Lewis structure of Cr2O72-

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Lewis structure of Cr2O72-
Lewis structure of Cr2O72- | Image: Root Memory

The Lewis structure of Cr2O72- contains four single bonds and four double bonds, with one oxygen and two chromiums in the center, and six other oxygens on either side. The center oxygen atom, each top oxygen atom, and each bottom oxygen atom have two lone pairs. The left oxygen atom and right oxygen atom have three lone pairs, and the chromium atom does not have any lone pair.

Plus, there is a negative (-1) charge on the left oxygen atom and right oxygen atom.

Steps

By using the following steps, you can easily draw the Lewis structure of Cr2O72-.

#1 Draw skeleton
#2 Show chemical bond
#3 Mark lone pairs
#4 Calculate formal charge and check stability (if octet is already completed on central atom)
#5 Convert lone pair and calculate formal charge again (if formal charges are not closer to zero)

Let’s one by one discuss each step in detail.

#1 Draw skeleton

In this step, first calculate the total number of valence electrons. And then, decide the central atom.

  • Let’s calculate the total number of valence electrons

We know that… chromium is a transition metal that has an electron configuration [Ar] 3d54s1, so chromium has six valence electrons. And oxygen is a group 16 element, so oxygen has six valence electrons.

Now Cr2O72- has two chromium atoms and seven oxygen atoms.

So the total number of valence electrons = (valence electrons of chromium atom × 2) + (valence electrons of oxygen atom × 7)

And Cr2O72- has a negative (-2) charge, so we have to add two more electrons.

Therefore, the total number of valence electrons = 12 + 42 + 2 = 56

  • Now decide the central atom

The atom with the least electronegative value is placed at the center. By looking at the periodic table, we get the electronegativity values for chromium and oxygen as follows:

Electronegativity value of chromium = 1.66
Electronegativity value of oxygen = 3.44

Obviously, chromium is less electronegative than oxygen. Hence, assume that left chromium is the central atom (as there are two chromium).

So now, put two chromium in the center and oxygens on either side. And draw the rough skeleton structure for the Lewis structure of Cr2O72- something like this:

Skeleton structure for Lewis structure of Cr2O72- | Image: Root Memory

#2 Show chemical bond

Place two electrons between the atoms to show a chemical bond. Since two chromium are surrounded by seven oxygens, use sixteen electrons to show eight chemical bonds as follows:

Eight chemical bonds shown between atoms | Image: Root Memory

#3 Mark lone pairs

As calculated earlier, we have a total of 56 valence electrons. And in the above structure, we have already used sixteen valence electrons. Hence, forty valence electrons are remaining.

Two valence electrons represent one lone pair. So forty valence electrons = twenty lone pairs.

Note 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.

Also, make sure that you start marking these lone pairs on outside atoms first. And then, on the central atom.

The outside atoms are oxygens and right chromium, so left oxygen, right oxygen, top oxygens, and bottom oxygens will get three lone pairs. The center oxygen will get two lone pairs. And right chromium and central atom (left chromium) will not get any lone pair, because all twenty lone pairs are used.

So the Lewis structure of Cr2O72- looks something like this:

Lone pairs marked on Lewis structure of Cr2O72- | Image: Root Memory

In the above structure, you can see that the octet is completed on the central atom (left chromium), and also on the outside atoms. Therefore, the octet rule is satisfied.

Now calculate the formal charge and check the stability of the above structure.

#4 Calculate formal charge and check stability

The following formula is used to calculate the formal charges on atoms:

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

Collect the data from the above structure and then, write it down below as follows:

  • For each chromium atom

Valence electrons = 6
Nonbonding electrons = 0
Bonding electrons = 8

Formal charge = 6 – 0 – ½ (8) = +2

  • For left oxygen, right oxygen, each top oxygen, and each bottom oxygen atom

Valence electrons = 6
Nonbonding electrons = 6
Bonding electrons = 2

Formal charge = 6 – 6 – ½ (2) = -1

  • For center oxygen atom

Valence electrons = 6
Nonbonding electrons = 4
Bonding electrons = 4

Formal charge = 6 – 4 – ½ (4) = 0

Mention the formal charges of atoms on the structure. So the Lewis structure of Cr2O72- looks something like this:

Formal charges are not closer to zero | Image: Root Memory

In the above structure, you can see that the formal charges of atoms are not closer to zero. Therefore, convert lone pair and calculate formal charge again.

#5 Convert lone pair and calculate formal charge again

As mentioned earlier, chromium is a period 4 element, so it can keep more than 8 electrons in its last shell.

So convert one lone pair from each top oxygen atom and each bottom oxygen atom to make a new bond with the chromium atom. And then, the Lewis structure of Cr2O72- looks something like this:

Lone pair of each top and each bottom oxygen is converted, and octet is completed on atoms | Image: Root Memory

Now one last thing we need to do is, calculate the formal charge again and check the stability of the above structure.

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

Collect the data from the above structure and then, write it down below as follows:

  • For each chromium atom

Valence electrons = 6
Nonbonding electrons = 0
Bonding electrons = 12

Formal charge = 6 – 0 – ½ (12) = 0

  • For each top oxygen, each bottom oxygen, and center oxygen atom

Valence electrons = 6
Nonbonding electrons = 4
Bonding electrons = 4

Formal charge = 6 – 4 – ½ (4) = 0

  • For left oxygen and right oxygen atom

Valence electrons = 6
Nonbonding electrons = 6
Bonding electrons = 2

Formal charge = 6 – 6 – ½ (2) = -1

Mention the formal charges of atoms on the structure. So the Lewis structure of Cr2O72- looks something like this:

Formal charges are calculated, and got the most stable Lewis structure of Cr2O72- | Image: Root Memory

In the above structure, you can see that the formal charges of atoms are closer to zero. Therefore, this is the most stable Lewis structure of Cr2O72-.

And each horizontal line drawn in the above structure represents a pair of bonding valence electrons.

Now Cr2O72- is an ion having a negative (-2) charge, so draw brackets around the above Lewis structure and mention that charge on the top right corner. And then, the Lewis structure of Cr2O72- looks something like this:

Lewis structure of Cr2O72- showing a negative (-2) charge | Image: Root Memory

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Deep

Rootmemory.com was founded by Deep Rana, who is a mechanical engineer by profession and a blogger by passion. He has a good conceptual knowledge on different educational topics and he provides the same on this website. He loves to learn something new everyday and believes that the best utilization of free time is developing a new skill.

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