These covalent bonded molecules do not have electrostatic charges like
those of ionic bonded substances. In general, covalent compounds are gases,
liquids having fairly low boiling points, or solids that melt at relatively low
temperatures. Unlike ionic compounds, they do not conduct electric currents.
When the electronegativity difference is between 0.4 and 1.6, there will not
be an equal sharing of electrons between the atoms involved. The shared
electrons will be more strongly attracted to the atom of greater electronegativity.
As the difference in the electronegativities of the two elements increases above
0.4, the polarity or degree of ionic character increases. At a difference of 1.7 or
more, the bond has more than 50% ionic character. However, when the difference
is between 0.4 and 1.6, the bond is called a polar covalent bond. An example:
TIP
Polar covalent bonds have unequal sharing of electrons. Their electro-negativity difference is
between 0.4 and 1.6.Notice that the electron pair in the bond is shown closer to the more
electronegative atom. When these nonsymmetrical polar bonds are placed around
a central atom, the overall molecule is polar. In the examples above, the chlorine
(in HCl) and oxygen (in H 2 O) are considered the central atoms. Both the bonds
and the molecules could be described as polar. Polar molecules are also referred
to as dipoles because the whole molecule itself has two distinct ends from a
charge perspective. Because of this unequal sharing, the molecules shown are
said to be polar molecules, or dipoles. However, polar covalent bonds exist in
some nonpolar molecules. Examples are CO 2 , CH 4 , and CCl 4. (See Figure 12.)