VSEPR theory and molecular geometry
The Valence Shell Electron Pair Repulsion Theory ( VSEPR ) is a molecular model for predicting the geometry of atoms making up a molecule where the electrostatic forces between valence electrons in a molecule are minimized around a central atom .
Also known as: Gillespie-Nyholm theory (the two scientists who developed it) – According to Gillespie, the Pauli exclusion principle is more important in determining molecular geometry than the effect of electrostatic repulsion.
Pronunciation: VSEPR is pronounced “ves-per” or “vuh-seh-per”
Examples: According to the VSEPR theory, the methane molecule (CH 4 ) is a tetrahedron because the hydrogen bonds repel and distribute themselves uniformly around the central carbon atom.
Use VSEPR to predict the geometry of molecules
You cannot use a molecular structure to predict the geometry of a molecule, although you can use the Lewis structure . This is the basis of the VSEPR theory. The pairs of valence electrons naturally arrange to be as far apart from each other as possible. This minimizes their electrostatic repulsion.
Take, for example, BeF 2 . If you look at the Lewis structure for this molecule, you see that each fluorine atom is surrounded by pairs of valence electrons, with the exception of the only electron, each fluorine atom of which is linked to the central beryllium atom. The fluorine valence electrons move away as much as possible or at 180 °, giving this compound a linear shape.
If you add another fluorine atom to make BeF 3 , the furthest from the valence electron pairs is 120 °, which forms a planar trigonal shape.
Double and triple links in VSEPR theory
Molecular geometry is determined by the possible locations of an electron in a valence shell, not by how many pairs of valence electrons are present.
To see how the model works for a molecule with double bonds, consider carbon dioxide, CO 2 . While carbon has four pairs of bonding electrons, there are only two places where electrons can be found in this molecule (in each of the double bonds with oxygen). The repulsion between the electrons is less when the double bonds are on the opposite sides of the carbon atom. This forms a linear molecule which has a bond angle of 180 °.
For another example, consider the carbonate ion , CO 3 2- . As with carbon dioxide, there are four pairs of valence electrons around the central carbon atom. Two pairs are in single bonds with oxygen atoms, while two pairs are part of a double bond with one oxygen atom. This means that there are three locations for the electrons. Repulsion between electrons is minimized when the oxygen atoms form an equilateral triangle around the carbon atom. Therefore, the VSEPR theory predicts that the carbonate ion will take a trigonal planar shape, with a bond angle of 120 °.
Exceptions to the VSEPR theory
The Valence pair electron repulsion theory does not always predict the correct geometry of molecules. Examples of exceptions include:
- transition metal molecules (e.g. CrO 3 is bipyramidal trigonal, TiCl 4 is tetrahedral)
- odd electron molecules (CH 3 is planar rather than trigonal pyramidal)
- some AX 2 E 0 molecules (for example, CaF 2 has a bond angle of 145 °)
- certain AX 2 E 2 molecules (for example, Li 2 O is linear rather than curved)
- some AX 6 E 1 molecules (for example, XeF 6 is octahedral pyramidal rather than pentagonal)AX 8 E 1 molecules
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