What is Molecular Geometry

Molecular geometry the name given to the geometry that describes the shape of a molecule. It can also be defined as the positions of the atomic nuclei in molecules. Not all molecules have the same shape of course. The shape of a molecule depends on the position of the molecules electrons. The molecular structure is a three-dimensional arrangement of atoms that create a molecule. The shape of a molecule is determined by the number of bonded electron pairs that are surrounding the atom, and by how many of the pairs are in a bond. The bonded atoms in a molecule are accountable for establishing the molecular geometry.

The most well-known theory for displaying and describing different molecular shapes is the VESPR Theory. The VESPR Theory clearly states that electrons in compounds will arrange themselves as far apart as possible. VESPR stands for Valence Shell Electron Pair Repulsion. The valence shell is the outer-most shell, occupied by electrons, of an atom that contains the electrons that are involved in a bond. According to the theory, molecular geometry is predicted by starting with the electron pair geometry of the central atom and then adding atoms to one or all of the electron pairs.

The VESPR Theory will result in six different molecular shapes: Linear, Trigonal Planar, Bent, Pyramidal, Tetrahedral, and Octahedral. A linear shape is between two atoms or three atoms that involve a multiple bond. The bond angle is 180 degrees like a flat line would measure to. A trigonal planar has a 120 degrees bond angle and consists of a central atom with three bonds and no unshared pairs. An example would be Aluminum Chloride (AlCl3). Bent has a central atom with two bonds and two unshared pairs. The bond angle is 105 degrees. If you were determining the molecular shape of PH3 you would find yourself with a central atom with three and one unshared pair, which can be described as pyramidal. This pyramid shape has a bond angle of 107 degrees. Another shape is tetrahedral. A tetrahedral has a bond angle of 109.5 degrees. It has a central atom with four bonds, no unshared pairs. The last molecular shape is octahedral. The geometry for this molecule has a central atom with six pairs of electrons.

So molecular geometry is an overall arrangement of atoms. The arrangement is displayed as different molecular shapes with bond angles and electron pairs. The shapes are determined from the percise position of the electrons.