Ionic character...and Partial charges?

Question

So for example with PCL3, would you just subract one O's value from P, or 3 O's? It isn't explained in my textbook.

i.e. 3(3.5)-2.1 or 3.5-2.1?

Also, how do you figure partial charge?

Answer 1

Although I don't know what an "O" is in this problem (did you mean Cl?), from your eqns. it looks like you're trying to figure out whether bonds are polar or nonpolar. To tell how polar a bond is, subtract the electronegativity of one element from the other that's involved in the bond. The larger the electronegativity difference, the more polar the bond. A difference of 0 means you have a nonpolar bond, with no partial charges (and essentially no ionic character). Finally, although it varies from book to book (and instructor to instructor), for very small differences in electronegativity (0.4 or less, usually), chemists will usually classify such bonds as nonpolar (with very little ionic character).

To apply to your problem, to tell whether the bonds in PCl3 are polar, you would do 3.0-2.1. (In other words, look at ONE of the bonds in the molecule.) This is a fairly sizable electonegativity difference...this IS a covalent bond, but it's polar...there's a fair amount of ionic character in the bond.

To determine the partial charges, look at the electronegatives of the atoms involved in the bond. The more electronegative element (in this case Cl) will have a partial negative charge...bonding electrons hang out around the Cl atom more than they do the P atom. The less electronegative element (P) will have a partial positive charge, because the bonding electrons hang out less around P when compared to Cl.