Electron Dot Structure of the Chromate Anion CrO4-2 / Example #47

A simple procedure for writing electron dot Lewis Structures was given in a previous article entitled “Lewis Structures and the Octet Rule”. 

Several worked examples relevant to this procedure were given in previous posts please see the Sitemap - Table of Contents (Lewis Electron Dot Structures).

Let us consider the case of the chromate anion  CrO₄^-2. Chromates and dichromates are used in chrome plating to protect metals from corrosion and to improve paint adhesion. Chromate and dichromate salts of heavy metals and alkaline earth metals are only very slight soluble in water and are thus used as pigments. Chromates are also used as rust and corrosion inhibitors in diesel engines. Dichromate is converted to chromic sulfate for tanning leather. The reaction of chromium with collagen raises the hydrothermal stability of the leather and makes it resistant to bacterial attack. The reaction with collagen is also a useful reaction in photography as a sensitizer for gelatin coatings.

Step 1: Connect the atoms with single bonds. 

Fig. 1:  The CrO₄^-2 atoms connected with single bonds

 Step 2:  Calculate the # of electrons in π bonds (multiple bonds) using  formula (1): 

Where n in this case is 4. Where V = (6 + 6 + 6 + 6 +6) – (-2) = 32 , V is the number of valence electrons of the molecule.

Therefore, P = 6n + 2 – V = 6 * 5 + 2 – 32  = 0     Therefore, there are no multiple bonds in the molecule.

 Step 3 & 4: The  resonance structures of  CrO₄^-2  are as follows:

Fig. 2:  Lewis electron dot structures of the chromate anion CrO₄^-2

The Cr atom can accommodate more than 8 valence electrons - elements above row 2 of the periodic table can accommodate more than 8 electrons. Lewis structure #3 is the most stable structure due to less charge separation even though the octet rule is not obeyed by the Cr atom.