KEY2CHEM

Bond Polarity

Polar covalent bonds result when the two atoms bonded together have a difference in electronegativity. Recall that the most electronegative elements are in the upper right hand corner of the periodic table and the least electronegative elements are in the lower left hand corner of the periodic table. The larger the difference in electronegativity between the two atoms, the more polar the bond. Increasing bond polarity is also referred to an increase in percent ionic character of the bond.

Example 1.

Which bond is most polar?

A. $$\require{mhchem}\ce{H-O}$$

B. $$\require{mhchem}\ce{H-N}$$

C. $$\require{mhchem}\ce{H-C}$$

Solution

A. $$\require{mhchem}\ce{H-O}$$

The elements hydrogen and oxygen have the greatest difference in electronegativity, so the $$\require{mhchem}\ce{H-O}$$ bond is most polar.

Example 2.

Which bond is least polar?

A. $$\require{mhchem}\ce{F-Br}$$

B. $$\require{mhchem}\ce{F-Cl}$$

C. $$\require{mhchem}\ce{F-F}$$

Solution

C. $$\require{mhchem}\ce{F-F}$$

Although $$\require{mhchem}\ce{F}$$ is highly electronegative, the $$\require{mhchem}\ce{F-F}$$ bond is nonpolar, since each $$\require{mhchem}\ce{F}$$ has the same electronegativity. Bond polarity is related to the difference in electronegativity between the two atoms bonded together, not just how electronegative one atom is.

Example 3.

Which bond is most polar?

A. $$\require{mhchem}\ce{Cl-P}$$

B. $$\require{mhchem}\ce{Cl-S}$$

C. $$\require{mhchem}\ce{Cl-As}$$

Solution

C. $$\require{mhchem}\ce{Cl-As}$$

The most polar bond is the one with the greatest electronegativity difference between the atoms in the bond. Since $$\require{mhchem}\ce{Cl}$$ and $$\require{mhchem}\ce{As}$$ are furthest apart on the periodic table, they will have the greatest difference in electronegativity and will therefore create the most polar bond.