Molecule

Molecules [ moleky ː l] (older also: molecule [ molecular ː kəl ], from Latin molecula, " small mass " ) are broadly two - or multi-atomic particles that are held together by chemical bonds and are at least as long stable that they, for as can be observed spectroscopically. It may be neutral particles, but also radicals, ions or ionic adducts. For example, many types of interstellar molecules under terrestrial conditions are not stable. IUPAC names such particles molecular structure (molecular entity).

Basics

In a narrow sense and in common usage of the chemical molecules are electrically neutral particles, which are composed of two or more atoms. The atoms form a self-contained, chemically dangling Association and are covalently linked. A so- defined molecule is the smallest particle of a given pure substance and has a determinable molecular mass. A molecule is not a rigid structure, occur in different molecular vibrations on the energy supply.

Molecules can be composed of atoms of a single chemical element, such as oxygen (O2) and nitrogen (N2) ( element molecules). Mostly, however, molecules are associations of atoms of different elements, such as water (H2O ) and methane ( CH4). The arrangement of the atoms ( its structure ) in a molecule, are fixed by the chemical bonds. So different, despite the same number of atoms involved ethanol ( H3C - CH2 -OH) of dimethyl ether ( H3C -O- CH3) and are represented by different chemical formulas. In certain cases, the molecules may be present, such as the molecules of the lactic acid forms with the same constitution but differing three-dimensional arrangement ( configuration). That same molecular formulas allow different molecules is commonly called isomerism.

A single molecule, the chemical properties of a substance. The physical properties such as boiling or melting point of a molecular substance are determined by intermolecular forces and can result in solids for formation of molecular lattices. Large molecules are called macromolecules. Consist of macromolecules plastics such as PET and biopolymers such as starch.

The size of diatomic molecules is in the range of 10-10 m (1 Å), relatively large molecules from very many atoms reach a diameter in the range of 10-9 m (10 Å ), the macromolecules can still be a bit bigger. Experimental can be estimated, for example, with the oil stain test, the size of molecules.

The bonding in molecules are explained, for example, with the VSEPR model or the MO theory and described.

Demarcation

Not all chemical compounds consist of individual molecules. No molecules are eg diamond-like materials, such as boron carbide ( B4C ) and silicon carbide ( SiC) ago. The atoms are indeed held together by covalent bonds, but a typical molecule can not be locked. The chemical formula is only a ratio formula. The arrangement of the atoms can be represented by a unit cell, which always recovers and end with formal open ( unused ) valence electrons on their surface.

No molecules are present in salts such as sodium chloride (NaCl), held together by ionic bonds. Again, the formula again the ratio of the atoms involved and also the association of atoms can in principle be of any size and reach the range of a few millimeters. Basic elements of this type of compound are particles ( here atoms) with a charge. Such particles are commonly called ions. The sodium atom is a cation (Na ), chlorine atom, an anion (Cl - ). In the case of sodium sulfate ( Na2SO4), the anion consists of one atom of SO42 - dressing which carries a charge. Atom associations with charges are not mentioned in the narrow sense molecules. This is also customary in organic chemistry: acetic acid consisting of molecules, the anion of the acid known as acetate ion. A special case is the mass, in which the term molecular ion is used.

In the narrow sense polyatomic radicals are not called molecules, because these particles are not chemically saturated. There is enough and clearly to call them radicals. Particularly in organic chemistry, they are highly reactive intermediates in certain chemical reactions. However, there are stable radicals such as nitric oxide or TEMPO. Here intermolecular forces lead to physical properties of the compounds and these compounds can be regarded as a molecular weight.

Modes of representation

There are different ways to display molecules. Basically between formula spellings, two-dimensional schemes (so-called Structural formulas ) can be distinguished and three-dimensional models (so-called stereo formulas ). Empirical formulas have the lowest, stereo formulas the highest information content. To close of abstracted spelling on the actual arrangement, you should be aware that the covalent bonds are arranged approximately in the form of a tetrahedron. (see Covalent bonds; Spatial orientation)

Common spatial models of molecules are the space-filling and stick model.