Cr2O72- Lewis structure

Cr₂O₇^2- (dichromate) has two chromium atoms and seven oxygen atoms.

In the Cr₂O₇^2- Lewis structure, there are two single bonds around the center oxygen atom, with two chromium atoms attached to it, and each chromium makes two single bonds and two double bonds with other three oxygen atoms. The oxygen atom with single bonds has three lone pairs, the oxygen atom with double bonds has two lone pairs, and the center oxygen atom also has two lone pairs.

Also, there is a negative (-1) charge on the two oxygen atoms with single bonds.

Alternative method: Lewis structure of Cr₂O₇^2-

Rough sketch

• First, determine the total number of valence electrons

In the periodic table, chromium is a transition metal having an electron configuration [Ar] 3d⁵4s¹, so chromium has six valence electrons. And oxygen lies in group 16, so oxygen also has six valence electrons.

Since Cr₂O₇^2- has two chromium atoms and seven oxygen atoms, so…

Valence electrons of two chromium atoms = 6 × 2 = 12
Valence electrons of seven oxygen atoms = 6 × 7 = 42

Now the Cr₂O₇^2- has a negative (-2) charge, so we have to add two more electrons.

So the total valence electrons = 12 + 42 + 2 = 56

Learn how to find: Oxygen valence electrons

• Second, find the total electron pairs

We have a total of 56 valence electrons. And when we divide this value by two, we get the value of total electron pairs.

Total electron pairs = total valence electrons ÷ 2

So the total electron pairs = 56 ÷ 2 = 28

• Third, determine the central atom

We have to place the least electronegative atom at the center.

Since chromium is less electronegative than oxygen, assume that the central atom is chromium.

Here, there are two chromium atoms, so we can assume any one as the central atom.

Let’s assume that the central atom is right chromium.

Therefore, place chromiums in the center and oxygens on either side.

• And finally, draw the rough sketch

Lone pair

Here, we have a total of 28 electron pairs. And eight Cr — O bonds are already marked. So we have to only mark the remaining twenty electron pairs as lone pairs on the sketch.

Also remember that chromium is a period 4 element, so it can keep more than 8 electrons in its last shell. And oxygen is a period 2 element, so it can not keep more than 8 electrons in its last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are oxygens.

So for center oxygen, there are two lone pairs. For the remaining six oxygens, there are three lone pairs (for each), and for chromium, there is zero lone pair because all twenty electron pairs are over.

Mark the lone pairs on the sketch as follows:

Formal charge

Use the following formula to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

For each chromium atom, formal charge = 6 – 0 – ½ (8) = +2

For center oxygen atom, formal charge = 6 – 4 – ½ (4) = 0

For remaining six oxygen atoms, formal charge = 6 – 6 – ½ (2) = -1

Here, both chromium and oxygen atoms have charges, so mark them on the sketch as follows:

The above structure is not a stable Lewis structure because both chromium and oxygen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.

Convert a lone pair of the left oxygen atom to make a new Cr — O bond with the left chromium atom as follows:

Since there are charges on chromium and oxygen atoms, again convert a lone pair of the top oxygen atom to make a new Cr — O bond with the left chromium atom as follows:

There are still charges on chromium and oxygen atoms, so again convert a lone pair of the right oxygen atom to make a new Cr — O bond with the right chromium atom as follows:

There are still charges on chromium and oxygen atoms, so again convert a lone pair of the top oxygen atom to make a new Cr — O bond with the right chromium atom as follows:

Final structure

The final structure of Cr₂O₇^2- comprises two central chromium atoms bridged by a single oxygen atom, with each chromium also bonded to three terminal oxygen atoms. In this configuration, each chromium atom utilizes an expanded valence shell to form double bonds with two of its terminal oxygens and a single bond with the third terminal oxygen, as well as a single bond to the central bridging oxygen. Within this layout, both chromium atoms, the bridging oxygen, and the four double-bonded terminal oxygens maintain a formal charge of zero, while the two single-bonded terminal oxygens each carry a -1 formal charge. Each double-bonded oxygen fulfills its octet with two lone pairs, the bridging oxygen retains two lone pairs, and the two negatively charged terminal oxygens each maintain three lone pairs. Accordingly, this specific electronic distribution serves as the definitive and most accurate Lewis representation of the dichromate ion.

To complete the representation, draw square brackets around the entire Lewis structure and place a “2-” sign as a superscript outside the upper right bracket. This notation signifies that the negative charge is a property of the whole ion, reflecting the presence of two additional electrons beyond the total valence count of the neutral atoms.

Next: C₂H₂F₂ Lewis structure

External video

• How to Draw the Lewis Dot Structure for CrO4 2- (Chromate ion) – YouTube • Wayne Breslyn

External links

• https://www.chemistryscl.com/general/Cr2O7-2–lewis-structure/index.php