Procedure

  1. Obtain a graph of fractional ionicity versus electronegativity difference.

  2. Look up electronegativities for the atoms in these six compounds: KBr, H2S, H2O, HI, RbF, and NaCl. Calculate six values for xA - xB.

  3. Locate the positions of these compounds on the graph. Mark each compound on the graph.

  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 = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5.

    (Teacher's Note: Many textbooks refer to xA - xB = 1.7 [50% ionicity] as an arbitrary demarcation between covalent and ionic bonds. Reference to the graph shows the futility of trying to find a "boundary" between the two types. For example, according to this arbitrary rule, HF would be ionic; however, experiment shows this compound is polar covalent. Other examples underscore the fact that there is really no way to classify most compounds as being either ionic or covalent. There is a gradual increase in ionicity when electronegativity difference increases, as shown by the graph. Thus, it's reasonable to state that when ionicity is greater than 50%, there is considerable ionic character to the bond. Typical ionic properties are usually encountered at greater values of ionicity. However, there's only one kind of "pure" bond, the homonuclear covalent bond.


What does all of this show?
  1. An electron is never completely transferred from one atom to another to form an ionic compound.

  2. An electron pair is always shared to some degree between two atoms.

  3. The more equal the sharing, the higher the covalent nature of that compound.

  4. Rule of Thumb: If a compound has an ionicity greater than 50%, (some authorities use an electronegativity difference of 2.0 representing about 63% ionicity) the compound is defined as "ionic". So, to simplify life, an arbitrary classification "ionic" is used.

At this point, it might be well to define ionic solid as operationally one whose water solution is a good conductor of electric current. If a compound is insoluble in water, and if in a fused (melted) form conducts electric current, then the compound may be classified as ionic.

Normally, DX means Xfinal state - Xinitial state, where X is any common thermodynamic state function such as P, T, V, H, E, and S. However, when bond energies are calculated, DH means total BEall reactants - total BEall products, where BE corresponds to the bond energies obtained from a table of bond energies for diatomic molecules. The DH obtained in this way is the energy required to break all bonds of reactant molecules minus the energy evolved when new bonds are made within product molecules.


Chemical Bonding (BOND)
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