# How are Lewis dot diagrams used to represent ionic compounds?

Jan 23, 2017

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

#### Explanation:

Let's take a simple example: $\text{sodium sulfate,}$ $N {a}_{2} S {O}_{4}$. 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 $2 \times N {a}^{+}$; the individual sodium ion has 10 electrons, and is thus a cation. Why?

For $\text{sulfate dianion}$, we have $6 + 4 \times 6 + 2$ 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 ${\left(O =\right)}_{2} S {\left(- {O}^{-}\right)}_{2}$ 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 ${\text{^(2+)S(-O)}}_{4}^{-}$ 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 ${\left(O =\right)}_{2} S {\left(- {O}^{-}\right)}_{2}$, and the parent sulfuric acid as ${\left(O =\right)}_{2} S {\left(- O H\right)}_{2}$ are what you will generally find in a text. The Lewis representation of $\text{sulfuric acid}$, and $\text{nitric acid}$, $O = {N}^{+} \left(- {O}^{-}\right) \left(O H\right)$, are thus both a bit problematic.