molecular geometry

Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. The chemical formula and the structure of a molecule are the two most important factors that determine its properties, particularly its reactivity. Structure also plays an important role in determining polarity, phase of matter, color, magnetism, and taste, among several other properties. Molecules, by definition, are most often held together with covalent bonds involving single, double, and/or triple bonds, where a "bond" is a shared pair of electrons (the other method of bonding between atoms is called ionic bonding and involves a positive cation and a negative anion). Isomers are types of molecules that share a chemical formula but have different geometries, resulting in very different properties.
A pure substance is composed of only one type of isomer of a molecule (all have the same geometrical structure).
Structural isomers have the same chemical formula but different physical arrangements, often forming alternate molecular geometries with very different properties.
Stereoisomers may have very similar physicochemical properties and at the same time very different biochemical activities.
Protein folding refers to the complex geometries and different isomers that proteins can take. Molecular geometries can be specified in terms of bond lengths, bonds angles and torsional angles. The bond length is defined to be the average distance between two atoms bonded together in any given molecule. A bond angle is the angle formed by three atoms bonded together. For four atoms bonded together in a straight chain, the torsional angle is the angle between the plane formed by the first three atoms and the plane formed by the last three atoms. Molecular geometry is determined by the type of bonds between the atoms that make up the molecule. Before atoms interact to form a chemical bond, the atomic orbitals mix in a process called orbital hybridisation. The two most common types of bonds are:

the sigma bond
the pi bond

The VSEPR model is one way to generally represent the geometric shape individual molecules will take. The AXE method is commonly used in formatting molecules to fit the VSEPR model. If the connectivity, bond lengths, bond angles and torsional angles are found, a molecules exact geometry is known. For any non-linear molecule with N atoms, 3N-6 internal coordinates need to be specified in order to know the exact geometry of the molecule.

Molecular Geometry

See also