Valence Shell Electron Pair Repulsion (VSEPR)

Valence Shell Electron Pair Repulsion (VSEPR) 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 molecules 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 cant 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 180Â °, 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 120Â °, 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 180Â ° bond angle. For another example, consider the carbonate ion, CO32-. 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 120Â ° 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 145Â °)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 molecules Source R.J. Gillespie (2008), Coordination Chemistry Reviews vol. 252, pp. 1315-1327, Fifty years of the VSEPR model