c) Polar Covalent Bonds

 Atoms, similar to preschoolers, don't forever play fair. This means that at times in a covalent bond the electrons are not joint equally between the two atoms. On regular, one of the atoms partially "pulls" the bonding electrons toward itself, creating an unequal sharing of those bonding electrons. This is called a POLAR COVALENT BOND. In order to determine whether a covalent bond is polar or not, it's necessary to understand electronegativity.

Recall that electronegativity is a measure of an atom's skill to draw its bonding electrons to itself. Each element has a numeric value corresponding to its electronegativity. The values used here were devised by Linus Pauling, however there are a few other scales of electronegativity values.

 

Fluorine was determined to be the most electronegative element and has an electronegativity value of 4.0. Francium, the least electronegative element, has a value of 0.7. It is imprtant to remember a general trend in the periodic table: electronegativity increases from left to right going across a period, and it increases from the bottom to the top of a group. For example, in period 2, nitrogen (group 5A) has an electronegativity value of 3.0, compared to 2.5 for carbon (group 4A). In group 5A, phosphorous (period 3) has an electronegativity value of 2.1, compared to 3.0 for nitrogen (period 2).

In an act that resembles toddlers tugging on a toy, a polar covalent bond occurs when one atom with a higher electronegativity draws the bonding electrons toward itself, pulling those electrons away from the atom with the lower electronegativity value. This creates an unequal sharing of electrons known as UNEQUAL CHARGE DISTRIBUTION, or charge separation. The charge separation makes the bond polar because the more electronegative atom becomes partially negatively charged and the atom with the lower electronegativity becomes partially positively charged.

Imagine H2 and HBr. In H2, the atoms have an equal "pull" on the bonding electrons, making the bond NONPOLAR.

In HBr, but, Br has an electronegativity of 2.8, compared to 2.1 for hydrogen. The Br atom pulls the bonding electrons toward itself, creating a partial negative charge      on itself and a partial positive charge on the hydrogen atom  

     

The bigger the absolute difference in electronegativity between two atoms, the additional polar that bond is. For instance, the electronegativity difference of a carbon-oxygen bond is -1.0, the result of 2.5 (the electronegativity value for carbon) minus 3.5 (the value for oxygen). The absolute value for the difference in electronegativity is the value without the minus sign (1.0 for a carbon-oxygen bond). For a carbon-chlorine bond, the difference in electronegativity is 0.5 (2.5 - 3.0 = -0.5). Thus, a carbon-oxygen bond is more polar (1.0) than a  carbon-chlorine bond (0.5) .