Polar Covalent Bonding

Covalent and Ionic bonding represent the two extremes in a continuum of bonding, the equal sharing and complete transfer of electrons respectively. There is a vast middle ground where the electrons are shared between the pair of atoms, just not equally. This type of bonding is termed Polar Covalent bonding. It arises when the two atoms have a Δχ between 0.4 and 1.7. The more electronegative atom pulls the bonding electrons closer to itself, resulting in a permanent partial negative charge on the atom. The other atom loses the electrons so gains a permanent partial positive charge. Formal charges can also result in polar covalent bonding.

	The more electronegative Cl atom (χ = 3.16) pulls the bonding electrons closer to itself, resulting in a permanent partial negative charge, indicated by the higher electron density (red color). The more electropositive H atom (χ = 2.20, Δχ = 0.96) loses electron density shown by the blue color. The permanent partial charges also result in a permanent dipole moment for the molecule. It will react to external electric fields (or other polar molecules) so that the charges align.		A second example is carbon monoxide. The Δχ difference is 0.89. Although the O atom is more electronegative, the triple bond results in a formal +1 charge on the O (-1 on the C). This can be seen by the higher electron density on the C atom (the red indicates the lone pair of electrons).
Hydrochloric Acid		Carbon Monoxide