Dichromate ion (Cr₂O₇^2-) Lewis Structure

Lewis structure of dichromate (Cr₂O₇^2-) ion is drawn step by step in this tutorial. Total valence electrons of two chromium atoms, seven oxygen atoms and two negative charges are considered to draw the Cr₂O₇^2- Lewis structure. There are double bonds and single bonds in the lewis structure of Cr₂O₇^2- ion.

Lewis Structure of dichromate ion

There are seven oxygen atoms and two chromium atoms in dichromate ion.

Now, we are going to learn, how to draw the Lewis structure of Cr₂O₇^2- ion step by step. You will learn all steps and rules of Lewis structure drawing.

Basic steps of drawing Cr₂O₇^2- Lewis structure

Following steps are required to draw Cr₂O₇^2-- lewis structure and they are explained in detail in this tutorial.

• How to find total number of electrons of the valance shells of chromium and oxygen atoms and including charge of the anion.
• How many electrons pairs in valence shells.
• Determine center atom from chromium and oxygen atom.
• Put lone pairs on atoms.
• Stability of lewis structure - Check the stability and minimize charges on atoms by converting lone pairs to bonds to obtain the best lewis structure.

Drawing correct lewis structure is important to draw resonance structures.

Total number of electrons of the valance shells of chromium and oxygen atoms and charge of the anion

There are two chromium atoms and seven oxygen atoms in the dichromate ion. Also, there is a -2 overall charge on the Cr₂O₇^2- ion.

Chromium and oxygen are located at 6 and 16 groups respectively in the periodic table. So, both chromium and oxygen atoms have six electrons in their valence shells.

There are seven oxygen atoms and two chromium atoms. Oxygen and chromium are located at 6 and 16 groups respectively in the periodic table. So, both chromium and oxygen atoms have six electrons in their valence shells. ion, Therefore to find total number of valence electrons given by oxygen, and chromium atoms multiply relevant atom's valence electrons by number of relevant atoms atoms. See followings to understand.

• Total valence electrons given by oxygen atoms = 6 * 7 = 42
• Total valence electrons given by chromium atoms = 6 * 2 = 12

Due to -2 charge, another 2 electrons are added to the total number of valence electrons.

• Due to -2 charge, received electrons to valence electrons= 2

• Total number of valence electrons = 42 + 12 + 2 = 56

Total valence electrons pairs

Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells.

Total electron pairs are determined by dividing the number total valence electrons by two. For, Cr₂O₇^2- there are 56 valence electrons, so total pairs of electrons are 28. In next steps, we are going to mark those 28 lone pairs on oxygen atoms and chromium atom as bonds and lone pairs.

Center atom and skeletal structure of Cr₂O₇^2- ion

To be the center atom, ability of having greater valance is important. Chromium can show maximum valence upto 6. But oxygen's maximum valence is 2. Therefore, chromium has more chance to be the center atom (See the figure). So, now we can build a sketch of Cr₂O₇^2-.

Mark lone pairs on chromium and oxygen atoms

There are already eight Cr-O bonds in the sketch. Therefore, only twenty valence electrons pairs are remaining to draw the rest of ion.

Next step is, marking those twenty valence electrons pairs on outside atoms (in this case, oxygen atoms) as lone pairs. One oxygen atom (the oxygen atom in middle will only take two lone pairs) will take three lone pairs following the octal rule (oxygen atoms cannot keep more than eight electrons in their valence shells). Therefore, twenty electrons pairs are marked on seven oxygen atoms. Now, all electrons pairs are finished due to marking on oxygen atoms.

So, there is no valence electrons pair to mark on chromium atom.

Mark charges on atoms

Check charges on atoms and mark them as below figure. Charges are important to decide the best lewis structure of the ion since in the best lewis structure, charges should be minimized.

Each oxygen atom except center oxygen atom has -1 charge and both chromium atom have +2 charge.

Check the stability of drawn structure of Cr₂O₇^2- ion and reduce charges on atoms by converting lone pairs to bonds

The drawn structure for Cr₂O₇^2- is not a stable structure because oxygen atoms and chromium atom have charges. Also, when charge of an atom (in chromium atom, there is a +2 charge) is large, that structure become more unstable and cannot be a good lewis structure. When an ion has so many charges on atoms, that structure is not stable. Now, we are going to find a more stable structure.

Now, we should try to minimize charges by converting lone pair or pairs which exist on oxygen atoms to bonds. So, we convert one lone pair of one oxygen atom as a Cr-O bond as in the following figure.

Now there is a double bond between one chromium and one oxygen atom. There are also five single bonds (Cr-O) with chromium atom and other five oxygen atoms. (The oxygen atom in middle already completed the valence shell.)

But there are still lot of charges on atoms and given structure is not stable yet. If possible, we should reduce charges furthermore. Another lone pair on another oxygen atom is transferred as a Cr-O bond.

We can do the same procedure to the other chromium atom too and will get the lewis structure of dichromate ion. In the end of this process, both chromium atoms should not have charges and only two oxygen atoms have charges (-1).