Limitations of Method
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Limitations of the Method

This method of predicting formulas  for compounds (described in the preceding "Predicting Formulas" section) is important but it is of limited value because it will predict only a limited number of the compounds that actually do exist. For example, carbon and oxygen form two compounds. Nitrogen and oxygen form six or seven compounds. Chlorine and oxygen form four or five compounds.
CH4 NH3 H2O HCl
SiH4 PH3 H2S HBr
CO2 N2O3 Cl2O
CCl4 CS2 PBr3 NI3

 

The limitations of this method stem from two assumptions. It assumes that the octet rule is completely valid, and that there are equal numbers of electrons from each element in the bonds. These assumptions work for the examples given so far, but are not true for many other compounds. In short, this method provides us with a good explanation of a limited number of simple compounds.

That leads us to another use of Lewis diagrams, making sense out of where the electrons are in formulas that we would not have predicted. That is the subject of the next section "Deducing Lewis Diagrams".

 

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