VSEPR Theory

VSEPR is the acronym for Valence Shell Electron Pair Repulsion theory. VESPR is a model used to predict the geometry of molecules based on minimizing the electrostatic repulsion of a molecule's valence electrons around a central atom.

Examples: 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.

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.

According to this theory

1. If a central atom is surrounded by two electron pairs, (one bonding and one anti-bonding or lone pair), the arrangement of them around the central atom will be such that these pairs are at a very far distance from each other. Thus, the arrangement would be linear (Diagonal) and the angle between them would be 180o.

2. When the central metal atom is surrounded by three electron pairs , these three are present in the form of a triangle to aid maximum distance between them. Thus, the spatial arrangement would be a Trigonal shape and the angle between them would be 120o for three bonding pairs.

3. The central atom surrounded by four electron pairs (Bonding + antibonding = 4) will having a geometry of a regular tetrahedral, the four electron pairs as apart from each other as possible, and at a bond angle of 109.5o.