Illustrated Glossary of Organic Chemistry

Polar covalent bond: A covalent bond in which the electron density is unevenly shared between the two bonded atoms, due to a difference in electronegativity or due to inductive effects. In general the electronegativity difference must be 0.5 or more before the bond is labeled as a polar covalent bond instead of nonpolar covalent bond.

ΔEN	Bonding	Bond Example
0.0 - 0.4	Nonpolar covalent bond	H-C, C-C
0.5 - 0.9	Slightly polar covalent bond	H-N, H-Cl
1.0 - 1.3	Moderately polar covalent bond	C-O, S-O
1.4 - 1.7	Highly polar covalent bond	H-O
1.8 - 2.2	Slightly ionic bond	H-F
2.3 - 3.3	Highly ionic bond	Na⁺ F^-

The relationship between electronegativity difference (ΔEN) of bonded atoms and bond polarity.