Molecule

Molecular compounds

A constitutional body of molecules is called molecular compound . The majority of the liquid or gas bodies (under the usual conditions of temperature and pressure) and the components of the living beings are molecular compounds.

Examples: the gas oxygenates, the hydrogen gas, the gas nitrogenizes, the Eau, the Carbon dioxide, the Méthane, the Butane, the Propane, the Ammoniac, the chloric gas and the ethanol is current molecular compounds.

In a few grams of such a body, there is million billion billion molecules (see Nombre of Avogadro).

The systematic chemical name of a molecular compound, unlike the common noun, makes it possible to know the constitution of the molecules, and thus to avoid confusion with the name given to the chemical elements. Example: the carbonic gas ( common noun ) for systematic name carbon dioxide indicating that its molecule () comprises two oxygen atoms.

A Element has molecules made up of only one kind of atoms. It should not be confused with the element. For example: the Dioxygène () and the trioxygene (), commonly called Ozone, are two elements made up of the only element oxygenates. The antonym of element is " made up Body ". The molecule of a made up body contains several type different of atoms.

A pure substance molecular contains only one kind of molecule; its chemical composition is then confused with that of the molecules which constitute it. The antonym of pure substance is " mixture ". The total chemical composition of a mixture depends on the proportion of the pure substances which constitute it and their nature.

Scheduling of the molecules

The molecules of a body are in permanent agitation (except with the Absolute zero). This agitation, called Brownian Movement, was described the first time by Robert Brown in 1821 in the liquids (but explained almost 100 years later).

When the body is with the state of Gaz, the molecules are very spaced, very agitated, with disordered movements caused by the shocks between them or with the solid bodies with which they are in contact (walls).

When the body is with the state Liquide, space between the molecules is much more restricted, agitation much slower.

With the solid state, the molecules are arranged according to a regular stacking, and vibrate around an average position.

The Température of a body gives an indication of the degree of agitation of the molecules.

The forces of interaction of very low intensity which are exerted remotely between the molecules, called Forces of Van der Waals condition these arrangements and consequently the physical properties of the molecular compounds. Thus, for example, the physical properties exceptional of the Eau are due for much to the Hydrogen bonds.

Stability of the molecules

The molecules are units a priori electrically neutral, in which the Atome S are dependant between them mainly by covalent bonds, (there exist many examples of assemblies supra-molecular by connections Van der Waals, hydrogen or ionic), where appear sometimes electronic dissymmetries being able to go until giving Ion S by Solvatation (polar solvents). Consequently, one must conclude that the dihydrogene (), the dichlore, the difluor and so much of other diatomic gases, are electrically neutral. What implies that when they are insulated, they are zérovalents, to respect the equivalence which there must be in any equation balanced in loads and overall neutral like: 2 =. Here, in the part of the réactants, the dihydrogene and the dioxygene are isolated molecules and thus do not have a clean load, like (although polar molecule). The chemical equation thus checks the neutrality of the total load.

The form and the size of a molecule (or one of its parts) can play a part in its aptitude to be reacted. The presence of certain atoms or groups of atoms inside a molecule plays an important role in its capacity to break or fix other atoms resulting from other bodies, i.e. to change to give rise to other molecules.

The various modes of Représentation of the molecules are intended to clarify the various reactive sites; certain sequences of atoms, called functional groups produce similarities of properties thus, particularly in the made up organics.

Macromolecules

The molecules having at least several tens of atoms are called Macromolécules.

Examples: The plastics are primarily made chains of carbon atoms which can contain several tens of thousands of atoms, from where physical properties completely interesting. The Proteins, the Lipids, the Sugars, the nucleic acids such DNA and others Biomolécules of big sizes are also macromolecules where the large variety of the internal chemical bonds induce a chemical reactivity often very selective playing an important role in the biological activity of the living beings.

Nonmolecular bodies

There exist two different main categories of nonmolecular pure substances:

the Métaux, which contain only one kind of atoms all related the ones to the others by pooling total and delocalized electrons, called metal Liaison,
the ionic compounds which gather atoms (or grouping of atoms) of which some support an excess of electrons (Anion S) distributed among others accepting a deficit of electrons (Cation S), the cohesion of the unit then is ensured by the electric forces present and is called ionic Connection.

Molecules in space

The external layers of stars contain, in spite of the extreme temperatures which reign there, of the very robust molecules like carbon monoxide.

The comets and the gas atmospheres of planets contain a larger variety of molecules.

In interstellar space, where the probability of meeting between atoms is very weak, one finds assemblies unstable (radical) of a diversity remained a long time unknown which is perhaps at the origin of the first molecules of the world of the alive one.

Historical elements

See also: Historical of the concept of molecule

The concept of molecule was presented the first time in 1811 by Amedeo Avogadro, which knew to overcome the confusion made at that time between atoms and molecules, because of the laws of the definite and multiple proportions of John Dalton (1803-1808).

The analysis of Avogadro was accepted per many chemists, with notable exceptions (Boltzmann, Maxwell, Gibbs). But the existence of the molecules remained under discussion open in the scientific community until the work of Jean Perrin (1911) which then confirmed in experiments the theoretical explanation of the Brownian Movement in terms of atoms suggested by Albert Einstein (1905). Jean Perrin also recomputed the number of Avogadro by several methods.

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