Molecular geometry or molecular structure is the three dimensional arrangement of the atoms that constitute a molecule, inferred from the spectroscopic studies of the compound. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.

A defined molecular geometry at equilibrium can only be expected at temperatures close to absolute zero because at higher temperatures the atoms will wobble around. The molecular geometry can be measured by X-ray crystallography and computed by quantum mechanical calculations or through semi-empirical molecular modeling. Larger molecules often exist in multiple stable chemical conformations that differ in their molecular geometry.

The position of each atom is determined by the nature of the chemical bonds by which it is connected to its neighboring atoms. The molecular geometry can be described by the positions of these atoms in space, evoking bond lengths of two joined atoms, bond angles of three connected atoms, and torsion angles of three consecutive bonds.

Bonding

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).

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 the centers of 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:

• Sigma bond
• Pi bond

An understanding of these bonds is in the domain of valence bond theory, which relies on an understanding of the wavelike behavior of electrons in atoms and molecules.

Isomers

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. The atoms are not bonded (connected) together in the same orders.
  • Functional isomers are special kinds of structural isomers, where certain groups of atoms exhibit a special kind of behavior, such as an ether or an alcohol.

• Stereoisomers may have many similar physicochemical properties (melting point, boiling point) and at the same time very different biochemical activities. This is because they exhibit a handedness that is commonly found in living systems. One manifestation of this chirality or handedness is that they have the ability to rotate polarized light in different directions.

• Protein folding concerns the complex geometries and different isomers that proteins can take.

See also

• Atomic orbital
• Covalent bond
• Electron configuration
• Extended Huckel method
• Molecular orbital
• Valence bond theory
• VSEPR

External links

• Covalent Bonds and Molecular Structure

Molecular geometry

هندسة جزيئية | Molekülstruktur | VSEPR