There is no sharp dividing line between ionic and covalent bonding. However,
a good rule of thumb is that an electronegativity difference of about 2 means that
the bond has so much ionic character that it is best regarded as ionic (FIG. 3.13).
For electronegativity differences smaller than about 1.5, a covalent description of
the bond is reasonable. For example, the electronegativities of carbon and oxygen
are 2.55 and 3.44, an electronegativity difference of 0.89, and CßO bonds are best
regarded as polar covalent. However, there are exceptions to these guidelines. For
instance, the electronegativity of magnesium is 1.31, and MgßCl bonds, with an
electronegativity difference of 1.85, are considered ionic.