# Chromate ion (CrO42-) Lewis Structure

Lewis structure of chromate ion (CrO42-) ion is drawn step by step in this tutorial. Total valence electrons of chromium atom, four oxygen atoms and two negative charges are considered to draw the CrO42- lewis structure.

Written by: Kumaragurunnanselage Damsarani, (undergraduate), Department of Civil Engineering, University of Peradeniya,

## Lewis Structure of Chromate ion

There are four oxygen atoms and chromium atom in chromate ion. Also, there is -1 charges on two oxygen atoms. In the lewis structure of chromate ion contains two double bonds.

Now, we are going to learn, how to draw the Lewis structure of CrO442- ion step by step. You will learn all steps and rules of Lewis structure drawing.

## Basic steps of drawing CrO42- Lewis structure

Following steps are required to draw CrO42-- 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 one chromium atom and four oxygen atoms in the chromate ion. Also, there is a -2 overall charge on the CrO42- ion which is also should be considered to draw the lewisstructure.

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.

• Total valence electrons given by chromium atom = 6*1 = 6

There are four oxygen atoms in CrO42- ion, Therefore to find total number of valence electrons given by oxygen, multiply valence electrons of oxygen atom by number of oxygen atoms.

• Total valence electrons given by oxygen atoms = 6 * 4 = 24

Due to -2 charge, another 2 electrons are added

• Due to -2 charge, received electrons to valence electrons= 2
• Total valence electrons = 6 + 24 + 2 = 32

### 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 CrO42- ion, there are 32 valence electrons, so total pairs of electrons are 16. In next steps, we are going to mark those 16 lone pairs on oxygen atoms and chromium atom as bonds and lone pairs.

### Center atom of CrO42- ion

To be the center atom, ability of having greater valance is important. Chromium can show maximum valence 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 CrO42- ion. .

### Lone pairs on atoms

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

Next step is, marking those twelve valence electrons pairs on outside atoms (in this case, oxygen atoms) as lone pairs. One oxygen atom will take three lone pairs following the octal rule (oxygen atoms cannot keep more than eight electrons in their valence shells). Therefore, twelve electrons pairs are marked on four 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. Charges are important to decide the best lewis structure.

Each oxygen atom has -1 charge and chromium atom has +2 charge.

### Check the stability of drawn structure of CrO42- ion and reduce charges on atoms by converting lone pairs to bonds

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

The drawn structure for CrO42- 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 a molecule or ion has so many charges on atoms, that structure is not stable. Now, we should try 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 chromium and one oxygen atom. There are also three single bonds (Cr-O) with chromium atom and other three oxygen atoms.

But there are still 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.

Now, there are twelve electrons around chromium atom. This is acceptable because chromium can keep more than eight electrons because chromium has unfilled 3d and 4s orbits. Now you understand this structure of CrO42- is more stable than previous structure due to less charges on atoms.