In chemistry, metals and nonmetals form ionic bonds, and two or more nonmetals form covalent bonds. These two bond types represent fundamentally different atomic interactions: covalent bonds involve the sharing of electrons between atoms, whereas ionic bonds result from the atoms possessing opposite charges. The truth, however, is more complicated, because few bonds exhibit purely ionic or purely covalent properties. That is, bonds tend to contain both ionic and covalent character. Linus Pauling deduced an equation to describe the fractional covalent character of a bond based on each atom’s electronegativity, or the ability of the atom to attract electrons to itself.
Determine the Pauling electronegativities of the two elements involved in the bond. Numerous print and online references provide this information (see Resources). For a bond between silicon and oxygen, for example, the electronegativity values would be 1.8 for silicon and 3.5 for oxygen.
Subtract the smaller electronegativity value from the larger value to determine the difference in electrongativity, X. Continuing the example from Step 1, the electronegativity difference is X = (3.5 - 1.8) = 1.7.
Substitute the value of X from Step 2 into the fraction-covalent equation: FC = exp (-0.25 * X^2). In the example presented in Steps 1 and 2, FC = exp (-0.25 * 1.7^2) = exp (-0.25 * 2.9) = exp (-0.72) = 0.49.
The notation exp (x) is the mathematical notation for “e to the power of x,” where e is the natural logarithm base, 2.718. Note also that the the notation x^2 indicates “x squared,” or “x to the power of 2.”
Remember to always follow the scientific order of operations when performing calculations: Perform operations in parentheses first, and calculate exponents before performing multiplication or division.