VSEPR THEORY

Valence Shell Electron Pair Repulsion Theory (VSEPR) is a molecular model to predict the geometry of the atoms making up a molecule where the electrostatic forces between a molecule's valence electrons are minimized around a central atom.

The theory is also known as Gillespie–Nyholm theory, after 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.

According to VSEPR theory, the methane (CH₄) molecule is a tetrahedron because the hydrogen bonds repel each other and evenly distribute themselves around the central carbon atom.

Using VSEPR To Predict Geometry of Molecules

You can't use a molecular structure to predict the geometry of a molecule, although you can use the Lewis structure. This is the basis for VSEPR theory. The valence electron pairs naturally arrange so that they will be as far apart from each other as possible. This minimizes their electrostatic repulsion.

Take, for example, BeF₂. If you view the Lewis structure for this molecule, you see each fluorine atom is surrounded by valence electron pairs, except for the one electron each fluorine atom has that is bonded to the central beryllium atom. The fluorine valence electrons pull as far apart as possible or 180°, giving this compound a linear shape.

If you add another fluorine atom to make BeF₃, the furthest the valence electron pairs can get from each other is 120°, which forms a trigonal planar shape.

Double and Triple Bonds in VSEPR Theory

Molecular geometry is determined by possible locations of an electron in a valence shell, not by how many how many pairs of valence electrons are present. To see how the model works for a molecule with double bonds, consider carbon dioxide, CO₂. While carbon has four pairs of bonding electrons, there are only two places electrons can be found in this molecule (in each of the double bonds with oxygen). Repulsion between the electrons is least when the double bonds are on opposite sides of the carbon atom. This forms a linear molecule that has a 180° bond angle.

For another example, consider the carbonate ion, CO₃^2-. 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 an oxygen atom. This means there are three locations for electrons. Repulsion between electrons is minimized when the oxygen atoms form an equilateral triangle around the carbon atom. Therefore, VSEPR theory predicts the carbonate ion will take a trigonal planar shape, with a 120° bond angle.

Exceptions to VSEPR Theory

Valence Shell Electron Pair Repulsion theory does not always predict the correct geometry of molecules. Examples of exceptions include:

• transition metal molecules (e.g., CrO₃ is trigonal bipyramidal, TiCl₄ is tetrahedral)
• odd-electron molecules (CH₃ is planar rather than trigonal pyramidal)
• some AX₂E₀ molecules (e.g., CaF₂ has a bond angle of 145°)
• some AX₂E₂ molecules (e.g., Li₂O is linear rather than bent)
• some AX₆E₁ molecules (e.g., XeF₆ is octahedral rather than pentagonal pyramidal)
• some AX₈E₁ molecule