AsCl3 Lewis structure

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AsCl3 Lewis Structure
AsCl3 Lewis structure

AsCl3 (arsenic trichloride) has one arsenic atom and three chlorine atoms.

In the AsCl3 Lewis structure, there are three single bonds around the arsenic atom, with three chlorine atoms attached to it. Each chlorine atom has three lone pairs, and the arsenic atom has one lone pair.


Here’s how you can easily draw the AsCl3 Lewis structure step by step:

#1 Draw a rough skeleton structure
#2 Mention lone pairs on the atoms
#3 If needed, mention formal charges on the atoms

Now, let’s take a closer look at each step mentioned above.

#1 Draw a rough skeleton structure

  • First, determine the total number of valence electrons
Periodic table

In the periodic table, arsenic lies in group 15, and chlorine lies in group 17.

Hence, arsenic has five valence electrons and chlorine has seven valence electrons.

Since AsCl3 has one arsenic atom and three chlorine atoms, so…

Valence electrons of one arsenic atom = 5 × 1 = 5
Valence electrons of three chlorine atoms = 7 × 3 = 21

And the total valence electrons = 5 + 21 = 26

  • Second, find the total electron pairs

We have a total of 26 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 = 26 ÷ 2 = 13

  • Third, determine the central atom

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

Since arsenic is less electronegative than chlorine, assume that the central atom is arsenic.

Therefore, place arsenic in the center and chlorines on either side.

  • And finally, draw the rough sketch
AsCl3 Lewis Structure (Step 1)
Rough sketch of AsCl3 Lewis structure

#2 Mention lone pairs on the atoms

Here, we have a total of 13 electron pairs. And three As — Cl bonds are already marked. So we have to only mark the remaining ten electron pairs as lone pairs on the sketch.

Also remember that arsenic is a period 4 element, so it can keep more than 8 electrons in its last shell. And chlorine is a period 3 element, so it can keep more than 8 electrons in its last shell.

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

So for each chlorine, there are three lone pairs, and for arsenic, there is one lone pair.

Mark the lone pairs on the sketch as follows:

AsCl3 Lewis Structure (Step 2)
Lone pairs marked, and got the stable Lewis structure of AsCl3

#3 If needed, mention formal charges on the atoms

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

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

For arsenic atom, formal charge = 5 – 2 – ½ (6) = 0

For each chlorine atom, formal charge = 7 – 6 – ½ (2) = 0

Here, both arsenic and chlorine atoms do not have charges, so no need to mark the charges.

In the above structure, you can see that the central atom (arsenic) forms an octet. And the outside atoms (chlorines) also form an octet. Hence, the octet rule is satisfied.

Therefore, this structure is the stable Lewis structure of AsCl3.

Next: CHBr3 Lewis structure

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