How are Lewis dot diagrams used to represent ionic compounds?

1 Answer
Jan 23, 2017

Most of the time, the Lewis structure of the individual ions are ignored.

Explanation:

Let's take a simple example: sodium sulfate, Na2SO4. As a salt, this is clearly electrostatically neutral, however, we can dig a bit deeper, and represent its Lewis structure.

For the positive sodium ions, we have 2×Na+; the individual sodium ion has 10 electrons, and is thus a cation. Why?

For sulfate dianion, we have 6+4×6+2 electrons to distribute, and this represents the 6 valence electrons from the 5 chalcogen atoms, plus the 2 electrons that constitute the negative charge.

A Lewis structure of (O=)2S(O)2 in which the neutral atoms are each associated with 6 valence electrons, and the anionic oxygens with 7 valence electrons (and hence the negative charges) is commonly invoked. Such a structure implies the equivalence of ALL of the oxygen atoms, in that we can draw resonance structures in which the negative charges can reside on any two oxygen atoms.

A representation as 2+S(O)4 is contraindicated by standard rules of Lewis structure determination, even tho it is a fact that all the oxygen atoms are equivalent. Representation of sulfate as (O=)2S(O)2, and the parent sulfuric acid as (O=)2S(OH)2 are what you will generally find in a text. The Lewis representation of sulfuric acid, and nitric acid, O=N+(O)(OH), are thus both a bit problematic.