Lewis| Electron Dot Structure of dichromate Cr2O7-2 | Chemistry Net

Lewis| Electron Dot Structure of dichromate Cr2O7-2

Lewis Electron Dot Structure of dichromate Cr<sub>2</sub>O<sub>7</sub><sup>-2</sup>

Lewis Electron Dot Structure of dichromate Cr2O7-2

A simple procedure for writing Lewis electron dot structures is 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).

Another example  for writing Lewis structures following the above procedure is given below.

Let us consider the case of the Lewis electron dot structures of the dichromate anion 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.

How can we draw the Lewis structures of Cr2O7-2 ?


Step 1: Connect the atoms with single bonds.

 Fig. 1:  The Cr2O7-2 atoms connected with single bonds

Step 2: Calculate the # of electrons in π bonds (multiple bonds) using  formula (1) in the article entitled “Lewis Structures and the Octet Rule”. 

Where n in this case is 9. 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.

So the structure of Step 1 is the Lewis structure.

Electrons are placed around each atom so that charge is minimized.

Step 3 & 4: The Lewis resonance structures of dichromate 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. 


Relevant Posts - Relevant Videos

Lewis Structures|Octet Rule: A Simple Method to write Lewis Structures

Electron Dot Structure of the Chromate Anion CrO4-2




  1. G.N. Lewis, J.A.C.S, 38, 762-785, (1916)
  2. E. C. McGoran, J. Chem. Educ., 68, 19-23 (1991)
  3. A.B.P. Lever, J. Chem. Educ., 49, 819-821, (1972)


Key Terms

resonance structures of dichromate Cr2O7-2, Lewis electron structures of dichromate , chemical formula of dichromate Cr2O7-2, simple procedure for drawing Lewis structures of dichromate,


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