Chemical bond

Overflight

In a way simplified in a Covalent bond, an electron or a pair of electrons can be located in space being between two core X atomic, because in this area the negative electrons are subjected to the positive load of each core instead of the load only of the one of them. At the same time, the electrons present between the cores prevent the repulsion between those and make that these cores are attracted towards the electrons which are closer and thus towards the other atom instead of being pushed back by this one. This situation tends to maintain the core and the electrons in a relatively fixed configuration although they are free to be driven in agreement with the constraints of the quantum Mécanique. Same manner, while simplifying, in a ionic Connection, one or more electrons is simply transferred from one atom to the other making that one of the atoms becomes a positive ion and the other of a negative ion. The connection is thus due to an electrostatic attraction between the atoms. This description is very simplified. The reason for which an atom transfers an electron towards another is a complicated subject which implies the quantum theory.

In theory, any connection can be explained by the quantum theory but, in practice, the chemical bonds are divided into several categories like both referred to above. Simplifications of the quantum theory were formulated to describe and predict the connections and their properties. Among these theories, we have, the Règle of the byte, the Théorie of the connections of valence, the theory of the hybridization of orbital the, the Théorie VSEPR, the Théorie of fields of ligands and the method LCAO. Electrostatic theories and other physical theories are used to describe the polarities of the connections and their effects on chemical substances. The real chemical bonds are not described exactly by these theories because of the Principe of uncertainty. However, as a whole, they constitute a powerful tool which can be applied in almost all chemistry. In quantum mechanics, in simplified terms, the electrons are localized on a atomic Orbitale but, in a chemical bond, they form a molecular Orbitale.

The orbital one can be

flexible; in this case, the electrons of connection have the greatest probability of being between the cores that elsewhere; the orbital one then tends to maintain the cores together
not-flexible: in this case, the electrons have more probability of being more close to one of the cores, the orbital one tends to disadvantage the connection.

Moreover, the orbital molecular ones are classified according to the type of orbital atomic hybridées to form the connection. These orbital atomic is due to the interactions electron-core caused by electromagnetic forces. Chemical substances will form a bond if their orbital will lower their energy when they interact one with the other. There are various chemical bonds according to the form of the electronic cloud and their energy levels.

History

The first thoughts on the nature of the chemical bond since the beginning of the 12th century supposed that certain types of chemical species were bound by certain types of chemical Affinité.

In the middle of the 19th century, Edward Frankland, F.A. Kekule, A.S. To cut, A.M. Butlerov and Hermann Kolbe, building the theory of the radicals, developed the theory of the valences, called at the beginning of " to be able of combiner" in which the compounds were dependant thanks to the attraction of poles positive and negative.

In 1916, the chemist Gilbert Lewis developed the idea of the connection per pair of electrons. Walter Heitler and Fritz London is the authors of the first explanation by the quantum mechanics of the chemical bond, especially that of molecular hydrogen, in 1927, using the theory of connections of valence. In 1930, the first quantum mathematical description of the simple chemical bond was developed in the thesis of doctorate of Edward Teller.

In 1931, the chemist Linus Pauling published what is sometimes regarded as the most important text of the history of chemistry: “One the Natural off the Chemical Jump”.

In this paper based on work of Lewis, Heitler and London, and on its own work, it presents six rules for the connection with divided electron; the three first were generally known:

the connection by division of electrons is formed by the interaction of a lone electron of each of the two atoms
the Spin S of the two electrons must be opposite
once coupled, the two electrons cannot enter another connection

Its three other rules were new:

the exchange of electrons for the connection includes/understands only one function of wave for each electron
the electrons available in the lower energy level form the strongest connections
of two orbital of an atom, it is that which can be superimposed more on orbital of another atom which will form the strongest connection

Connections in the chemical formulas

Like the atoms, the molecules and their orbital are three-dimensional, it is difficult to use simple techniques to represent them. In the molecular formulas, the chemical bond (orbital flexible) between two atoms is indicated various manners according to the needs.

Sometimes, she is completely ignored. For example, in Organic chemistry, the chemists are sometimes interested by the functional Groupe of the molecule. Thus, according to the need, the molecular formula of ethanol can be written on paper

according to the configuration,
in three dimensions,
in two full dimensions (indicating each connection without the three-dimensional directions),
in two compressed dimensions (CH₃-CH₂-OH),
by separating the functional group from the remainder of the molecule (C₂H₅OH), or
by atomic components (C₂H₆O).

Sometimes, even the cloud of electrons not-binder is sometimes indicated. (with the directions in 2 dimensions approximate. For example elementary carbon:: _.^'C_.^' Certain chemists indicate also the orbital ones; for example, the hypothetical anion ethene^-4 (_\^/C=C_/^\ ^-4) indicating the possibility of formation of connection.

Strong chemical bonds

These chemical bonds are intramolecular forces which maintain the atoms together in the molecules and the solids. These connections can be simple, double or triple i.e. the number of electrons participating (or contained in the orbital one of connection) is of two, four or six. An even number of electrons is usual because the paired electrons have a lower energy. In fact, theories more advanced on the connections show than the bonds are not always caused by an integer of electrons and, this, depend on the distribution of those in each atom concerned in the connection. For example, carbons in the Benzène are related one to the other by approximately 1,5 connections and the two atoms in the nitric Oxyde NO are connected by approximately 2,5 connections.

Quadruple connections are not impossible but very rare. The type of connection depends on the difference of electronegativity and the distribution of orbital possible in the dependant atoms. With more electronegativity is important, with more the electron is attracted by a particular atom and with more the connection ionic is. If electronegativity is low, the connection is covalent.

Covalent bond

The covalent bond is the common type of connection where the difference in electronegativity between the dependant atoms is small or null. In this last case, one speaks sometimes about purely covalent connection. See the articles Connection sigma and Connection pi for usual explanation LCAO of the not-polar connections.

Polar covalent bond

The polar covalent bond is by nature an intermediate type of connection between the covalent bond and the ionic connection. In more advanced theories, one considers that all the connections are of this type.

Ionic connection

The ionic connection is an electrostatic type of interaction between atoms of which the difference in electronegativity is higher than 1,6 (this limit is conventional). With such a difference in electronegativity, the electronic transfer is regarded as total, the most electronegative atom becomes a Anion and electropositive Cation. There exist polyatomic ions which carry an electric charge and in addition contain covalent bonds between the atoms the component. The ionic loads, in general, have values between 4th and +7e.

Coordinative covalent bond

The coordinative covalent bond is a special connection in which the electrons of connection come from only one of the atoms, but are roughly shared also by both in orbital. This configuration is different from an ionic connection with weak difference in electronegativity.

Polyatomic ions

A type different of connections between two or more atoms appears in the ions. The connection is located in the middle of the three (or more) atoms.

Chemical bonds including/understanding more than two atoms.

Aromatic connection

Often the orbital ones have a form and a density complexes and very often the position of the electrons cannot be reduced with simple features (localization of two electrons) or points (localization of an electron). It is the case when the compound obeys the Règle of Hückel (rule of the 4n +2). In the Benzene, the aromatic compound example, 18 electrons of connection connect 6 carbons to form a ring plane. The Ordre of connection (average of the number of connections) between various carbons can be considered (18/6) /2=1.5 but in this case all the connections are identical from the chemical point of view. They can sometimes be like simple connections alternating with double connections, but it is more correct to say than all the connections are equivalent of value. In the case of heterocyclic aromatic Compound and of substituted benzene, the differences in electronegativity of the parts of the ring influences the chemical behavior which if not would be equivalent.

Metal connection

A metal connection just like exists only in one solid (or in the liquid state) the ionic connection. In the metal connection, the electrons are mobile in a whole network whereas in an ionic connection, the loads are almost static.

Intermolecular connections

Connections between permanent dipoles

Hydrogen bond

The hydrogen bond or hydrogen bridge is a chemical bond of low intensity which connects the molecules, and which implies an atom of Hydrogène.

Connection of van der Waals

The connection of van der Waals is an interaction of low intensity between atoms, molecules, or a molecule and a crystal.

Interactions between cations and connections pi

Electrons in the chemical bonds

Limitation of the theory of connections of valence

Determination of the chemical properties

See too

Covalent bond: Connection π, Connection σ, osidic Connection
Hydrogen bond,
ionic Connection,
metal Connection,
Connection of van der Waals.
the Interaction orbitalaire.
nuclear Energy binding

Simple: Chemical jump

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