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This section explores how we predict the molecular and electron-pair shapes of molecules using the VSEPR (Valence Shell Electron Pair Repulsion) theory. We will first go over what VSEPR theory is and how it defines an electron-pair geometry and a molecular geometry.
Make a model of an ammonia molecule using one blue ball and attaching three white balls to it using the rigid gray pegs. Draw a diagram of the model and a Lewis structure of ammonia below.
Valence shell electron pair repulsion (VSEPR) theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie-Nyholm* theory after its two main developers.
Molecular models are designed to reproduce molecular structures in three dimensions. If models are correctly assembled, many subtle features concerning shapes of molecules – such as dipole moment, polarity, bond angle, and symmetry – will become clearer. One aspect of molecular structure is isomerism.
In the ammonia molecule, the three hydrogen atoms attached to the central nitrogen are not arranged in a flat, trigonal planar molecular structure, but rather in a three-dimensional trigonal pyramid (Figure \(\PageIndex{6}\)) with the nitrogen atom at the apex and the three hydrogen atoms forming the base.
The objectives of this laboratory are to: Write Lewis structure representations of the bonding and valence electrons inmolecules. Use the VSEPR model to predict the molecular geometries (shapes) of molecules. Determine whether a molecule is polar (has a dipole moment) or not.
The VSEPR model can predict the structure of nearly any molecule or polyatomic ion in which the central atom is a nonmetal, as well as the structures of many molecules and polyatomic ions with a central metal atom.
