A bond between neutraly charged atoms (usualy non-metals). The atoms share electrons. Structures made from covalent bonds are called Molecules and include Organic Compounds, Diatomic Gases, Water, and many other compunds. Covalent compounds are seperated into two groups: Polar Covalent, and Non-Polar Covalent.
See Also
Metallic Bond
Ionic Bond
A covalent bond is one where the electrons responsible for the bond are shared between the two atoms in the bond. This is achieved when the orbitals of the two electrons are hybridised to form a bonding orbital that extends over both atoms.

Unless the two atoms are the same (and have the same atoms bonded beyond them) there is polarisation of the bond. Whether the resultant molecule is polarised (has a dipole) depends upon whether the polarisations of the individual bonds cancel or not.

The situation becomes more complex if the bond is not a single bond. In this case there has to be a further hybridisation of orbitals where the electrons are not directly between the atomic centres but are displaced to either side. The electrons are still associated with the two atoms unless that are other double bonds. It the multiple double bonds can form an aromatic system then the electrons for the aromatic bonds extend over all the atoms of the aromatic system.

The Covalent Bond is the bond found usually between non-metals, but can occur between non-metals and metals. It is the result of constructive interference between the valence or outer shell electrons. This interference creates an are of high probability for the electrons and hence creates an area of high electronegativity, which in turn attracts the two nuclei towards itself, forming the bond. And thus for many covalent bonds this results in uneven distribution of charge, creating dipoles which allow for hydrogen bonds. This is the case in water.

Covalent bonding is successfully described with quantum physics, and the quantum model of the atom, unlike rutherfords solar system analogy or bohrs model.

Covalent bonds are chemical bonds where electrons are shared between two atoms. Covalent bonding forms molecules. It occurs when the difference of electronegativity between the two atoms is less than 1.67, except when a metallic bond is formed. If the difference is 1.67 or greater, then an ionic bond is formed. Covalent bonds are formed to let each atom have full outer electron configurations. This consists of typically 8 electrons in the outer electron configuration, however, a configuration of only two electrons also is chemically stable.

Covalently bonded molecules have (in general) low melting points, are in the forms of brittle solids or gases, and are easily dissolved in water, however, when they are dissolved, they do not typically conduct electricity. Covalent bonds are generally weaker than ionic bonds.

Covalent bonds formed between two atoms of the same element are said to be "perfectly covalent" because their electronegativity. This is most common in the seven elements that occur natrually in diatomic form: H, N, O, F, Cl, Br, and I. This causes the only truly nonpolar covalent molecules.

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