AsO2- Lewis Structure in 6 Steps (With Images)

So you have seen the above image by now, right?

Let me explain the above image in short.

AsO2- lewis structure has an Arsenic atom (As) at the center which is surrounded by two Oxygen atoms (O). There is 1 double bond and 1 single bond between the Arsenic atom (As) and each Oxygen atom (O). There is a -1 formal charge on the single bonded Oxygen atom (O).

If you haven’t understood anything from the above image of AsO2- lewis structure, then just stick with me and you will get the detailed step by step explanation on drawing a lewis structure of AsO2- ion.

So let’s move to the steps of drawing the lewis structure of AsO2- ion.

Steps of drawing AsO2- lewis structure

Step 1: Find the total valence electrons in AsO2- ion

In order to find the total valence electrons in AsO2- ion, first of all you should know the valence electrons present in the arsenic atom as well as oxygen atom.
(Valence electrons are the electrons that are present in the outermost orbit of any atom.)

Here, I’ll tell you how you can easily find the valence electrons of arsenic as well as oxygen using a periodic table.

Total valence electrons in AsO2- ion

→ Valence electrons given by arsenic atom:

Arsenic is a group 15 element on the periodic table. ^[1] Hence the valence electrons present in arsenic is 5.

You can see the 5 valence electrons present in the arsenic atom as shown in the above image.

→ Valence electrons given by oxygen atom:

Oxygen is group 16 element on the periodic table. ^[2] Hence the valence electrons present in oxygen is 6.

You can see the 6 valence electrons present in the oxygen atom as shown in the above image.

Hence, 

Total valence electrons in AsO2- ion = valence electrons given by 1 arsenic atom + valence electrons given by 2 oxygen atoms + 1 more electron is added due to 1 negative charge = 5 + 6(2) + 1 = 18.

Step 2: Select the central atom

For selecting the center atom, you have to remember that the atom which is less electronegative remains at the center.

Now here the given ion is AsO2- ion and it contains arsenic atom (As) and oxygen atoms (O).

You can see the electronegativity values of arsenic atom (As) and oxygen atom (O) in the above periodic table.

If we compare the electronegativity values of arsenic (As) and oxygen (O) then the arsenic atom is less electronegative.

So here the arsenic atom (As) is the center atom and the oxygen atoms (O) are the outside atoms.

Step 3: Connect each atoms by putting an electron pair between them

Now in the AsO2 molecule, you have to put the electron pairs between the arsenic atom (As) and oxygen atoms (O).

This indicates that the arsenic (As) and oxygen (O) are chemically bonded with each other in a AsO2 molecule.

Step 4: Make the outer atoms stable. Place the remaining valence electrons pair on the central atom.

Now in this step, you have to check the stability of the outer atoms.

Here in the sketch of AsO2 molecule, you can see that the outer atoms are oxygen atoms.

These outer oxygen atoms are forming an octet and hence they are stable. 

Also, in step 1 we have calculated the total number of valence electrons present in the AsO2- ion.

The AsO2- ion has a total 18 valence electrons and out of these, only 16 valence electrons are used in the above sketch.

So the number of electrons which are left = 18 – 16 = 2.

You have to put these 2 electrons on the central arsenic atom in the above sketch of AsO2 molecule.

Now let’s proceed to the next step.

Step 5: Check the octet on the central atom. If it does not have octet, then shift the lone pair to form a double bond or triple bond.

In this step, you have to check whether the central arsenic atom (As) is stable or not.

In order to check the stability of the central arsenic (As) atom, we have to check whether it is forming an octet or not.

Unfortunately, the arsenic atom is not forming an octet here. Arsenic has only 6 electrons and it is unstable.

Now to make this arsenic atom stable, you have to shift the electron pair from the outer oxygen atom so that the arsenic atom can become more stable.

After shifting this electron pair, the central arsenic atom will get 2 more electrons and thus its total electrons will become 8.

You can see from the above picture that the arsenic atom is forming an octet as it has 8 electrons.

Now let’s proceed to the final step to check whether the above lewis structure is stable or not.

Step 6: Check the stability of lewis structure

Now you have come to the final step in which you have to check the stability of lewis structure of AsO2.

The stability of lewis structure can be checked by using a concept of formal charge.

In short, now you have to find the formal charge on the arsenic atom (As) as well as oxygen atoms (O) present in the AsO2 molecule.

For calculating the formal charge, you have to use the following formula;

Formal charge = Valence electrons – (Bonding electrons)/2 – Nonbonding electrons

You can see the number of bonding electrons and nonbonding electrons for each atom of AsO2 molecule in the image given below.

For Arsenic (As) atom:
Valence electrons = 5 (because arsenic is in group 15)
Bonding electrons = 6
Nonbonding electrons = 2

For single bonded Oxygen (O) atom:
Valence electrons = 6 (because oxygen is in group 16)
Bonding electrons = 2
Nonbonding electrons = 6

For double bonded Oxygen (O) atom:
Valence electrons = 6 (because oxygen is in group 16)
Bonding electrons = 4
Nonbonding electrons = 4

From the above calculations of formal charge, you can see that the single bonded oxygen (O) atom has -1 charge and the other atoms have 0 charges.

So let’s keep these charges on the respective atoms in the AsO2 molecule.

This overall -1 charge on the AsO2 molecule is represented in the image given below.

In the above lewis dot structure of AsO2- ion, you can also represent each bonding electron pair (:) as a single bond (|). By doing so, you will get the following lewis structure of AsO2- ion.

I hope you have completely understood all the above steps.

For more practice and better understanding, you can try other lewis structures listed below.

Try (or at least See) these lewis structures for better understanding:

About author

Jay Rana

Jay is an educator and has helped more than 100,000 students in their studies by providing simple and easy explanations on different science-related topics. He is a founder of Pediabay and is passionate about helping students through his easily digestible explanations.

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