A pair of electrons shared between two bonded atoms, attracting each of the positively charged nuclei of the atoms and so bonding them together, constitutes a single covalent bond. Each of the bonded atoms provides one electron of the shared pair. Sometimes, both electrons of the shared pair can be provided by just one of the bonded atoms forming a covalent bond that is also known as a 'dative' or 'co-ordinate' bond. Once a dative bond is formed it is indistinguishable from any other covalent bonds between the same atoms. For example, in the ammonium ion, NH4+, the non-bonding pair (lone pair) of electrons on the nitrogen atom is donated to an H+ ion.
:NH3 + H+ ® NH4+
Two shared pairs of electrons between bonded atoms consitutes a double covalent bond, and three shared pairs forms a triple covalent bond.
See also Ionic Bond | Molecule.