A hypothetical molecule, X–Y, has a dipole moment of 1.83 D and a bond length of 111 pm. Calculate the percent ionic character of this molecule.

Answer 1

This is a rather arcane question (It was entirely new to me!), but with a little research, here is what you need to know.

The % ionic character of a bond is sometimes found by comparing the observed dipole moment for that bond to the dipole moment you would get for fully separated charges at the bond distance--which does make some sense. This is expressed as:

%ionic character = (µ/eR)*100

Where µ is the observed dipole moment, e is the charge on one electron, and R is the bond length. The quantity eR is simply the dipole moment you would see for the fully separated charge.

All of this must be done using the correct units (cgs-esu), recognizing that 1 Debye = 1x10^-18 esu cm and that the charge on the electron is 4.80 x 10^-10 esu.

So, in this case, you have (after doing the needed conversions):

% ionic character = (1.83 x 10^-18 esu cm)/{(4.8 x 10^-10 esu)x(1.11x10^-8 cm)} x 100

or

100x{1.83/(4.8x1.11) = 34.3 %

Be sure you know how to do this for another example (pay particular attention to the units)!