AsF3 Lewis structure

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AsF3 Lewis Structure
AsF3 Lewis structure

AsF3 (arsenic trifluoride) has one arsenic atom and three fluorine atoms.

In the AsF3 Lewis structure, there are three single bonds around the arsenic atom, with three fluorine atoms attached to it. Each fluorine atom has three lone pairs, and the arsenic atom has one lone pair.


Use these steps to correctly draw the AsF3 Lewis structure:

#1 First draw a rough sketch
#2 Mark lone pairs on the atoms
#3 Calculate and mark formal charges on the atoms, if required

Let’s discuss each step in more detail.

#1 First draw a rough sketch

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

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

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

Since AsF3 has one arsenic atom and three fluorine atoms, so…

Valence electrons of one arsenic atom = 5 × 1 = 5
Valence electrons of three fluorine 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 fluorine, assume that the central atom is arsenic.

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

  • And finally, draw the rough sketch
AsF3 Lewis Structure (Step 1)
Rough sketch of AsF3 Lewis structure

#2 Mark lone pairs on the atoms

Here, we have a total of 13 electron pairs. And three As — F 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 fluorine 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 fluorines.

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

Mark the lone pairs on the sketch as follows:

AsF3 Lewis Structure (Step 2)
Lone pairs marked, and got the stable Lewis structure of AsF3

#3 Calculate and mark formal charges on the atoms, if required

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 fluorine atom, formal charge = 7 – 6 – ½ (2) = 0

Here, both arsenic and fluorine 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 (fluorines) also form an octet. Hence, the octet rule is satisfied.

Therefore, this structure is the stable Lewis structure of AsF3.

Next: BrCl3 Lewis structure

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