A chemical bond is a lasting attraction between these atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the electrostatic force of attraction between oppositely charged ions as in ionic bonds or through the sharing of electrons as in covalent bonds. Therefore, the electromagnetic force plays a major role in determining the internal properties of most objects encountered in daily life.
A covalent bond is a chemical bond formed by shared electrons. Valence electrons are shared when an atom needs electrons to complete its outer shell and can share those electrons with its neighbor. The electrons are then part of both atoms and both shells are filled. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding.
The simplest and most common type is a single bond in which two atoms share two electrons. Other types include the double bond (e.g. H2C=CH2), the triple bond, one- and three-electron bonds, the three-center two-electron bond and three-center four-electron bond.
Covalency is greatest between atoms of similar electronegativities. Thus, covalent bonding does not necessarily require that the two atoms be of the same elements, only that they be of comparable electronegativity (i.e. elements that lie near one another in the periodic table). There is no precise value that distinguishes ionic from covalent bonding, but an electronegativity difference of over 1.7 is likely to be ionic while a difference of less than 1.7 is likely to be covalent.