Chemical bond is the strength which holds together two or several atoms, ions, molecules, or a combination of them. In its nature, it is an electrostatic force of attractive between negatively charged electrons and positively charged nuclei. Modern views on the nature of a chemical bond are based on the electron valency theory, which was developed independently by G. N. Lewis and W. Kossel

According to the electron valency theory, atoms, while forming bonds, tend to achieve the most stable (that is, having the lowest energy) electronic configuration. Atoms can achieve this by two means. They can lose or gain electrons and form ions. If atoms gain electrons, then they form anions. If they lose electrons, they become cations. A chemical bond appears between an anion and a cation. This chemical bond represents electrostatic attractive force. Chemical bond of this type is called ion bond.

Atoms can form stable inner electronic configurations through collectivization of electrons. The chemical bond, which is formed in this case, is known as a covalent bond. A covalent bond is formed as the result of collectivization of a pair of electrons, one electron from each atom. However, in some molecules and polyatomic ions both of these electrons can be supplied by only one atom. This sort of covalent bond is called donor-acceptor (coordinating) bond.

The chemical bond in metals presents an especial case. It can be ascribed neither to the ion type, nor to the covalent type. In the solid state metals consist of positively charged ions, which are tightly packed into crystal lattice and kept together by electrons "flowing" around ions. This type of bond is called metallic bond.

There are two more types of chemical bonds, which are hydrogen bond and Van der Waals- forces. Bonds of these two types are much weaker.

Chemical bond differs from intermolecular interactions by considerable redistribution of electronic density in the area of chemical bond in comparison with simple overlapping of electronic densities of unbonded atoms or atomic fragments, which are brought together at a distance of a bond.

All kinds of chemical bonds can be divided into two groups:

One can hardly distinguish between strong and weak chemical bonds. Anyway, there are certain differences between them. Covalent interactions are accomplished when there is overlapping between the electronic clouds of the subsystems. Weak interactions appear between particles, which are rather distant from each other, when overlapping is negligible.

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