, however, agree well for simple molecules, the bond angles are not in accordance with the model. For compounds with delocalized electrons, the application of the model can also be fraught with difficulties, then here the involvement of molecular orbital theory is necessary.