Egon wrote:
Q: Why is it that formal charges can range from -4 to +7 but partial charges are only in the range -1.0 to +1.0 ?
The formal charge on a molecule represents the overall charge (in electrons) on that species. Molecules and atoms can lose or gain several electrons. A common example is the Fe atom which can give rise to Fe2+ and Fe3+. This gives rise to a concept called oxidation state. Thus in FeCl2 (a neutral molecule) the Cl has a conventions formal charge of -1 so the iron has an oxidation sate of +2. In MnO4- (permanganate) the oxygens have a formal oxidation state of -2 so to create electrical balance the Mn has an oxidation state of +7. This is a useful housekeeping and descriptive device but no one would imagine that there is actually a 7+ species.
The partial charge is an attempt to describe the electron distribution in a molecule by partitioning the molecule into atoms and estimating how many electrons are associated with each. There are many ways of calculating the electron distribution and several or partitioning it so there are tens, if not hundreds of methods for estimating the charge associated with an atom. (Note this is the difference from neutral atoms). It turns out that almost all calculations give results in the range you quote.
P.
Miguel
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