Lewis| Electron Dot Structure of dichromate Cr2O7-2 - #48

A simple procedure for writing 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 dichromate anion. The chemical formula is Cr₂O₇^-2. Consider that there is a Cr-O-Cr bond.
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.  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 Cr₂O₇^-2atoms connected with single bonds

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

Where n in this case is 7. Where V = (6 + 6 + 6 + 6 + 6 + 6 + 6 + 6 + 6 ) – (-2) = 56 , V is the number of valence electrons of  the dichromate ion.

Therefore, P = 6n + 2 – V = 6 * 9 + 2 – 56  = 0   So, there are no multiple bonds in the molecule.

 

Step 3 & 4: The  resonance structures of Cr₂O₇^-2  are as follows:

Fig. 2:  Lewis electron dot structures of Cr₂O₇^-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 #4 is the most stable structure due to less charge separation even though the octet rule is not obeyed by the Cr atom.