The nature of a covalent bond depends on the relative electronegativities of the atoms sharing the electron pairs. Covalent bonds are polar or nonpolar depending on the difference in electronegativities between the atoms.
Polar covalent bonding occurs between atoms with small differences in electronegativity. The bonding electron pair is not shared equally, but pulled more toward the element with the higher electronegativity. Yet the difference in electronegativity is not high enough for complete electron transfer (ionic bonding) to take place. As a result, the more electronegative atom acquires a partial negative charge, δ–, and the less electronegative atom acquires a partial positive charge, δ+, giving the molecule partially ionic character. For instance, the covalent bond in HCl is polar because the two atoms have a small difference in electronegativity. Chlorine, the more electronegative atom, attains a partial negative charge and hydrogen attains a partial positive charge. This difference in charge is indicated by an arrow crossed (a plus sign) at the positive end pointing to the negative end:
This small separation of charge generates what is known as a dipole moment.
Nonpolar covalent bonding occurs between atoms that have the same electronegativities. The bonding electrons are shared equally, with no separation of charge across the bond. Not surprisingly, nonpolar covalent bonds occur in diatomic molecules with the same atoms. Certain elements exist under normal conditions only as diatomic molecules: N2, O2, F2, Cl2, Br2, I2, H2. Their positions in the periodic table form an inverted L-shape towards the top right, excluding the noble gases.
In a coordinate covalent bond, the shared electron pair comes from the lone pair of one of the atoms in the molecule. Once such a bond forms, it is indistinguishable from any other covalent bond. Distinguishing such a bond is useful only in keeping track of the valence electrons and formal charges. Coordinate bonds are typically found in Lewis acid-base compounds (see chapter 13 on Acids and Bases). A Lewis acid is a compound that can accept an electron pair to form a covalent bond; a Lewis base is a compound that can donate an electron pair to form a covalent bond. For example, in the reaction between boron trifluoride (BF3) and ammonia (NH3):
NH3 donates a pair of electrons to form a coordinate covalent bond; thus, it acts as a Lewis base. BF3 accepts this pair of electrons to form the coordinate covalent bond; thus, it acts as a Lewis acid.