Illustrated Glossary of Organic Chemistry

Covalent bond: A chemical bond formed by the overlap of atomic orbitals. The electron pair between the atoms is shared to some extent. A bond is considered to be covalent when the electronegativity difference between the bonded atoms is 1.7 or less.

	In the case of molecular chlorine the electronegativity difference between the bonded atoms is zero (Cl EN = 3.0), and so this is an example of a covalent bond.
	In the case of hydrogen chloride the electronegativity difference between the bonded atoms is 0.9 (Cl EN = 3.0 and H EN = 2.1), and so this is an example of a polar covalent bond.
	In the case of sodium chloride the electronegativity difference between the bonded atoms is 2.1 (Cl EN = 3.0 and Na EN = 0.9), and so this is an example of an ionic bond.