Dipole Moment and Partial Charges

The shift of electron density in a polar bond is symbolized by placing a crossed arrow (a dipole aiTow) above the Lewis structure to indicate the direction of the shift. For example. 

A quantitative measure of the polarity of a bond is its dipole moment (p.), which is calculated as the product of the charge (Q) and the distance (r) between the charges  

The dstance, r between partial diarges in a polar datomic rnolecule is the bond length expressed in meters. Bond lengths are usually given in angstroms (A), or picometers (pm), so it IS generally necessary to convert to meters. 

In order for a diatomic molecule containing a polar bond to be electrically neutral, the partial positive and partial negative charges must have the same magnitude. Therefore, the Q term in Equation 8.1 refers to the magnitude of the partial charges and the calculated value of p is always positive. Dipole moments are usually expressed in debye units (D). named for Peter Debye. In terms of more familiar SI units, 

TABLE 8.5 1 Bond Leii hs and Dipole Moments uf tlte Hydmac i Halido.s 

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