1.3.2 Polar Covalent Bonds

In many covalent bonds the electrons are not shared equally between two bonded atoms. For example, in hydrogen chloride the electrons are unevenly distributed between the two atoms because the atoms that share the electrons in the molecule are different and have different electronegativities. Electronegativity is the tendency of an atom to pull bonding electrons toward it. It increases as you go from left to right across a row of the periodic table or down to up in any of the columns. A bond in which electrons are shared unevenly is called a polar bond or polar covalent bond. A polar bond has a slight positive charge on one end and a slight negative charge on the other end. The greater the difference in electronegativity between the bonded atoms, the more polar the bond will be. The direction of bond polarity can be indicated with an arrow. The head of the arrow is at the negative end of the bond; a short perpendicular line near the tail of the arrow marks the positive end of the bond.

In this notation, the delta can be read as “partially” that indicates the hydrogen atom of HCl is “partially positive,” and the chlorine atom is “partially negative.” The uneven electron distribution in a compound containing covalent bonds is measured by a quantity called the dipole moment (µ). The dipole moment is commonly given in derived units called debyes , abbreviated D. The dipole moment is defined by the following equation:

µ = q r

In this equation, q is the magnitude of the separated charge and r is a vector from the side of positive charge to the site of negative charge. For example, the HCl molecule has a dipole moment of 1.08 D. Molecules that have permanent dipole moments are called polar molecules. Some molecules contain several polar bonds. In such case, each polar bond has associated with it's a dipole moment contribution, called a bond dipole. The net dipole moment of such a polar molecule is the vector sum of its bond dipoles. Carbon dioxide molecule is not a polar molecule, even though it has polar bonds. Carbon dioxide is linear, and the C-O bond dipoles are oriented in opposite directions. So they cancel out each other and the dipole moment is zero.