Several different equations can be used to estimate the degree to which a chemical
bond between two atoms is ionic or covalent. The electronegativity difference between
the atoms (xA – xB) is a central key. xA is the electronegativity of the more
electronegative element. One of these equations was developed by the famous
chemist, Linus Pauling. When this equation is plotted, it has the shape of a stretchedout S. Such a graph is shown below where fractional ionicity represents the fractional
ionic character (a value of 1 indicates 100% ionic).
1. Consider the following compounds: KBr, H2S, H2O, HI, RbF, and NaCl. Consider
whether each of these compounds should be classified as ionic or covalent in their
bonding nature.
2. Look up the electronegativities for the atoms in these six compounds. Calculate the
six values for xA – xB.
3. Locate the positions of these compounds on the graph. Mark each compound on the
graph. List the six compounds in order of increasing ionicity.
4. Estimate from the curve the value for fractional ionicity when xA – xB = 4.
5. Estimate from the curve the value for fractional ionicity when xA – xB = .5, 1.0, 1.5,
2.0, 2.5, 3.0 and 3.5.
6. Carefully inspect an electronegativity table that yields the values of electronegativity
for all the elements. Is it possible to have a bond that is 100% ionic? Explain.
7. Recall, last year, you learned when the value of xA – xB was <.5, the bond type was
called covalent; when the value of xA – xB was ≥ .5 but < 2.0 the bond type was called
polar covalent and any value of xA – xB ≤ 2.0 was called ionic. What can you conclude
about this “rule” now?